Behavioral Ecology Vol. 7 No. 3: 235-242
Functional shifts in the use of parasitized
hosts by a tephritid fly: the role of host quality
Daniel R. Papaj" and Russell H. Messing15
a
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA, and
Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA
b
Superparasitism, a phenomenon in which parasitic insects lay eggs in already-exploited hosts, provides a useful context in which
to examine the dynamics of parental investment. This study explored conditions under which female Mediterranean fruit flies
(Ceratitis capitata) shift from avoiding superparasitism of host fruit to preferring it, even placing eggs direcdy into existing egglaying cavities. An a priori hypothesis of costs and benefits was used to predict how use and avoidance of parasitized fruit would
change in -response to changes in fruit size and ripeness. We predicted that avoidance would decrease with increasing fruit size,
while use would increase with decreasing ripeness. Using afield-cageassay, ripeness was held constant and the size of host coffee
berries manipulated. Avoidance of parasitized berries was significandy less pronounced on large berries than on small ones. In
a second experiment, size was held constant and ripeness manipulated. On unripe berries, females deposited the majority of
clutches direcdy into existing egg-laying cavities. On ripe berries, by contrast, die same females deposited most clutches in
previously unparasitized fruit. Parallel patterns in the frequency of female-female contests were observed, supporting die nodon
diat a fruit's value is determined by an interaction between fruit size or ripeness, on one hand, and die prior occurrence of
eggs, on die odier. Laboratory assays suggested that use of existing sites had advantages in terms of time savings; female behavior
thus constitutes a relatively uncommon example of adaptive superparasitism in which parasitized hosts are actually preferred
over unparasitized ones. Key words: Ceratitis capitata, competition, host-marking pheromone, parent-offspring conflict, parental
investment, superparasitism, Tephritidae. [Behav Ecol 7:235—242 (1996)]
S
hifts in patterns of parental investment in animal populations, particularly under real-world conditions, have been
poorly documented (Clutton-Brock, 1991). Recendy, such
shifts have been examined intensively widi respect to die occurrence of superparasitism, a phenomenon in which parasitic insects lay eggs in hosts already parasitized. For example,
females in a number of phytophagous and entomophagous
species avoid laying eggs at sites diat have been exploited by
conspecifics (reviews by Godfray, 1994; Papaj, 1993; Roitberg
and Prokopy, 1987), a behavior diat minimizes die level of
intraspecific competition incurred by a female's progeny.
Avoidance of previously exploited hosts is usually mediated by
a chemical mark, termed a host-marking pheromone, put
down by die ovipositing female on or in the host (reviews by
Roitberg and Prokopy, 1987). Given costs to die female in
terms of pheromone production and additional search effort,
avoidance of parasitized hosts appears to represent a legitimate (albeit simple) form of maternal investment in die interests of her offspring. Avoidance of superparasitism, as die
deposition of eggs in parasitized hosts is often termed, is ideal
for die analysis of maternal investment because it is easily
quantified and has clear fitness consequences.
Superparasitism is not avoided absolutely by many parasitic
insects. A number of factors including egg load, nutritional
status, presence of conspecifics, and even weather conditions
influence the extent to which females of a given parasite species avoid use of parasitized hosts (Godfray, 1994; Papaj, 1993;
Roitberg et al., 1992, 1993; Visser et al., 1990). Recent dieoretical effort indicates that superparasitism can at least sometimes be adaptive, a perspective diat has received significant
empirical support (van Alphen and Visser, 1990; Visser et al.,
1990). For example, if unparasitized hosts are rare and dierefore hard to find, it can benefit a female to use parasitized
Received 27 December 1994; revised 17 July 1995; accepted 13 August 1995.
1045-2249/96/S5.00 © 1996 International Society for Behavioral Ecology
hosts. Consistent widi diis prediction, female tephritid fruit
flies progressively deprived of die opportunity to lay eggs
avoid parasitized host fruit to progressively lesser degrees
(Roitberg and Prokopy, 1983).
Notwidistanding die adaptiveness of superparasitism under
certain conditions, a parasitized host has almost always been
presumed to represent a relatively low-quality resource to a
female. Given a simultaneous choice between parasitized and
unparasitized hosts similar in all odier respects, a female is
presumed to prefer to lay eggs in die unparasitized host (van
Alphen and Visser, 1990). This presumption is implicit in
models of die evolution of host-marking pheromone communication systems (review by Godfray 1994; Roitberg and
Mangel, 1987) and, to date, has been supported by behavioral
data for a variety of insect species (review by Papaj, 1993).
In this context, the behavior of die Mediterranean fruit fly,
Ceralilis capitata (Diptera: Tephritidae), is particularly intriguing. As in taxonomically related tephritid species, the Mediterranean fruit fly (hereafter referred to as medfly) marks die
fruit after egg laying. As in those species, the host-marking
pheromone (when applied experimentally to intact fruit and
tested against an unmarked control) has a deterrent effect on
egg laying (Papaj et al., 1989a, 1992; Prokopy et al., 1978).
Since larvae are constrained to complete development widiin
the fruit in which they are deposited, the deterrency of hostmarking pheromone presumably functions to reduce die level
of competition incurred by a female's young. This inference
is supported by abundant evidence that medfly larvae compete within fruit (Debouzie, 1977a,b, 1978, 1981; Papaj etal.,
1989b). Yet despite die deterrency of host-marking pheromone and evidence of its functional significance, medfly females sometimes prefer to lay eggs in egg-infested hosts, often
depositing clutches direcdy into egg-laying punctures established by anodier female (Back and Pemberton, 1915; Papaj
etal., 1989a, 1992; Papaj, 1993 and references within; Silvestri,
1914). Use of existing egg-laying sites has also been documented in walnut-infesting Rhagolelis flies (Lalonde and Man-
Behavioral Ecology Vol. 7 No. 3
236
gel, 1994; Papaj, 1993, 1994b) and several entomophagous
parasitoids (review by Godfray, 1994; Price, 1970; Takasu and
Hirose, 1988).
Deposition of clutches into existing egg-laying cavities, as
observed in medfly and walnut flies, holds several possible
advantages for females (Lalonde and Mangel, 1994; Papaj,
1993, 1994). In medfly, use of existing sites is associated with
increases in the proportion of egg-laying attempts that result
in successful deposition of a clutch (Papaj etal., 1992) as well
as a reduction in the time required to deposit a clutch (Papaj,
1993). If females are time-stressed (cf. Papaj, 1993), time savings might translate to increases in the rate at which eggs are
laid. Although firm evidence on this point is lacking, use of
existing sites may also reduce ovipositor wear and improve the
success with which a female is able to establish her own sites
(Lalonde and Mangel, 1994; Papaj, 1993, unpublished data).
In short, we expect females to use existing sites when the
benefits of doing so in terms of time savings or reduced ovipositor wear exceed costs in terms of added competition incurred by their young. In the following study, we were particularly interested to know if a female medfly's use of existing
sites might change from a preference for such sites to avoidance of them in accordance with changes in perceived costs
and benefits. Such a finding would amount to a demonstration that, under one set of environmental conditions, females
act overtly in their offspring's interests and avoid superparasitism, while under other conditions, females act overtly
against those interests and superparasitize. In our study, we
took advantage of the fact that it is relatively easy to manipulate the fly's host environment and to observe host foraging
behavior under conditions closely approximating natural ones.
We employed an experimental approach to the dynamics of
host use in which the presumed costs and benefits of using
existing egg-laying sites were manipulated in realistic ways and
a priori predictions about female behavior evaluated directly.
METHODS
Experimental rationale and predicted effects
We first set out to show that females behave on test fruit as
though larvae in successive clutches compete with one another. We were not interested in quantifying larval competition;
rather, we were interested in manipulating factors known to
affect levels of larval competition in tephritid flies and assaying female response to these manipulations. The presumed
cost of larval competition associated with use of parasitized
fruit was manipulated experimentally by varying fruit size. In
another RhagoUlis species, R. pomonella, small fruit provide
less substrate for developing tephritid fly larvae than large
fruit and are associated with correspondingly higher levels of
competition at a given larval density (Averill and Prokopy,
1987a). We predicted that females foraging on large fruit
would avoid laying eggs in parasitized fruit to a lesser degree
than females on small fruit.
We next set out to show that, despite a capacity to act to
reduce effects of larval competition, females sometimes act to
increase levels of competition by using existing sites. The presumed benefits of use of existing sites associated with time
savings and/or ovipositor wear were manipulated by varying
fruit penetrability or, equivalendy, fruit ripeness. On a ripe
fruit whose soft surface is penetrated quickly by a female's
ovipositor, the success in depositing clutches should be relatively high, the time saved by exploiting existing punctures
relatively low, and ovipositor wear suffered by establishing new
sites relatively slight (Papaj, 1993; Papaj et al., 1992). For all
of these reasons, we predicted that females foraging on ripe
fruit would use existing sites to a lesser degree than females
on unripe fruit.
General protocol
Experiments were conducted on the grounds of the Kauai
Research Station in Kapa'a, Hawaii. All flies used in tests were
of wild origin, collected as larvae in loquats from the island
of Maui, and held in 25 cm X 25 cm X 25 cm cages at ambient
temperatures and photoperiod. All assays were conducted using previously unparasitized coffee berries (Coffea arabica, Rubiaceae) collected from an unmanaged grove near Kapa'a.
Coffee was used because, of the over 250 species of fruits,
nuts, and vegetables attacked by medfly over its currendy cosmopolitan distribution, coffee is one of the hosts used by medfly widiin its original range in eastern and southern Africa
(Christenson and Foote, 1960; Hagen et al., 1981; Liquido et
al., 1991). Coffee is also used in the Hawaiian islands from
which our flies were collected, and fruit was readily available
for our experiments.
A covered, cylindrical, nylon-screen field cage measuring 3
m high and 3 m in diameter was placed over a small tree
located in the station arboretum. At the beginning of each
experiment, pairs of parasitized and unparasitized coffee berries were distributed uniformly throughout the foliage. Berries were hung from branches with the aid of small metal clips
and wire threaded through the stem. In this way, we avoided
damaging the fruit surface in a way that might influence fly
behavior. We then introduced approximately 50 previously
mated medfly females of wild origin (age approximately 14 to
21 days) into the field cage and permitted them to forage in
the trees for host fruit
Observations of fly activity on a particular fruit were made
as follows. The foliage was surveyed until a female was observed on a berry. The pair of berries to which this berry
belonged became a focal berry pair on which fly activity was
monitored until such time as no flies remained on the fruit
or 10 min had elapsed. In this way, a record was obtained not
only of the first fly noted on a member of the pair, but also
of any flies that landed subsequendy on either berry until the
end of an observation. For each fly, all oviposition attempts,
host-marking bouts, encounters, and contests with other females were recorded with the aid of an audio cassette recorder. Oviposition attempts are characterized by the insertion of
the ovipositor tip into the fruit followed by a stereotyped boring behavior. Host-marking bouts (which are unambiguous indications of clutch deposition; Prokopy et al., 1978) are characterized by dragging the ovipositor over the surface of the
fruit, usually for 1 to 3 min. Contests were characterized by
head-to-head confrontations in which flies approach one another, often with wings folded back. Typically, one fly lunges
at the other, often butting the other fly with its head. Most
contests end with one of the flies leaving the fruit. Encounters
were defined arbitrarily as occurring when twoflieswere within about 0.5 cm of one another and when at least one member
of the pair was facing die other.
Any berries in which clutches were deposited by test flies
were removed at the end of the observation and replaced with
fresh ones. Between the two test days, flies remained in the
field cage, but berries were removed. Flies were supplied at
all time with food in the form of sucrose and protein hydrolysate placed on a small petri dish positioned within the foliage. Water was supplied in \ials with cotton wicks distributed
throughout the foliage.
Effect of fruit size
In this field-cage experiment, females were permitted to forage freely for either large (mean fresh weight = 1.91 g, SE =
0.033, N = 62) or small (mean fresh weight = 0.90 g, SE =
0.023, N = 69) ripe coffee berries. The mean fresh weight for
Papaj and Messing • Dynamics of superparasitism in fruit flies
a haphazardly collected sample of ripe coffee berries from the
same grove over the same time period, by comparison, was
1.34 g (SE = 0.032, N = 240). Within each size class, fruit
were of two types: half of the berries were unmanipulated and
unparasitized (i.e., control fruit) and half were egg parasitized
and marked. Test berries were infested with eggs and marked
by permitting females in a laboratory cage access to previously
unparasitized berries. After a female bored into the fruit, deposited a clutch of eggs, and marked the fruit by dragging
her ovipositor over the fruit surface, a small adhesive tag was
placed over the site. Another female was then permitted to
lay eggs at a new site and to mark the fruit The berry was
then removed, the tag removed, and the twice-infested, twicemarked fruit used in assays that same day. In each test, 15
pairs of unparasitized and parasitized berries of a particular
size class were used at a time. Females were presented first
with large berries and then with small berries in successive
trials on a given day. The order in which berries of each size
were presented to females was alternated on successive days.
Focal fruit observations were made as described above.
Effect of fruit ripeness
The second field-cage experiment was similar to the first except that we manipulated berry ripeness rather than size.
Ripeness was judged by color: ripe berries are uniformly dark
red in coloration, whereas unripe berries are mainly green
with occasional streaks of red. All berries were intermediate
in size to the large and small berries used in the previous
experiment Twice-parasitized berries were prepared as described above, except that egg-laying punctures were circled
with ballpoint pen so that reuse of punctures could be more
easily confirmed. As a control, circles of similar size were
drawn on unparasitized berries. After sufficient test fruit were
generated, 30 pairs of berries of a particular ripeness class
were distributed throughout the foliage. On day 1, females
were presented in the morning with unripe berries and in the
afternoon with ripe berries in successive trials. On day 2, females were presented in the morning with ripe berries and
in the afternoon with unripe berries in successive trials. Focal
fruit observations were made as described above.
Advantages of using existing oviposition sites
In laboratory assays, we investigated possible advantages of using existing oviposition sites for fruit of differing size or ripeness. In a small screen cage (25 cm on a side), individual
females were permitted to deposit eggs in berries into which
two clutches had been deposited previously at separate sites.
We counted the number of egg-laying attempts and clutches
laid at new sites versus existing sites on the fruit. Deposition
of a clutch was inferred from the occurrence of host-marking
(Prokopy et al., 1978). We also recorded the time required to
deposit a clutch at new versus existing sites. Because, in an
earlier study (Papaj, 1993), clutch size did not vary between
new and existing sites on the same fruit, a difference in the
time of clutch deposition was taken here to reflect a difference in the time required to penetrate the fruit surface and
begin deposition of eggs (i.e., setup time).
Statistical analysis
In field cage experiments, our focal berry pair procedure
guaranteed that fruit in both treatments in a given experiment were observed for the same overall amount oftime.The
relatively short tenure on fruit combined with the relatively
long time between observations of the same fruit pair made
it unlikely that repeated observations on a focal pair involved
237
(a)
50
40
0) 30
|
20
O
x 10
with
eggs
without
eggs
Small Fruit
with
eggs
without
eggs
Large Fruit
(b)
E 120
I
5
s
90
60
30
HI
6
0
with
eggs
without
eggs
Small Fruit
with
eggs
without
eggs
Large Fruit
(c)
with
eggs
without
eggs
Small Fruit
with
eggs
without
eggs
Large Fruit
Figure 1
Effects of fruit size on use of egg-occupied versus unoccupied fruit
(a) number of clutches laid, (b) number of egg-laying attempts, and
(c) percent egg-laying attempts that result in egg deposition.
the same individual flies. Pseudoreplication in this respect was
thus not a major factor in our experiments. For this reason
and owing to the frequent occurrence of zero data for one
member of a focal pair, we felt justified in grouping data over
focal fruit within a treatment Because data for some variables
were relatively scarce and because no obvious effects of test
day were detected, we further grouped data within a treatment over all test days. Resulting frequencies were analyzed
with log-linear models (Sokal and Rohlf, 1981; SAS, 1985).
RESULTS
Effect of fruit size
Regardless of fruit size, females preferred to deposit clutches
in unparasitized fruit (Figure la). Nevertheless, the degree to
238
Behavioral Ecology Vol. 7 No. 3
Table 1
Results of log-linear analysis of fruit size and level of infestation as
sources of variation in female behavior on fruit
Source
df
Table 2
Frequency of female-female encounters and contests on coffee
berries of different sizes or ripeness levels and different levels of
infestation
p value
Propensity to lay eggs
Clutch
Size
Clutch X size
Infest
Clutch X infest
Size X infest
Clutch X size X infest
18.12
0.11
0.44
27.65
7.26
32.92
0.80
<.0001
.74
.50
<.0001
.007
<.0001
.3700
Propensity to engage in contests
Contests
Size
Contests X size
Infest
Contests X infest
Size X infest
Contests X size X infest
24.48
0.66
0.37
11.30
1.49
11.30
1.49
<.0O01
.42
.54
.0008
.22
.0008
.22
Type of
berry
Level of
infestation
Number of
encounters
Number of
contests
Encounters
with
contests (%)
Large
Parasitized
Unparasitized
Parasitized
Unparasitized
Parasitized
Unparasitized
Parasitized
Unparasitized
30
30
15
79
15
37
57
36
23
23
9
64
5
30
47
15
77
77
60
81
33
83
82
42
Small
Ripe
Unripe
which parasitized fruit were avoided depended on berry size;
females deposited relatively fewer clutches in parasitized berries when berries were small (Figure la; G test, G = 9.38, df
= 1, p < .005).
Fruit size influenced clutch deposition in two ways. First,
size influenced the number of attempts initiated into parasitized versus unparasitized fruit (Figure lb). When fruit were
small, many more attempts were initiated into unparasitized
berries than punctured, marked ones; when fruit were large,
by contrast, slightly more attempts were intitiated into punctured, marked berries than unparasitized ones. Second, size
influenced the frequency of egg-laying attempts that resulted
in egg deposition (Figure lc). When fruit were large, proportionally more attempts resulted in egg deposition on unparasitized berries than on punctured, marked ones; when fruit
were small, the pattern was in the same direction but less
pronounced.
These patterns were evaluated with a log-linear analysis of
the following factors: clutch deposition given attempt (yes versus no), infestation level (parasitized versus unparasitized),
and size (large versus small). Trie interaction among all three
factors was not significant (Table 1), indicating that the manner in which level of infestation affected egg-laying success
did not depend significantly on berry size. Nevertheless, several of the two-way terms were significant. There was a highly
significant size X infestation term (Table 1) indicating that,
as noted above, the overall number of oviposition attempts on
parasitized versus unparasitized fruit depended significantly
on berry size. In addition, there was a significant clutch deposition X infestation term (Table 1) indicating that, as noted
above, the propensity of females to deposit eggs given initiation of oviposition depended on level of infestation.
Patterns in female-female interactions
On large fruit, the frequency of female-female encounters as
well as the proportion of encounters that involved a contest
was identical on parasitized and unparasitized berries (Table
2). On small fruit, by contrast, encounters among females
were far more frequent on unparasitized fruit. Moreover, on
small fruit, the proportion of encounters on unparasitized
fruit that involved a contest exceeded that on parasitized fruit
(Table 2).
These patterns were evaluated with a log-linear analysis of
three factors: occurrence of contest given encounter (yes ver-
•
Also shown is the percentage of encounters that involve a contest.
sus no), infestation level (parasitized versus unparasitized),
and size (large versus small). The interaction among all three
factors was not significant (Table 1), indicating that the manner in which previous infestation influenced the occurrence
of a contest did not depend significantly on berry size. However, there was a highly significant size X infestation term (Table 1) indicating that, as noted above, the relative frequency
of female-female encounters on parasitized versus unparasitized fruit depended on fruit size.
Effect of fruit ripeness
Use of parasitized fruit and, particularly, existing sites on
those fruit depended critically on berry ripeness. When fruit
were ripe, clutches were deposited mainly in unparasitized
berries (Figure 2a). When fruit were unripe, by contrast, all
but one clutch was deposited in punctured, marked berries
(Figure 2a; G test, G = 67.02, p < .0001).
Ripeness influenced clutch deposition in two ways. First,
ripeness influenced the number of attempts initiated into parasitized versus unparasitized fruit (Figure 2b). When fruit
were ripe, more attempts were initiated into unparasitized
berries than parasitized ones; when fruit were unripe, by contrast, more attempts were intitiated into parasitized berries
than unparasitized ones. Second, ripeness influenced the tendency for an oviposition attempt to result in egg deposition
(Figure 2c). When fruit were ripe, proportionally more attempts on unparasitized berries resulted in egg deposition
than on parasitized berries. When fruit were unripe, by contrast, just the opposite was true. Proportionally more attempts
on parasitized berries resulted in egg deposition than on unparasitized berries.
These inferences were supported by log-linear analysis of
three factors: clutch deposition given attempt (yes versus no),
infestation (parasitized versus unparasitized), and ripeness
(unripe versus ripe). There was a significant interaction
among the three factors (Table 3), indicating that the manner
in which level of infestation affected success of egg deposition
depended significantly on the ripeness of the berries. There
was also a significant ripeness X infestation term (Table 3)
indicating that, as noted above, the relative frequency of attempts on parasitized and unparasitized fruit depended on
ripeness.
Patterns in puncture use
All clutches deposited in unripe parasitized berries were deposited in punctures (Figure 3a). Moreover, the preponderance of egg-laying attempts on such berries were initiated in
239
Papaj and Messing • Dynamics of superparasitism in fruit flies
(a)
50
40
TableS
Results of log-linear analysis of fruit ripeness and level of
infestation as sources of variation in female behavioral responses
on fruit
Source
df
Propensity to lay eggs
Clutch
Ripeness
Clutch X ripeness
Infest
Clutch X infest
Ripeness X infest
Clutch X ripeness X infest
1
1
1
1
1
1
1
42.18
0.02
12.29
0.53
1.43
35.55
13.19
<.0001
.89
.0005
.47
.23
<.0001
.0003
1
6.63
5.04
0.17
5.28
0.22
10.80
26.18
.01
.02
.68
.02
.64
.001
•C.OOOl
p value
30
20
O
10
O
Y////A
with
eggs
withoul
eggs
Unripe Fruit
with
eggs
without
eggs
Ripe Fruit
(b)
„ 150
£ 120
CD
I
90
c
3 60
§ 30
with
eggs
without
eggs
Unripe Fruit
with
eggs
without
eggs
Ripe Fruit
(C)
50
§- 40
<
Propensity to engage in contests
Contests
Ripeness
Contests X ripeness
Infest
Contests X infest
Ripeness X infest
Contests X ripeness X infest
30
8 20
§ 10
CO
existing sites. However, the pattern of attempts is illustrative.
Proportionally fewer attempts were made in punctures on ripe
berries than on unripe ones (Figure 3b; G = 20.17, p <
.0001), as one might expect if punctures were particularly
valuable to females when the intact fruit surface was difficult
to penetrate. The difference in placement of attempts on ripe
versus unripe berries notwithstanding, a substantial proportion of attempts were made in punctures on ripe berries
(>50%; Figure 3b). This result indicates that, even on ripe
fruit, flies behaved as though there was an advantage to using
punctures.
Finally, on both ripe and unripe fruit, proportionally more
attempts made in existing punctures resulted in egg deposition than did attempts made at new sites (Figure 3c). The
difference appears greater on unripe berries than on ripe
ones. However, too few clutches were deposited on parasitized
berries in the ripe fruit treatment to permit a statistical test
of that difference.
Patterns in female-female interactions
with
eggs
without
eggs
Unripe Fruit
with
eggs
without
eggs
Ripe Fruit
Figure 2
Effects of fruit ripeness on use of egg-occupied versus unoccupied
fruit (a) number of clutches laid, (b) number of egg-laying
attempts, and (c) percent egg-laying attempts that result in egg
deposition.
punctures (Figure 3b), suggesting that the observed tendency
for clutches to be deposited in punctures was due to an active
preference for punctures on the part of females on unripe
berries (and not, for example, to a trial-and-error process in
which females repeatedly attempted egg laying in an unripe
berry until they blundered into a puncture and successfully
laid eggs). Finally, almost a quarter of all egg-laying attempts
in punctures on unripe berries resulted in egg deposition,
compared to none of the attempts made at new sites on such
fruit
Too few clutches were deposited on parasitized berries in
the ripe fruit treatment to assess their distribution at new and
Contests were generally more common on the preferred berry type within a ripeness category (Table 2). On ripe fruit,
most contests took place on unparasitized berries; on unripe
fruit, by contrast, most contests took place on parasitized berries. This pattern was due in part to the fact that females on
the preferred fruit type simply tended more often to encounter another fly than did females on the other fruit type (Table
2). However, results also suggest that, given an encounter with
another female, females tended more often to contest fruit of
the preferred type than did females on fruit of the other type
(Table 2). On unripe fruit, the proportion of encounters on
parasitized fruit that involved a contest exceeded that on unparasitized fruit. On ripe fruit, just the opposite pattern was
observed: the proportion of encounters on unparasitized fruit
that involved a contest exceeded that on parasitized fruit In
short, females behaved as though the more preferred substrate for egg laying within a given ripeness category was worth
a relatively greater investment in terms of defense from other
females.
These inferences were supported by a log-linear analysis of
three factors: occurrence of contest given encounter (yes versus no), infestation (parasitized versus unparasitized), and
ripeness (unripe versus ripe). There was a significant inter-
240
Behavioral Ecology Vol. 7 No. 3
(a)
25
•g 20
Note: all data for occupied
fruit only
® 15
o
J2
action among the three factors (Table 3), indicating that the
manner in which infestation influenced the occurrence of a
contest depended significantly on the ripeness of the berries.
There was also a significant infestation X ripeness term (Table
3), indicating that the relative frequency of female-female encounters on parasitized and unparasitized fruit depended on
berry ripeness.
10
Puncture use and time saved on ripe versus unripe fruit
O
6
5
0
existing new
site
site
Unripe Fruit
Ripe Fruit
(b)
£125
E
|
I 75
50
S
25
existing
site
new
site
Unripe Fruit
existing
site
new
site
Ripe Fruit
25
(c)
Q.
20
5
CD
10
CO
a?
existing
site
new
site
Unripe Fruit
existing
site
new
site
Ripe Fruit
Figure 3
Effects of fruit ripeness on placement of clutches at new sites versus
existing egg-laying sites: (a) number of clutches laid, (b) number of
egg-laying attempts, and (c) percent egg-laying attempts that result
in egg deposition. Note that the data are for punctured, marked
fruit only.
As in thefield-cageexperiment, the tendency for females presented with punctured, marked fruit in a laboratory assay to
attempt egg laying in punctures depended on fruit ripeness.
On ripe fruit, only about one-third of egg-laying attempts were
made directly into existing punctures; on unripe fruit, by contrast, over three-quarters of the attempts were made directly
into punctures (Table 4; Gtest, G = 23.66, df = \,p< .0001).
Unripe coffee berries were evidently so impenetrable that
no test female successfully deposited clutches at new sites in
a single attempt (Table 4). Rather, new sites were established
in steps. First, a small wound was established by a female followed by repeated attempts at the same site until eggs were
laid successfully. By contrast, ripe berries were penetrable
enough that almost one-quarter of our test females successfully laid eggs in their first attempt (Table 4). More attempts
in both ripeness treatments resulted in egg deposition in both
treatments when those attempts were made at existing sites
(Table 4).
Because females failed to lay eggs in single attempts at new
sites on unripe fruit, the time required to deposit eggs in
single attempts at such sites could not be estimated. However,
we obtained precise estimates for new sites on ripe fruit and
in punctures on ripe and unripe fruit. There were significant
differences in these measures (Table 4; one-way ANOVA, F237
= 25.44, p < .0001). Contrasts indicated that the time required to deposit a clutch into a puncture, whether on ripe
or unripe fruit, was highly significantly shorter than the time
required to deposit a clutch at a new site (Table 4; means
comparisons by Tukey-Kramer method, p < .0001 for each
comparison). Thus, even on ripe fruit there was an advantage
in depositing eggs in punctures in terms of time savings. For
clutches deposited in punctures, by contrast, time for deposition in ripe fruit was not significantly different from that in
unripe fruit (Table 4; means comparison by Tukey-Kramer
method, p > .5).
DISCUSSION
Functional aspects of dynamics in host acceptance
Recent efforts to understand patterns of host use in insects
(reviews by Godfray, 1994; van Alphen and Visser, 1990) are
filling gaps in our knowledge about the dynamics of parental
investment. Our own study contributes to these efforts in at
least two ways. First, in most previous work, females have been
Table 4
Percentage of attempts made at existing sites, percentage of attempts resulting in egg deposition at new versus existing egg-laying sites, and
time required to deposit a clutch in new versus existing egg-laying sites for ripe and unripe coffee berries
Attempts made
at existing sites
Attempts ending in egg deposition
Time to oviposition (s)
At new sites
At existing sites
At new sites
In existing sites
Berry type
%
N
%
N
%
N
Median (SE)
A'
Median (SE)
N
Ripe
Unripe
36
82
53
50
24
0
34
9
63
32
19
41
175 (24.4)
12
76.5 (7.8)
51.5 (6.9)
16
12
Papaj and Messing • Dynamics of superparasitism in fruit flies
shown to change from a tendency to discriminate against parasitized hosts to, at best, a failure to discriminate between parasitized and unparasitized hosts. In this study, by contrast, an
experimental population of medfly was observed to shift from
almost absolute discrimination against parasitized hosts to almost absolute discrimination in favor of such hosts. In a sense,
our results extend the range of alternative behavioral strategies known to be adopted by females of a given parasite species (see also Takasu and Hirose, 1988).
Second, the noted shifts in use and avoidance of parasitized
hosts were wholly consistent with a priori predictions about
the costs and benefits of the alternative strategies. In other
words, as has also been found in other studies of dynamics in
superparasitism (Roitberg et al. 1992, 1993; van Alphen and
Visser, 1990; Visser et al., 1990), shifts were functional in form.
On small ripe fruit, for instance, females appear to act overtly
in the interests of their young, avoiding parasitized fruit and
reducing competition to those young. In this case, females
avoided superparasitism under conditions in which we expected avoidance to be functional. On unripe fruit, in contrast, females appear to act overtly against those interests, using existing sites and thereby increasing competition incurred
by those young. In the latter instance, females are potentially
increasing the rate at which eggs are laid at the expense of
the fitness of any one offspring. In this case, females superparasitized under conditions in which we expected superparasitism to be functional.
Maternal versus offspring interests
Egg-laying decisions in parasitic insects have been viewed as
products of parent-offspring conflict (Godfray and Parker,
1992; Roitberg and Mangel, 1993; Rosenheim, 1993). As
framed above, the observed shifts in female fly behavior reflect changes in the degree of conflict between parent and
offspring. Critical to the notion that parent-offspring conflict
in our system is dynamic is the premise that larvae of one
clutch compete with members of another clutch. For example, if larvae of successive clutches cooperate in feeding within
the fruit, a female's decision to place clutches in the company
of other clutches would not constitute a conflict between her
and her offspring. While it is not inconceivable that larvae
within a clutch benefit by being in the company of other
clutches (Papaj, 1993), there is substantial evidence of larval
competition for the medfly (Debouzie, 1977a,b, 1978, 1981;
Papaj et al. 1989a). Moreover, medfly females behave as if
superparasitism is costly. Host-marking pheromone laid down
on a fruit after eggs are deposited has a consistently deterrent
effect even in situations where females are attracted to existing egg-laying sites (Prokopy et al., 1978; Papaj et al., 1989b,
1992). In other words, even as medfly females use existing
sites, they deposit a chemical signal that reduces further use
of those sites. Females may even use the level of marking pheromone on the fruit as an indication of the number of previously laid clutches and thus of the magnitude of competition
likely to be incurred by their offspring (Averill and Prokopy,
1987b, 1988; Papaj, 1993).
In addition, our finding that avoidance was less pronounced
on large berries than small ones is exactly what would be expected if larvae compete with one another and if competition
at a given egg density declines as fruit size increases. Since it
would presumably always be in an individual offspring's interest to be deposited in unparasitized fruit, the fact that females
adjust avoidance of parasitized fruit according to fruit size implies that some female interest (perhaps having to do with the
advantages of using existing sites) is traded off against the
interests of the female's offspring.
Virtually all evidence for larval competition is based on stud-
241
ies with ripe fruit. It is conceivable that the restriction in use
of existing sites to unripe fruit reflects a pattern in which larvae
compete in ripe fruit but facilitate each other's growth and
survival in unripe fruit While unripe fruit are less suitable for
larval development than ripe fruit (Mourikis, 1965; Tsitsipis,
1989), there is no evidence that the pattern of fitness effects
of prior clutches changes from one of competition in ripe fruit
to one of facilitation in unripe fruit While the issue should be
examined further, it is worth pointing out that growing evidence indicates that use of existing sites confers direct benefits
upon females in terms of time savings and egg-laying success
(Table 4; Papaj, 1993; Papaj et al., 1992). Surely such benefits
account, at least in part, for the patterns observed here.
Clutch size and patterns of egg allocation
In addition to deciding where to deposit clutches, parasitic
insects modify the size of clutches laid in hosts in functional
ways (review by Godfray, 1987, 1994; Papaj et al. 1989b, 1990).
We did not evaluate patterns of clutch size in these experiments, primarily because it is difficult to measure the size of
a clutch deposited at a site already containing eggs. Owing to
this constraint, we are left to assume that the patterns observed here in terms of allocation of clutches translate to similar patterns in terms of the overall allocation of eggs. This
assumption seems reasonable. In medfly, clutch size (which
averages about three to four) appears to be highly constrained. An earlier study (Papaj, 1990), for example, failed
to find any adjustment to clutch size over a range of fruit sizes
an order of magnitude greater than that examined here. Papaj et al. (1989b, 1990) did find that clutches in infested fruit
were smaller than those in uninfested fruit. This effect, if it
occurred in our fruit ripeness experiment, would have tended
to make the difference in the total numbers of eggs laid in
infested versus uninfested fruit less pronounced than the observed difference in the number of clutches laid (Figure 2a).
However, since the observed preference for infested fruit in
that experiment was absolute (i.e., no eggs laid at all in uninfested fruit), any effect of infestation on clutch size would
not diminish the preference of females for infested fruit In
short, while one might expect smaller clutches to be deposited
on infested fruit, such reductions do not diminish the significance of the patterns of clutch allocation observed here.
Closing remarks
Our findings are consistent with correlational data reported
recendy by Lalonde and Mangel (1994) who found that use
of existing sites by females of another tephritid fly, RhagoUtis
complela, waned over the course of a season as host walnuts
ripened and the penetrability of walnut husks increased. In
that study, female behavior was not observed direcdy but was
inferred from the distribution of eggs in fruit Our experimental manipulations offer direct experimental support for
Lalonde and Mangel's (1994) contention that use of existing
sites by individual females varies as a function of fruit ripeness.
It would be useful to look in turn for temporal as well as
spatial variation in medfly's use of existing sites. Our experiments raise the intriguing possibility that individual females
can change from avoidance of superparasitism to preference
for superparasitism over relatively short periods of time. Such
flexibility might be especially beneficial for a generalist like
the medfly whose many host species probably vary gready in
ways that affect the decision to superparasitize.
We thank Henar Alonso-Pimentel, Laurie Henneman, Robert Lalonde, Cesar Nufio, Bob Smith, and an anonymous reviewer for comments on earlier drafts.
242
REFERENCES
Averill AL, Prokopy RJ, 1987a. Intraspecific competition in the tephritid fruit fly, Rhagoletis pomonetia. Ecology 68:878-886.
Averill AL, Prokopy RJ, 1987b. Residual activity of oviposition-deterring pheromone in Rhagoletis pomonella and female response to
infested fruit. J Chem Ecol 13:167-177.
Averill AL, Prokopy RJ, 1988. Factors influencing the release of hostmarking pheromone by Rhagoletis pomonella flies. J Chem Ecol 14:
95-111.
Back EA, Pemberton CE, 1915. Susceptibility of citrous fruits to the
attack of the Mediterranean fruit fly. J Agric Res 3:311-330.
Christenson LD, Foote RH, 1960. Biology of fruit flies. Annu Rev
Entomol 5:171-192.
Clutton-Brock TH, 1991. The evolution of parental care. Monographs
in behavior and ecology. Princeton, New Jersey: Princeton University Press.
Debouzie D, 1977a. Effect of initial population size on Ceratihs productivity under limited food conditions. Ann Zool Ecol Anim 9:
367-381.
Debouzie D, 1977b. Etude de la competition larvaire chez Ceratitis
capilata (Diptere, Trypetidae). Arch Zool Exp Gen 118:315-334.
Debouzie D, 1978. Variabilite a l'interieur d'une population de Ceratitis capitata elevee sur un milieu naturel non renouvele (Diptera:
Trypetidae). Ann Zool Ecol Anim 10:515-524.
Debouzie D, 1981. Analyse experimentale de l'utilisation des resources dans un systeme simphfe forme d'une banane attaquee par
la mouche mediterraneenne des fruits Ceratitis capitata. ActaOecol
Gen 2:371-386.
Godfray HCJ, 1987. The evolution of clutch size in parasitic wasps.
Am Nat 129:221-233.
Godfray HCJ, 1994. Parasitoids: behavioral and evolutionary ecology.
Princeton, New Jersey: Princeton University PTess.
Godfray HCJ, Parker GA, 1992. Sibling competition, parent-offspring
conflict and clutch size. Anim Behav 43:473-490.
Hagen KS, Allen WW, Tassan RL, 1981. Mediterranean fruit fly: the
worst may be yet to come. Calif Agric 35:5-7.
Lalonde RG, 1992. Host selection behavior of a thistle-feeding fly:
choices and consequences. Oecologia 90:534-539.
Lalonde RG, Mangel M, 1994. Seasonal effects on supcrparasitism by
Rhagoletis completa. J Anim Ecol 63:583-588.
Liquido N, Shinoda LA, Cunningham RT, 1991. Host plants of the
Mediterranean fruit fly (Diptera: Tephritidae): an annotated review. Lanham, Maryland: Entomological Society of America.
Mourikis P, 1965. Data concerning the development of the immature
stages of the Mediterranean fruit fly (Ceratitis capitata (Wiedemann) (Diptera: Trypetidae)) on different host fruits and on artificial media under laboratory conditions. Ann Benaki Phytopathol
Inst 7:59-105.
Papaj DR 1990. Fruit size and clutch size in Ceratitis capitata. Entomol
Exp Appl 54:195-198.
Papaj DR, 1993. Use and avoidance of occupied hosts as a dynamic
process in tephritid flies. In: Insect-plant interactions, vol. 5 (Bernays EA, ed). Boca Raton, Florida: CRC Press; 25-46.
Papaj, DR, 1994a. Optimizing learning and its effect on evolutionary
change. In: Behavioral mechanisms in evolutionary ecology (Real
LA, ed). Chicago: University of Chicago Press; 133-153.
Behavioral Ecology Vol. 7 No. 3
Papaj DR, 1994b. Oviposition-site guarding by male walnut flies and
its possible consequences for mating success. Behav Ecol Sociobiol
34:187-195.
Papaj DR, Averill AL, Prokopy RJ, Wong TTY, 1992. Host-marking
pheromone and use of previously established oviposition sites by
the Mediterranean fruit fly (Diptera: Tephritidae). J Insect Behav
5:583-598.
Papaj DR, HendrichsJ, Katsoyannos B, 1989b. Use of fruit wounds in
oviposition by the Mediterranean fruit fly. Entomol Exp Appl 53:
203-209.
Papaj DR, Roitberg BD, Opp SB, 1989a. Serial effects of host infestation on egg allocation by the Mediterranean fruit fly: a rule of
thumb and its functional significance. J Anim Ecol 58:955-970.
Papaj DR, Roitberg BD, Opp SB, Prokopy RJ, Aluja M, Wong TTY,
1990. Effect of marking pheromone on clutch size in the Mediterranean fruit fly. Physiol Entomol 15:463-468.
Price PW, 1970. Biology and host exploitation by Pleolopkus indistinctus (Hymenoptera: Ichneumnonidae). Ann Entomol Soc Am 63:
1502-1509.
Prokopy RJ, ZieglerJR, Wong TTY, 1978. Deterrence of repeated oviposition by fruit-marking pheromone in Ceratitis capitata. J Chem
Ecol 4:55-63.
Roitberg BD, Mangel M, 1987. On the evolutionary ecology of marking pheromones. Evol Ecol 2:289-315.
Roitberg BD, Mangel M, 1993. Parent-offspring conflict and life-history consequences in herbivorous insects. Am Nat 142:443-456.
Roitberg BD, Mangel M, Lalonde RG, Roitberg CA, van Alphen JJM,
Vet L, 1992. Seasonal dynamic shifts in patch exploitation by parasitic wasps. Behav Ecol 3:156-165
Roitberg BD, Prokopy RJ, 1983. Host deprivation influence on response of Rhagoletis pomonella to its oviposition-deterring pheromone. Physiol Entomol 8:69-72.
Roitberg BD, Prokopy RJ, 1987. Insects that mark host plants. BioScience 37:400-406.
Roitberg BD, Sircom J, Roitberg CA, Mangel M, van Alphen JJM,
1993. Life expectancy and reproduction. Nature 364:108.
RosenheimJA, 1993. Single-sex broods and the evolution of nonsiblicidal parasitoid wasps. Am Nat 141:90-104.
SAS Institute, 1985. SAS user's guide. Cary, North Carolina: SAS Institute.
Silvestri F, 1914. Report of an expedition to Africa in search of the
natural enemies of fruit flies (Trypaneidae), with descriptions, observations and biological notes. Hawaii Board of Agriculture and
Forestry Division of Entomology Bulletin No. 3.
Sokal RR, Rohlf FJ, 1981. Biometry. San Francisco: WH Freeman.
Takasu K, Hirose Y, 1988. The parasitoid Ooencyrtus nezarae (Hymenoptera; Encyrtidae) prefers hosts parasitised by conspecifics over
unparasinzed hosts. Oecologia 87:319-323.
Tsitsipis JA, 1989. Nutrition, requirements. In: World crop pests, vol.
3A, Fruit flies: their biology, natural enemies and control (Robinson
AS and Hooper G, eds). New York: Elsevier.
van Alphen JJM, Visser ME, 1990. Superparasitism as an adaptive strategy for insect parasitoids. Annu Rev Entomol 35:59-79.
Visser M, ran Alphen JJM, Nell HW, 1990. Adaptive superparasitism
and patch time allocation in solitary parasitoids: the influence of
the number of parasitoids depleting a patch. Behaviour 114:21-36.