POPULATION ECOLOGY
Protracted Emergence of Overwintering Amyelois transitella
(Lepidoptera: Pyralidae) From Pistachios and Almonds in California
L.P.S. KUENEN1
AND
J. P. SIEGEL
United States Department of AgricultureÐAgricultural Research Service, San Joaquin Valley Agricultural Sciences
Center, 9611 South Riverbend Avenue, Parlier, CA 93648
Environ. Entomol. 39(4): 1059Ð1067 (2010); DOI: 10.1603/EN09277
ABSTRACT The navel orangeworm, Amyelois transitella (Walker), is the primary insect pest of
pistachios and almonds in California. Four years of research (2002Ð2006) were conducted in Madera
and Kern Counties to elucidate the pattern of adult emergence of the overwintering navel orangeworm population. Springtime emergence from unharvested (mummy) nuts was protracted (600
degree-days or more from 1 January of each year) and in 2004 and 2006 extended to mid-July. The
population structure, sex ratio, and timing of emergence differed between pistachio and almond
mummies. Pistachio populations had a signiÞcantly greater proportion of late stage individuals
compared with almond mummies, 85.7 versus 34.1%. The sex ratio of adults emerging from pistachio
mummies was signiÞcantly skewed with a ratio 57:43 male:female compared with 50:50 in almond
mummies. Emergence from mummies held outdoors (variable temperature) began in early March and
continued through early June in both pistachio mummies and almond mummies. The adult emergence
pattern from pistachio mummies contained a single emergence peak, whereas emergence from almond
mummies occurred in multiple peaks. These same patterns occurred when mummies were held at
constant temperature, and the emergence peak from pistachio mummies occurred sooner. The impact
of these Þndings on understanding navel orangeworm population dynamics and current control
recommendations is discussed.
KEY WORDS navel orangeworm, pistachios, almonds, overwintering emergence, sex ratio
The navel orangeworm (Amyelois transitella Walker),
Þrst found in the United States on navel oranges in
Arizona (Mote 1922), is a multivoltine scavenger on
fallen or damaged fruits, and infests sound nuts when
their hulls have split. Since its appearance in CaliforniaÕs Central Valley in the late 1940s (Wade 1961),
commercially grown almonds, Þgs, pistachios, and
walnuts have been infested by navel orangeworm, and
this moth is currently the primary pest of pistachios
and almonds in California. Over the period 1995Ð2004,
there was a dramatic expansion of both pistachio and
almond plantings (54 and 31%, respectively), and currently there are over 251,000 ha of almonds and 50,000
ha of pistachios in California (Anonymous 2005). The
value and importance of these crops have substantially
increased (⬇2.92 billion dollars combined in 2005),
and concurrent with this rise in value there is an
increased demand to reduce navel orangeworm damage below current levels.
The navel orangeworm overwinters in the previous
seasonÕs crop residue (mummies) as larvae or pupae
without apparent diapause. Development occurs sporadically through the winter and spring whenever
temperatures exceed the lower threshold of 12.8⬚C
1
Corresponding author, e-mail: [email protected].
(Engle and Barnes 1983a, Sanderson et al. 1989a).
Adults emerge from the mummies on the ground and
in the trees, and females lay their eggs on other mummies or new crop nuts when available to continue their
life cycle. Adults that subsequently emerge from these
mummies oviposit on the new crop when available, or
remaining mummies (Sanderson et al. 1989a). In subsequent ßights, females oviposit on new crop nuts
(or on mummy nuts remaining on the trees or
ground), and oviposition continues through November (Kuenen and Rowe 2003). The timing of almond
susceptibility depends on variety, and new crop Nonpareil almonds, the most common variety grown, are
susceptible to infestation when the hulls split and
expose the nuts, typically beginning during late June
or early July (Kuenen and Barnes 1981). Development
on new crop almonds is faster than on mummy almonds (Seaman and Barnes 1984, Sanderson et al.
1989b), and the adults that emerge may infest more
almonds before harvest, depending on variety and
harvest date. In contrast to almonds, the pistachio shell
opens inside the intact hull; therefore, most pistachios
are not susceptible to navel orangeworm infestation
until their hulls split, typically starting in late August
to early September (Beede et al. 1984). However, a
small fraction of the pistachio crop known as early
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ENVIRONMENTAL ENTOMOLOGY
splits becomes vulnerable to infestation by early August, because both hull and shell split together, exposing the kernel (Doster and Michailides 1995). In
addition, there are numerous small pistachios that we
call pea-split nuts. These nuts are approximately the
size of green peas (4 Ð 8 mm diameter) and have incompletely formed shells, such that they can readily
pop open from internal or external pressure. We have
found that pea-split nuts often split in late June or July
apparently from high heat or later as the nut meats
grow, and thus provide an even earlier new crop food
source for navel orangeworm.
To reduce the levels of overwintering navel orangeworm, orchard sanitation is recommended for navel
orangeworm during the winter months because it both
destroys the overwintering population and reduces
the resources available for the Þrst ßight of navel
orangeworm (Curtis 1976, Engle and Barnes 1983b,
also Sibbett and Van Steenwyk 1993). This cultural
practice in almonds consists of shaking mummies from
the trees after attachment has been weakened by
winter rain and fog, and then destroying the mummies
on the ground by ßailing before mid-February (Bentley et al. 2002). In pistachios, cultural control consists
of blowing fallen mummy nuts off the berm (under
certain circumstances the mummies may also be
knocked off the tree after attachment to the rachis is
weakened by rain and fog) and disking the mummies
into the soil of the drive rows (Bentley et al. 2008).
These practices are not 100% efÞcient, and insecticides are used to protect these crops because the
population of navel orangeworm increases during the
summer and the acceptable percentage of damaged
nuts is low in both crops.
Missing from this view of navel orangeworm biology
is the progression of navel orangeworm emergence
from mummies, and our goal in conducting this research was to quantify the emergence patterns from
pistachio and almond mummies, and to determine
whether females from the overwintering population
(Þrst ßight) can oviposit on new crop nuts. In this
study, we present data on the overwintering population structure, sex ratio in adults emerging from pistachio and almond mummies, and emergence pattern
of overwintering navel orangeworm, including overlap with new crop nuts.
Materials and Methods
Mummy Collection and Holding. All of the pistachio mummies used in these studies were the Kerman
variety (this variety constitutes ⬎95% of all pistachios
currently grown in California), and almond mummies
used were Nonpareil and Carmel varieties. The mummies used in 2002Ð2006 were collected from orchards
in southern Madera County. Pistachio mummies and
almond mummies were also collected in January 2006
from heavily infested orchards in Kern County. Two
temperature regimes were evaluated in these studies,
a variable regime from 2002 to 2004 and constant
temperature in 2004 Ð2006. In the variable temperature regime, pistachio mummies were collected from
Vol. 39, no. 4
both the ground adjacent to the trunks (berm) and the
canopy in early January, and almond mummies were
collected from the ground the same week. Plant debris
was removed, and enough mummies were saved to Þll
emergence cages (below). A total of 8,600 and 6,400
pistachio mummies was pooled and used to assess
navel orangeworm emergence in 2002Ð2003 and 2003Ð
2004, respectively, and 1,100 and 1,000 almond mummies were used in 2002Ð2003 and 2003Ð2004, respectively. For the constant temperature regime, we used
30,000 pistachio mummies collected from berms and
9,250 mummies collected from trees in December
2004, in Madera County, and in January 2006, we used
pistachio mummies collected from the berm and trees
in Madera and Kern Counties (50,000 and 30,000,
respectively), and almond mummies collected from
the ground in Madera and Kern Counties (25,895 and
16,765, respectively).
Variable Temperature Exposure. In 2002 and 2003,
pistachio and almond mummies were held outdoors
⬇60 cm above the ground on wooden frames covered
with hardware cloth (0.64-cm mesh). These frames
were fully exposed to ambient light, temperature, rain,
and fog. In early March, mummies were transferred
(one-layer deep) to aluminum-framed cages with
window-screen sides and a sleeve opening on one side.
These cages were held on the same wooden frames
and moved to the north side of a water tower, where
they were sheltered from direct sunlight for ⱕonethird of the photoperiod to mimic partial exposure to
sunlight in an orchard; however, no other protection
from the elements was provided. Adults were collected twice weekly, and their emergence date was
recorded until no moths were collected for 2 wk. All
adults were frozen, and their sex was subsequently
determined by examination of their genitalia under a
dissecting microscope.
Constant Temperature Exposure. In these experiments conducted in 2004 Ð2005 and 2006, pistachio and
almond mummies were placed in 18.9-liter plastic
buckets covered with screens and held at 26.7⬚C, and
emergence was monitored weekly, as described by
Siegel et al. (2008). All adults were counted and placed
in 50-ml plastic tubes for subsequent sex determination, as described above. When emergence was heavy
in the almond mummies in 2006, six to eight tubes were
Þlled with ⬇600 Ð1,000 adults for sex determination
and the remaining moths were counted as they ßew
from the buckets. These experiments were terminated
after 60 d when 740 degree-days (DD) were accumulated in the incubator. Differences in the average DD
accumulated for all adult emergence from pistachio
mummies and almond mummies were evaluated with
one-way analysis of variance using JMP 7.02 (SAS
Institute, Cary, NC).
Population Structure. In late January 2003, 500 randomly selected pistachio and 500 randomly selected
almond mummies were dissected, and all larvae and
pupae were recorded and classiÞed into two age categories. One category, late instars, consisted of Þfthand sixth-instar larvae plus pupae, and the second
category consisted of the remaining instars; no un-
August 2010
KUENEN AND SIEGEL: EMERGENCE OF OVERWINTERING NAVEL ORANGEWORM
hatched eggs were found on any of the mummies. The
differences in age structure between almond and pistachio mummies were analyzed using 2 ⫻ 2 contingency ␹2 analysis.
Sex Ratio Analysis. The data from all experiments
were pooled to maximize sample size. Deviation from
a 50:50 sex ratio for each nut species was evaluated by
␹2 analysis with one degree of freedom, and differences in the sex ratio between pistachio mummies and
almond mummies were evaluated using 2 ⫻ 2 contingency ␹2 analysis.
Determining Prevalence of Navel Orangeworm in
Pistachios. Assessing navel orangeworm infestation in
pistachio mummies using the total number of mummies collected is misleading because as many as 30%
of the pistachios on the tree are blank nuts that contain
no kernels and therefore cannot be infested (Goldhamer and Beede 2004). These blank pistachio mummies may predominate in the orchard after harvest
(Siegel et al. 2008). In this study, a subset of mummy
pistachios was dissected to establish the proportion of
mummies that could be infested, and this percentage
was multiplied by the total number of mummies collected to estimate the number of available mummies.
In 2002 and 2003, 500 pistachio mummies were dissected each year, and in 2004 Ð2005 and 2006, 5,000
pistachio mummies were dissected each year for this
calculation.
Degree-Day Accumulation. DD accumulation from
1 January of each year was determined using the
University of California integrated pest management
web-based DD calculator (Anonymous 2009). The
lower developmental threshold was set to 12.8⬚C, and
the upper developmental threshold was set to 34.4⬚C
with a horizontal upper development cutoff (Zalom et
al. 1998). DD accumulation for the mummies held
outside our laboratory used California Meteorological
Instrument Station (CMIS 39) ⬇0.5 km distant; DD
accumulations for Madera County used CMIS 145
(⬇18 km from the collection site); and DD accumulations for Kern County used CMIS 54 (⬇18 km from
the collection site). For the mummies held at constant
temperature, DD accumulation was determined as
follows: (incubator temperature ⫺ 12.8⬚C) ⫻ days
elapsed; incubator temperatures were conÞrmed
with HOBO data loggers (Onset Computer, Pocasset, MA).
Results
The proportion of late instars and pupae was signiÞcantly higher in pistachio mummies, 85.7%, than in
almond mummies, 34.1% (Table 1; ␹2 ⫽ 26, df ⫽ 1, P ⬍
0.001). Only a single third-instar larva was found in the
dissected pistachio mummies, whereas in almond
mummies there were three equal groups of larvae
comprising third instars, fourth instars, and late instars
plus pupae. The prevalence of navel orangeworm was
substantially lower in pistachio mummies than in almond mummies based on dissection and emergence
from the cages. In 2002, prevalence was 1.0% in pistachio mummies and 36.9% in almond mummies, and
1061
Table 1. Instar distribution of navel orangeworm dissected
from 500 mummies of pistachio and almond collected in early
January 2003
Instar
Almonds
Pistachios
Second
Third
Fourth
Fifth and sixth
Pupa
Total collected
9
63
73
71
4
220
0
1
3
22
2
28
in 2003, prevalence was 7.2% in pistachio mummies
and 67.8% in almond mummies. The prevalence of
navel orangeworm in the 2004-collected mummies
was twice as high in ground mummies, 19.9% (11,460
Þlled, split mummies), compared with 9.8% in tree
mummies (3,352 Þlled, split mummies). In 2006, the
prevalence of navel orangeworm was again greater in
almond mummies than in pistachio mummies, 7.8%
compared with 4% in Madera County and 50.2% compared with 15% in Kern County. The sex ratio of
emerged adults from 2002 to 2006 was skewed from
pistachio mummies with a male:female ratio of 57:43
(␹2 ⫽ 42, df ⫽ 1, P ⬍ 0.001), whereas almond mummies had a 50:50 sex ratio (Table 2).
In the variable temperature regime experiment in
2002Ð2003, adult emergence began in late March
and continued through the Þrst week of June (Fig.
1). There was a single emergence peak from pistachio mummies, and when it ended, 75% of the adults
had emerged, whereas three emergence peaks were
evident from almond mummies and it was only at the
end of the third peak that 75% of the adults had
emerged. The DD accumulation for 50 and 75%
adult emergence occurred earlier in pistachio mummies than in almond mummies, corresponding to a
difference of 1 and 3 wk, respectively. It took three
times longer (in DD) to go from 50 to 75% emergence from almond mummies than from pistachio
mummies. In 2003Ð2004, a similar pattern of a single
emergence peak from pistachio mummies and multiple emergence peaks from almond mummies occurred (Fig. 2), although the dates for 50 and 75%
emergence differed from 2002. That is, from pistachio mummies, 50% emergence occurred 1 wk earlier than in the previous year, and from almond
mummies 50% emergence occurred 2 wk later than
the previous year. The date for completion of 75%
emergence followed this same pattern.
In 2006, navel orangeworm emergence from pistachio mummies and almond mummies at constant temperature followed the same pattern as emergence under variable temperature regimes. Pistachio mummies
collected from Madera County had a single emerTable 2. Total male and female navel orangeworm collected
from pistachio mummies and almonds, 2002–2006
Host
Male
Female
Total
Almonds
Pistachios
3,817
2,176
3,790
1,668
7,607
3,844
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ENVIRONMENTAL ENTOMOLOGY
Vol. 39, no. 4
16
14
Pistachios
Adult Emergence
12
10
8
6
4
2
50%
75%
0
0
100
200
March
300
400
April
500
600
June
Degree Days
70
60
Adult Emergence
Almonds
50
40
30
20
10
50%
75%
0
0
100
March
200
April
300
400
Degree Days
500
600
June
Fig. 1. Adult navel orangeworm emergence from pistachio mummies and almond mummies collected in January 2002 from
Madera County, California, and held outdoors at Parlier. DD calculated using CMIS 39.
gence peak, whereas almond mummies had two distinct emergence peaks (Fig. 3). The DD accumulated
for 50 and 75% emergence were similar in both groups
of mummies, perhaps because the orchards were adjacent. In the Kern County mummies (Fig. 4), both 50
and 75% emergence occurred sooner from pistachio
mummies than from almond mummies, and emergence from almond mummies lasted an additional 150
DD. When the data from both counties were combined, mean emergence (⫾SD) occurred 19.7%
sooner from pistachio mummies than from almond
mummies (F ⫽ 1,230; df ⫽ 1, 17,434; P ⬍ 0.0001),
283.4 ⫾ 84.7 DD (7,004 adults) compared with 353.1 ⫾
151.1 DD (10,431 adults). In these pooled datasets,
over 9% of the adults emerged during the DD equivalent of early to late June, and in pistachio mummies
the last recorded emergence was on the DD equivalent of 15 July.
In 2004 Ð2005, both the berm and tree-collected
pistachio mummies had a single emergence peak
(Fig. 5), and the DD accumulated for 50 and 75%
emergence were similar. Emergence ceased at the
DD equivalent of 15 July and was more protracted
than in the samples from the other years. Approximately 10% of emergence occurred at the DD
equivalent of 1Ð15 July. To assuage our own concerns about possible reinfestation of our sampled
mummies by newly emerged females, we conducted
preliminary studies in laboratory jars that were
monitored daily to characterize development extremes and found no development of new adults in
the time frame used in this study. We continued this
type of monitoring during the course of our experiments described in this study and found no evidence of moth emergence from eggs laid by females
emerged in this study. Final support comes from
Sanderson et al. (1989b), who found the mean navel
orangeworm development rate on mummy almonds
was ⬇514 DD for onset of emergence (add ⬇75 DD
for our Þrst observed emergence ⬇600 DD; any
emergence after this time was decreasing rather
than increasing if ever increasing numbers of males
and females had mated and laid eggs on the mummy
samples). Furthermore, the difÞculty in establishing
a laboratory colony from feral moths is generally
known for most insects, most likely because of the
activity of insects trying to escape the cage(s) rather
than trying to mate; thus, establishment of larvae
arising from mated females is even more remote.
August 2010
KUENEN AND SIEGEL: EMERGENCE OF OVERWINTERING NAVEL ORANGEWORM
1063
60
Pistachios
Adult Emergence
50
40
75%
30
20
10
50%
0
0
100
March
200
300
April
400
500
600
June
Degree Days
60
Almonds
Adult Emergence
50
40
30
75%
20
10
50%
0
0
100
March
300
200
April
Degree Days
400
500
600
June
Fig. 2. Adult navel orangeworm emergence from pistachio mummies and almond mummies collected in January 2003 from
Madera County, California, and held outdoors at Parlier. DD calculated using CMIS 39.
Discussion
We have demonstrated in this study that the pattern
of navel orangeworm adult emergence differed between overwintering hosts, pistachio mummies, and
almond mummies when held outdoors under a variable/ambient temperature regime, and that the age
structure of the overwintering immature navel orangeworm in the mummies appeared to be the underlying cause. The single cluster of late-instar larvae
and pupae in pistachio mummies corresponded to the
observed single emergence peak, and both were consistent with the earlier emergence observed from pistachio mummies. In almond mummies, the three larval
age group clusters corresponded to the multiple adult
emergence peaks, and the substantial number of
younger larvae was consistent with the increased time
required for 50 and 75% adult emergence. For both
years of the outdoor studies, adult emergence continued into June. When we subsequently held much
larger samples of pistachio and almond mummies under constant temperature conditions, we observed an
even more protracted emergence and the last moths
emerged in the DD equivalent of June and into early
July in 2 yr.
It is unclear what causes the different navel orangeworm age structures observed in pistachio compared
with almond mummies; several factors alone or in
concert may be responsible. First, the timing of nut
susceptibility plays a major role in determining infestation in the Þeld. Under current management
schemes, new crop pistachios may only be susceptible
for half the time that new crop Nonpareil almonds are
(⬇4 versus 9 wk, respectively) because of differences
in both hull split and harvest date. In addition, other
varieties of almonds become susceptible after Nonpareils and navel orangeworm larvae develop in the
hulls of most or all almond varieties, further expanding
the age spread of navel orangeworm in almonds versus
pistachios. The shorter and later susceptibility of pistachios most likely led to the reduced age spread of
immature navel orangeworm in pistachio mummies,
which in turn engendered the single emergence peak
observed from pistachio mummies at both variable
and constant temperature.
Second, nutritional suitability among nut varieties
and between species, as well as nutritional stability
over time, will affect age structure because neonates
that hatch on the same day on different hosts will
develop at different rates. Navel orangeworm development rate is dependent on nut maturity, initially
increasing as immature nuts age and then decreasing
as mummy nuts degrade (unpublished data). Devel-
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ENVIRONMENTAL ENTOMOLOGY
600
Vol. 39, no. 4
Pistachios
Adult Emergence
500
400
300
200
100
50%
75%
250
300
0
0
50
100
150
200
April
350
400
450
500
550
Degree Days
May
600
650
June
Almonds
450
Adult Emergence
400
350
300
250
200
150
100
50
50%
75%
0
0
50
100
150
April
200
250
300
350
400
May Degree Days
450
500
550
600
650
June
Fig. 3. Comparison between adult navel orangeworm emergence from almond mummies and pistachio mummies
collected from the ground (January 2006) in adjacent orchards in Madera County, California. Emergence continued from
almond mummies and pistachio mummies through the DD equivalent of 26 June, using CMIS 145.
opment can be almost twice as long in almond mummies (Sanderson et al. 1989b) and pistachio mummies
(unpublished data) than in new crop nuts. Legner
(1983) reported that it took 5 mo for adults to complete adult emergence (⬎1,900 DD⬚C) from infested
Nonpareil almonds collected in July and held at
25.6⬚C. Prolonged development like this also occurs in
other almond varieties, but does not occur in pistachios (unpublished data).
Third, allelochemical differences between pistachio mummies and almond mummies may affect the
timing and sex ratio of emerging adults. For example,
the skewed sex ratio observed in adults emerging from
pistachio mummies indicates that female mortality
was higher, assuming that male and female eggs were
laid without bias. Because this did not occur in almond
mummies and because this skewed sex ratio does not
occur when adults emerge from new crop pistachios
(unpublished data), our Þnding underscores that nuts
are a dynamic resource whether fresh or mummies.
The suite of fungi that grow on these mummies may
be quite different, and fungal metabolites may inßuence navel orangeworm development and mortality.
For example, we reported that substantial navel orangeworm mortality occurred in mummy pistachios
between December and February because of undetermined factors (Siegel et al. 2008); the skewed sex
ratio observed in this study may result from mortality
as a result of these factors.
Previously, we have noted that navel orangeworm
males were captured in female-baited traps throughout the growing season (Kuenen and Rowe 2003), and
a major goal of the current study was to determine
whether elements of the Þrst ßight (overwintering
navel orangeworm) could bridge directly into new
crop nuts, or whether there is an obligate cycle of
development in mummies. Our data for 2002 and 2003
are consistent with an obligate Þrst in-season generation of navel orangeworm on mummy nuts, because
there are no new crop nuts available before late June.
August 2010
KUENEN AND SIEGEL: EMERGENCE OF OVERWINTERING NAVEL ORANGEWORM
800
1065
Pistachios
Adult Emergence
700
600
500
400
300
200
100
50%
75%
0
0
50
100
150
200
April
250
300
350
400
450
500
550
600
650
700
June
May
Degree Days
1000
900
Almonds
Adult Emergence
800
700
600
500
400
300
200
100
50%
75%
0
0
50
100 150 200 250 300 350 400 450 500 550 600 650 700
April
May
June
750 800 850
July
Degree Days
Fig. 4. Comparison between adult navel orangeworm emergence from almond mummies and pistachio mummies
collected from the ground (January 2006) in Kern County, California. Emergence continued from almond mummies until
the DD equivalent of 15 July and from pistachio mummies until the DD equivalent of 22 June 2006, using CMIS 54.
However, we recognized that this conclusion may
have been a function of insufÞcient sample size.
Therefore, we moved to the second protocol (Siegel
et al. 2008) that could incorporate much larger sample
sizes. For instance, in the 39,250 pistachio mummies
collected in 2004, a portion of the Þrst ßight (9%)
emerged as late as mid-July; by this time new crop
almonds are available (Kuenen and Barnes 1981). In
2006, ⬇10% of the adults emerged through the degreeday equivalent of the month of June when Nonpareil
almond hull split begins. In pistachios, both pea-split
and malformed small nuts containing kernels that can
support navel orangeworm development are susceptible as early as mid-June; consequently, late emerging
adults from the overwintering generation can Þnd
suitable new crop nuts in both pistachios and almonds.
The dates for 50 and 75% emergence will vary among
counties and years because DD are accumulated at
different rates throughout the San Joaquin Valley.
Focusing on navel orangeworm developmental DD is
only one piece of the puzzle because it is not
necessarily closely linked to nut development/availability, and further research is needed on the phenology of pistachios, because there is no current model.
Consequently, in a given year there are undoubtedly
orchards where a portion of the Þrst ßight oviposits on
new crop nuts, whereas in other orchards there is
obligate development of a generation of navel orangeworm on mummies only.
Damaging levels of infestation may occur in an area
when something is altered from the norm, such as
increased or early availability of early split pistachios,
so that a higher percentage of the Þrst ßight successfully oviposits on new crop nuts, which in turn speeds
navel orangeworm population growth. Furthermore,
the combination of differential development rates in
pistachios and almonds and movement between these
two crops result in overlap between the navel orange-
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ENVIRONMENTAL ENTOMOLOGY
Vol. 39, no. 4
900
800
Adult Emergence
700
600
500
Berm
Tree
400
300
50%
75%
250
350
200
100
0
50
150
April
May
450
Degree Days
550
650
750
July
Fig. 5. Contrasting navel orangeworm emergence from pistachio mummies collected from the ground (berm) and from
trees in an orchard in Madera County, December 2004. Emergence continued until the DD equivalent of 15 July 2005 using
CMIS 145.
worm ßights, which increases as the summer progresses.
Thus, from late June forward, it is impossible to assign
eggs laid on ovitraps, males captured in female-baited
traps, or larvae collected from nuts to a particular generation, thus clouding the view of discrete generations of
navel orangeworm in the Þeld.
In conclusion, this study has several implications for
navel orangeworm control in pistachios and almonds.
Our data of prolonged emergence, differential development on mummy and new crop nuts, and subsequent overlap among generations, as well as the lack
of developmental synchronization by means of diapause, question the traditional view that navel orangeworm populations consist of discrete generations.
Functionally, it may make more sense to refer to separate ßights rather than separate generations; and
even this may be misleading, because navel orangeworm are present (males can be trapped) throughout
the summer (Kuenen and Rowe 2003).
Control of this insect is particularly challenging
because the Þrst ßight (adults from overwintering
immatures) may last as long as 5 mo, and in almonds
the prolonged, multipeak emergence of navel orangeworm makes it difÞcult, and probably economically
unfeasible, to target the entire Þrst ßight with currently available insecticides. Pistachios may be a better target for spring insecticide treatment(s) because
peak emergence is compressed, but sanitation in pistachios is problematic and pistachio blocks have a
larger resident population than their almond counterparts (Burks et al. 2008). Movement between these
two nut crops (Higbee and Siegel 2009) necessitates
the development of a coordinated control strategy,
and it is likely that several strategies will need to be
combined to effectively suppress navel orangeworm
populations in pistachios and almonds.
Acknowledgments
We thank James Bettiga, Kevin Olsen, and Bradley Higbee
for their help in providing mummies, and Richard Gill, Patricia Noble, and Heather Rowe for their assistance in
mummy collection and monitoring emergence. This work
was supported in part by the California Pistachio Commission.
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Received 29 September 2009; accepted 2 March 2010.