BEHAVIOR
Effects of Foliar Surfactants on Host Plant Selection Behavior of
Liriomyza huidobrensis (Diptera: Agromyzidae)
FRASER R. MCKEE,1 JOSHUA LEVAC,2
AND
REBECCA H. HALLETT3
School of Environmental Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Environ. Entomol. 38(5): 1387Ð1394 (2009)
ABSTRACT The pea leafminer, Liriomyza huidobrensis (Diptera: Agromyzidae), is a highly polyphagous insect pest of global distribution. L. huidobrensis feeds and lays its eggs on leaf tissue and reduces
crop marketability because of stippling and mining damage. In Þeld insecticide trials, it was observed
that stippling was reduced on plants treated with surfactant alone. The objectives of this study were
to determine the effect of surfactants on host selection behaviors of female L. huidobrensis and to assess
the phytotoxicity of two common surfactants to test plants. The application of the surfactant Sylgard
309 to celery (Apium graveolens) caused a signiÞcant reduction in stippling rates. The application of
Agral 90 to cucumber leaves (Cucumis sativus) resulted in changes to the amount of effort invested
by females in speciÞc host plant selection behaviors, as well as causing a signiÞcant reduction in the
amount of stippling damage. The recommended dose of Sylgard 309 does not induce phytotoxicity on
celery over a range of age classes nor does Agral 90 cause a phytotoxic effect in 35-d-old cucumber.
Thus, reductions in observed stippling and changes to host selection behaviors were caused by an
antixenotic effect of the surfactant on L. huidobrensis rather than a toxic effect of the surfactant on
the plant. The presence of surfactant on an otherwise acceptable host plant seems to have masked host
plant cues and prevented host plant recognition. Results indicate that surfactants may be used to
reduce leafminer damage to vegetable crops, potentially reducing the use of insecticides.
KEY WORDS pea leafminer, Agral 90, Sylgard 309, preoviposition behaviors, antixenosis
The pea leafminer, Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae), is an agricultural pest
endemic to South America (Spencer 1973, Scheffer
and Lewis 2001). Since the early 1980s, L. huidobrensis
has been recorded in numerous countries around the
world, presumably associated with the global trade of
ornamental plants (Shepard et al. 1998, Rauf et al.
2000, Scheffer et al. 2001, Weintraub 2001, Banchio et
al. 2003). Because of its current worldwide distribution, L. huidobrensis has become one of the most
economically important pests of open Þeld and greenhouse crops (Weintraub and Horowitz 1997, Shepard
et al. 1998, Chen and Kang 2004). Despite its common
name, this species of leafminer is highly polyphagous
and has host plants in 16 different families of vegetable
crops, including celery (Apium graveolens L.) and
cucumber (Cucumis sativus L.) (Shepard et al. 1998,
Rauf et al. 2000, Wei et al. 2000, Banchio et al. 2003,
Martin et al. 2005). The polyphagous nature of L.
huidobrensis, combined with high reproductive rates
and rapid development of insecticide resistance, con1 Current address: Department of Ecosystem Science and Management, University of Northern British Columbia, Prince George, British
Columbia, Canada V2N 4Z9.
2 Current address: Department of Biological Sciences, University of
Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.
3 Corresponding author, e-mail: [email protected].
tribute to the success of L. huidobrensis as an invasive
species (Shepard et al. 1998).
Both larval and adult L. huidobrensis cause injury to
plants. Mature females use their ovipositor to puncture the leaf surface, creating a “stipple” in which to
feed and/or oviposit (Parrella 1987). On hatching,
larvae mine into the leaf and feed on the chloroplastrich mesophyll. Damage from adult and larval activities disrupts photosynthesis and leaf physiology, causing leaf wilt and reducing plant health (Parrella 1987).
Both crop yield and marketability are reduced, resulting in high economic losses to vegetable producers
around the world (Weintraub and Horowitz 1997,
Rauf et al. 2000).
Studies that investigate the cues responsible for
mediation of host selection behavior in L. huidobrensis
are limited within the published literature. Chemical
cues associated with gustation are important to Liriomyza species in the determination of suitable host
plants (Parrella 1987). Plant surface exudates and tactile stimuli have been linked to host acceptance and
rejection in L. huidobrensis (Wei et al. 2000, Pang et
al. 2006). Plant-based extracts have been studied for
larvicidal activity against larval L. huidobrensis (Weintraub and Horowitz 1997, Banchio et al. 2003) and
have also reduced feeding damage and oviposition by
adult L. huidobrensis (Banchio et al. 2003). In Þeld
0046-225X/09/1387Ð1394$04.00/0 䉷 2009 Entomological Society of America
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ENVIRONMENTAL ENTOMOLOGY
trials, foliar applications of surfactants without an insecticide seemed to reduce stippling damage and impair normal host selection behaviors (Hallett et al.
2004). Surfactants lower the surface tension of a solvent and may be included as wetting agents in tank
mixes of pesticide sprays to ensure proper foliar coverage (Jones and Atkins 2000). There are currently no
known studies that speciÞcally address the effect of
surfactants on ovipositional behavior of L. huidobrensis. The objectives of this study, therefore, were to
determine the effect of surfactants on host selection
behaviors of female L. huidobrensis and to assess the
phytotoxicity of surfactants on two host plants.
Materials and Methods
Plants. Celery and cucumber were selected for experimentation as representatives of Þeld and greenhouse-grown vegetables, respectively, commonly infested by L. huidobrensis in Ontario. All cultivars used
were obtained from Stokes Seeds (Thorold, Ontario,
Canada). All plants were grown in Pro-Mix B (Premier
Horticultural, Dorval, Quebec, Canada) and maintained at 25⬚C with a 16:8 light period and 50 Ð70% RH.
Celery cultivar Florida 683 was either seeded directly
into 10-cm pots (experiments 1 and 2) or seeded in 128
cell plug trays and transplanted to 12.7-cm pots when
seedlings reached ⬇8 cm in height (experiment 3).
Cucumber cultivar Calypso 146A was grown in 128 cell
plug trays and transplanted to 10-cm pots when seedlings reached ⬇5 cm in height (experiments 4 Ð 6).
Plants were watered ad libitum and fertilized three
times per week with 15Ð30-15 Miracle-Gro vegetable
fertilizer (Miracle-Gro, Marysville, OH).
Surfactants. Surfactants were selected on the basis
of the agricultural contexts in which they are registered for use in Canada. Sylgard 309 (siloxylated polyether 76%; Dow Corning Canada, Georgetown, Ontario, Canada) is registered for use with herbicides
applied to Þeld grown crops, i.e., including celery
(Pest Management Regulatory Agency 2008b). Agral
90 (nonylphenoxypolyethoxyethanol 90%; Syngenta
Crop Protection Canada, Guelph, Ontario, Canada) is
registered for use with insecticides that are registered
for use in greenhouse cucumbers (Pest Management
Regulatory Agency 2008a). Neither surfactant had a
detectable odour when applied to plants as described
below.
Insects. Leafminers used in all experiments were
obtained from a laboratory colony maintained according to procedures outlined in Martin et al. (2005).
Four-day-old adults, reared primarily on lettuce (Lactuca sativa L. cultivar Ithaca M.I.), were used in experiments 1 and 2 (2004). Leafminers used in experiments 5 and 6 (2006 Ð2007) were from colonies reared
on lettuce, cucumber, peas (Pisum sativum L. cultivar
Bolero), and Chinese broccoli (Brassica alboglabra
cultivar Guy Lon 422A). Three- to 4-d-old mated females (i.e., with abdomens visibly distended with
eggs) were used in experiments 5 and 6.
Experiment 1: Effect of Sylgard on Stippling. A
choice experiment was conducted to determine the
Vol. 38, no. 5
effect of Sylgard on stippling by the pea leafminer. Ten
pairs of celery plants (2 mo after seeding), selected for
approximately equal size, were sprayed with either 6
ml of water or 6 ml of Sylgard solution (2.5 ml/liter;
i.e., labeled rate) using hand-held spray bottles (E-Z
Sprayer; Continental Industries, Brampton, Ontario,
Canada) at a distance of 30 cm to cover the surface of
the plant. Plants were left to dry for 20 min and pairs
of plants were placed in 100 by 60 by 40-cm Plexiglas
cages. Fifty adult leafminers were introduced into the
cages by aspirator. After 5 d, the Þrst and third petioles
from the center were removed from each plant, all
leaßets examined (Þve leaßets for Þrst petioles and
seven leaßets for third petioles) and the number of
stipples counted on each leaßet.
Experiment 2: Effect of Sylgard on Female Behavior. To assess the effect of surfactant on preoviposition
behavior, an observational choice study was conducted with individual females. Observation chambers were made from 1.7-liter white plastic buckets
(15 cm diameter by 10 cm height), which had their lids
replaced with a transparent sheet. The third oldest
petiole was selected from celery plants of equal age
and approximately equal size. One petiole was treated
with 6 ml of Sylgard solution as above (2.5 ml/liter),
and the other was sprayed with water. Both petioles
were allowed to air dry for 20 min and placed inside
the observation chamber. A single female was released
into the observational chamber and observed for 1 h.
The number of landings on control and treated petioles were recorded, along with the duration of the
landing and whether or not probing and stippling
occurred. Probing was deÞned as the extension of the
femaleÕs mouthparts to make physical contact with the
plant surface. Eight replicates were conducted.
Experiment 3: Phytotoxicity of Sylgard on Celery.
Five treatments of Sylgard (5.0, 2.5, 1.25, 0.63, and 0
ml/liter [control]) were applied to celery plants that
were 2, 4, and 8 mo old. Ten, three, and Þve replicates
were conducted for the 2-, 4-, and 8-mo age classes,
respectively. The three healthiest stalks on each plant
were tagged before the experiment and a total of 6 ml
of the treatment solution was applied to the entire
plant surface with hand-held spray bottles from a
distance of 30 cm. Plants were air dried and arranged
in a completely randomized design.
Phytotoxicity was assessed every 3 d for a 21-d
period using a foliar damage rating based on the percentage of chlorotic and/or necrotic leaf surface area
with 0 ⫽ 0% damage, 1 ⫽ 1Ð20% damage, 2 ⫽ 21Ð 40%
damage, 3 ⫽ 41Ð 60% damage, 4 ⫽ 61Ð 80% damage, and
5 ⫽ 81Ð100% damage. Damage ratings were assessed
visually by overlaying a leaf with a transparent grid
with squares equal to 1 cm2. Grid squares partially
Þlled with necrotic foliar tissue were estimated to the
nearest 0.25 cm2.
Experiment 4: Phytotoxicity of Agral on Cucumber.
Five treatments of Agral (0.6 ml/liter water, 0.3 ml/
liter [labeled rate], 0.15, 0.08, and 0 ml/liter [control]) were applied to 35-d-old cucumber plants. The
Þrst three leaves of the vines were tagged before
treatment. Experimental protocols were identical to
October 2009
MCKEE ET AL.: SURFACTANTS AND HOST SELECTION BY PEA LEAFMINER
experiment 3, except that 10 replicates per treatment
were conducted, and plants were assessed for damage
every 3 d for 15 d, as normal senescence of cucumber
leaves began before 21 d.
Experiment 5: Effect of Agral on Female Behavior
on Cucumber. A no-choice experiment was conducted with 35-d-old cucumber plants treated with 0.3
ml/liter Agral (n ⫽ 25) and control plants treated with
water (n ⫽ 25). After treatment, a single cucumber
plant was placed in the center of a vented Plexiglas
observation cage (30 by 30 by 30 cm). A single female
was released into the cage, which was placed inside a
white fabric tent (50 by 50 by 130 cm) to minimize
external interference. Overhead lighting was provided
by two 100-W spotlights placed outside of the tent and
equidistant from the center of the tent. Observations
of behavior were made every 20 s for 30 min through
small windows cut into the fabric. Time to Þrst plant
contact and time spent on plant were recorded. Behaviors were recorded for 30 min; however, observation of the female continued as long as was needed for
her to make initial contact with the plant. Between
trials, air inside the cage was exchanged using a vacuum pump to ensure removal of any plant volatiles
that might inßuence subsequent test subjects.
Observed behaviors were stippling, sitting, ßying,
walking, grooming, and push-ups. “Stippling” incorporated the previously described probing behavior
and was deÞned as the puncturing of the leaf surface
with the ovipositor and feeding and removal of tissue
with the mouthparts because these behaviors occur
together and result in formation of a stipple or wound
on the plant. The term “grooming” was used to describe rubbing together of two or more appendages,
including antennae. The term “push-up” was used to
describe repeated lifting and lowering of the body.
Behavioral recordings made at 20-s intervals were
compiled into six 5-min periods for each 30-min observation period, and data for number of observations
per behavior, time until Þrst contact, and time spent on
plant were analyzed for differences between treated
and control plants. Female behaviors were separated
based on “on” and “off” plant observations. For this
study, only female behaviors occurring while in contact with plants were analyzed. Because the time taken
to contact plants varied by female, observations of on
plant behaviors were standardized as percentages of
the total number of 20-s time intervals each female
spent on a plant.
Experiment 6: Effect of Agral on Female Host Preference. A choice experiment, with 25 replicates, was
conducted with cucumber plants treated as in experiment 5. Treated and control plants were randomly
assigned to each end of a vented Plexiglas cage (36 by
61 by 46 cm), which was placed within the white fabric
tent, as above. Ten mated females were released at the
center of the cage. After 2 h, plants were removed, and
the number of stipples on each plant counted and
recorded. The number of stipples on each plant was
converted to proportion of total stipples to reduce
variability arising from differences among females in
stippling rates.
1389
Table 1. Comparisons of the behavior of female pea leafminers
(L. huidobrensis) on celery (A. graveolens) treated with Sylgard 309
(2.5 ml/liter) (treated) or with water (control)
Treated
Control
P value
Experiment 1
No. stipples per
4.67 ⫾ 0.28
29.14 ⫾ 1.29
⬍0.0001
leaßeta
Experiment 2
1.13 ⫾ 0.23
5.75 ⫾ 0.70
⬍0.0001
No. landingsb
1.00 ⫾ 0.19
3.75 ⫾ 0.53
0.0002
No. probing eventsc
0.13 ⫾ 0.13
2.38 ⫾ 0.42
0.0002
No. stippling eventsd
384.88 ⫾ 115.82 1,180.00 ⫾ 246.93 0.0113
Time on plant (s)e
307.90 ⫾ 57.97
205.22 ⫾ 36.70
NS
Time per landing
(s)f
Data analysed by ANOVA, with ␣ ⫽ 0.05. Values are means ⫾ SE.
df ⫽ 1,455; F ⫽ 348.95.
df ⫽ 1,14; F ⫽ 39.44.
c
df ⫽ 1,14; F ⫽ 24.20.
d
df ⫽ 1,14; F ⫽ 26.37.
e
df ⫽ 1,14; F ⫽ 8.50.
f
df ⫽ 1,40; F ⫽ 2.18.
NS, not signiÞcant.
a
b
Statistical Analyses. Analyses of variance (ANOVA)
were conducted using PROC GLM (SAS v 9.1; SAS
Institute, Cary, NC) to examine differences in the
number of stipples on treated and control celery plants
in experiment 1, and differences in the number of
landings, probes, and stipples and time spent on
treated and control plants in experiment 2. Phytotoxicity (experiments 3 and 4) and behavioral data (experiments 5 and 6) were analyzed by ANOVA followed by means separations with TukeyÕs studentized
range (honestly signiÞcant difference [HSD]) test
with ␣ ⫽ 0.05. In experiment 6, proportion of stipples
data were subject to arcsine transformation before
analyses to satisfy the assumptions of ANOVA.
Results
Experiment 1: Effect of Sylgard on Stippling. The
number of stipples per leaßet differed signiÞcantly
between treated and control plants (Table 1). Replicate was found to be a signiÞcant source of variation
(df ⫽ 9,455; F ⫽ 2.26; P ⫽ 0.018), but no signiÞcant
treatment by replicate interaction was found (df ⫽
4,455; F ⫽ 1.19; P ⫽ 0.32). Petioles were observed to
be dying on treated plants at the time of stippling
assessments; however, this observation was not quantiÞed.
Experiment 2: Effect of Sylgard on Female Behavior. Statistically signiÞcant differences were found between treated and control plants in the number of
landings, number of probing events, number of stipples, and time spent on the plant (Table 1). Replicate
was not found to be a signiÞcant source of variation for
any of these measures. No signiÞcant difference was
found between treatments in time per landing event.
Replicate was found to be a signiÞcant source of variation (df ⫽ 7,40; F ⫽ 3.85; P ⫽ 0.003) for this measure;
however, no signiÞcant treatment by replicate interaction was found (df ⫽ 7,40; F ⫽ 1.56; P ⫽ 0.177).
1390
ENVIRONMENTAL ENTOMOLOGY
Vol. 38, no. 5
Table 2. Phytotoxicity ratings for three ages of celery (A. graveolens) plants 21 d after treatment with Sylgard 309 and for 35-d-old
cucumber (C. sativus) plants 15 d after treatment with Agral 90
Sylgard 309
(ml/L)
5.00
2.50
1.25
0.63
0
Foliar damage rating (mean ⫾ SE)
Agral 90 (ml/liter)
2-mo celerya
4-mo celeryb
8-mo celeryc
1.06 ⫾ 0.06 a
0.78 ⫾ 0.04 b
0.73 ⫾ 0.07 b
0.71 ⫾ 0.08 b
0.76 ⫾ 0.09 b
2.25 ⫾ 0.17 a
1.54 ⫾ 0.11 b
1.22 ⫾ 0.08 b
1.34 ⫾ 0.08 b
1.38 ⫾ 0.07 b
1.78 ⫾ 0.06 a
1.63 ⫾ 0.06 a
1.35 ⫾ 0.07 b
1.72 ⫾ 0.08 a
1.32 ⫾ 0.05 b
Foliar damage rating
(mean ⫾ SE)
35-d cucumberd
0.60
0.30
0.15
0.08
0
1.17 ⫾ 0.07 a
0.74 ⫾ 0.06 b
0.66 ⫾ 0.04 b
0.64 ⫾ 0.04 b
0.69 ⫾ 0.04 b
Mean damage ratings within a column followed by the same letter are not signiÞcantly different (TukeyÕs studentized range test, ␣ ⫽ 0.05).
ANOVA, df ⫽ 4,1031; F ⫽ 5.93; P ⬍ 0.001.
ANOVA, df ⫽ 4,286; F ⫽ 14.98; P ⬍ 0.001.
c
ANOVA, df ⫽ 4,611; F ⫽ 11.43; P ⬍ 0.001.
d
ANOVA, df ⫽ 4,861; F ⫽ 39.44; P ⬍ 0.001.
a
b
Experiment 3: Phytotoxicity of Sylgard on Celery.
The highest rate (5.0 ml/liter) of Sylgard caused signiÞcant damage to the leaves of 2- and 4-mo-old celery
plants (Table 2). On 8-mo-old celery plants, the 5.0,
2.5, and 0.63 ml/liter treatments of Sylgard caused
signiÞcantly more damage than the 1.25 and 0 ml/liter
treatments over the 21 d of post-treatment assessments. No treatment by assessment day interaction
was found for any of the plant ages examined.
Experiment 4: Phytotoxicity of Agral on Cucumber.
The highest rate (0.60 ml/liter) of Agral caused signiÞcantly more damage to the leaves of 35-d-old cucumber plants than all other treatments (Table 2).
Damage in the 0.60 ml/liter treatment was present on
all leaves from 3 d after application but progressed
more slowly on leaves in other treatments (data not
shown). No treatment by assessment day interaction
was found.
Experiment 5: Effect of Agral on Female Behavior
on Cucumber. SigniÞcant differences in the frequency of behaviors across time periods were found
largely within treatments, with signiÞcant differences
in stippling occurring between control and treated
plants in the later time periods (Fig. 1). On control
plants, stippling was the most frequently observed
behavior (as measured by percent of observations),
followed by sitting across all time periods. On treated
plants, stippling was the most common behavior with
the exception of the Þnal two time periods when
sitting became the most common behavior. Grooming
and push-ups were infrequent behaviors and each
comprised ⬍2% of observations in any 5-min time
period. Replicate was found to be a signiÞcant source
of variation in the Þrst, second, third, and fourth 5-min
time periods for control plants, as well as the second,
Þfth, and sixth 5-min time periods for the treated
plants, and was attributed to variation among activity
levels of individual females.
In the Þrst 5 min, the only behaviors observed on
control plants were stippling and walking. During this
time, these behaviors occurred infrequently and were
not signiÞcantly different from one another (df ⫽ 4,96;
F ⫽ 1.00; P ⫽ 0.41; Fig. 1a). No behaviors were observed on treated plants during the initial 5 min.
From 5 to 10 min, stippling occurred more frequently than push ups and walking on control plants
(df ⫽ 4,96; F ⫽ 3.41; P ⫽ 0.012). Sitting, walking, and
stippling all occurred at low, but equal, frequency on
treated plants during this time (df ⫽ 4,96; F ⫽ 1.87; P ⫽
0.122; Fig. 1b). No signiÞcant differences were found
in the frequencies of individual behaviors between
control and treated plants in this time period.
From 10 to 15 min, stippling occurred more frequently than all other behaviors, except sitting, on
control plants (df ⫽ 4,96; F ⫽ 4.85; P ⬍ 0.002; Fig. 1c).
A similar, but nonsigniÞcant, pattern of behavioral
frequencies was recorded on treated plants (df ⫽ 4,96;
F ⫽ 1.50; P ⬍ 0.210). No signiÞcant differences were
found in the frequencies of individual behaviors between control and treated plants in this time period.
From 15 to 20 min, stippling was the most frequent
behavior on control plants (df ⫽ 4,96; F ⫽ 12.21; P ⬍
0.001; Fig. 1d). A similar, but nonsigniÞcant, pattern of
behavioral frequencies was observed on treated plants
within this time period (df ⫽ 4,96; F ⫽ 1.66; P ⫽ 0.165).
There was no difference in the frequencies of individual behaviors between treatments.
From 20 to 25 min, stippling occurred more frequently than sitting, which occurred more frequently
than all other behaviors on control plants (df ⫽ 4,96;
F ⫽ 22.30; P ⬍ 0.001), whereas sitting occurred more
frequently than all other behaviors on treated plants
(df ⫽ 4,96; F ⫽ 9.20; P ⬍ 0.001; Fig. 1e). During this
time period, stippling occurred signiÞcantly more often on control than on treated plants (df ⫽ 1,48; F ⫽
13.06; P ⬍ 0.001). No other behaviors differed significantly in frequency between control and treated
plants.
In the Þnal time period (25Ð30 min), stippling was
again the most frequently observed behavior on control plants, followed by sitting, which was more frequent than all other behaviors (df ⫽ 4,96; F ⫽ 27.34;
P ⬍ 0.001). On treated plants sitting was the most
frequent behavior (df ⫽ 4,96; F ⫽ 10.63; P ⬍ 0.001; Fig.
1f). Stippling was signiÞcantly less frequent on treated
than control plants (df ⫽ 1,48; F ⫽ 22.38; P ⬍ 0.001),
whereas no other signiÞcant differences occurred in
the frequencies of all other behaviors.
Of the Þve observed behaviors exhibited on plants
by female L. huidobrensis, the occurrence of stippling
had the largest change in frequency across the 30-min
observation period on control versus treated cucum-
October 2009
MCKEE ET AL.: SURFACTANTS AND HOST SELECTION BY PEA LEAFMINER
Grooming
Mean percent of recordings / behavior
50
Push-up
Sitting
50
A ) 0 -5 m in u te s
40
40
30
30
20
20
10
10
Stippling
50
a
a
a
a
a
A
A
A
A
A
0
50
B ) 5 -1 0 m in u te s
40
40
30
30
20
20
0
50
ab
a
ab
b
a
A
A
A
A
A
a
a
a
A
A
A
A
E ) 2 0 -2 5 m in u te s
c
b
B
*
A
a
0
50
C ) 1 0 -1 5 m in u te s
A
b
10
10
Walking
D ) 1 5 -2 0 m in u te s
a
0
1391
a
a
A
A
A
F ) 2 5 -3 0 m in u te s
c
40
40
30
30
20
20
B
A
10
0
a
a
ab
b
C o n tro l P la n ts
a
A
A
A
b
*
A
10
A
T re a te d P la n ts
a
0
a
a
C o n tro l P la n ts
A
A
A
T re a te d P la n ts
Fig. 1. Frequency (mean percent of recordings per behavior) of observed behaviors of female pea leafminer (L.
huidobrensis) in no-choice experiments with cucumber (C. sativus) plants treated with Agral 90 (0.30 ml/liter) (treated) or
with water (control). Observations made at 20 s intervals from introduction of the female to the arena were compiled into
six 5-min periods (A-F) for analyses. Frequencies of behaviors with the same letter and case are not signiÞcantly different
from each other; frequencies of behaviors marked with an asterisk are signiÞcantly different from the same behavior on treated
plants (TukeyÕs studentized range test, ␣ ⫽ 0.05).
ber plants. Over the duration of the observation period, the frequency of stippling by females increased
signiÞcantly on control but not treated plants (Fig. 2).
Stippling on control plants began at a frequency of
0.2% and steadily increased to become the principal
behavior exhibited in minutes 25Ð30 (38.8%). In contrast, stippling rates on treated plants were much
lower than on control plants and never exceeded 7%
of recorded observations. Sitting, rather than stippling,
became the most engaged-in behavior for female L.
huidobrensis exposed to treated plants (18.9%).
Females took longer to make contact with treated
(1,942.16 ⫾ 290.41 s) than with control (1,120 ⫾ 86.50
s) cucumber plants (df ⫽ 1,48; F ⫽ 7.36; P ⫽ 0.009).
There were signiÞcantly fewer observations made of
females being situated on treated plants (21.7%) than
on control plants (36.2%; df ⫽ 1,48; F ⫽ 4.69; P ⫽
0.035).
Experiment 6: Effect of Agral on Female Host Preference. After the 2-h exposure period, a signiÞcantly
lower percentage of stipples were found on treated
plants (34.1%) than on control plants (65.9%; df ⫽ 1,24;
F ⫽ 15.06; P ⬍ 0.001). Replicate was not a signiÞcant
source of variation.
Discussion
Surfactants reduced normal preoviposition behaviors of L. huidobrensis on otherwise acceptable celery
host plants. This effect does not seem to be related to
possible negative effects of surfactants on plant health,
as recommended rates of the surfactants used did not
1392
ENVIRONMENTAL ENTOMOLOGY
Vol. 38, no. 5
Mean percent of recordings
50
45
40
c
Treated
Control
*
35
30
25
20
b
15
A
10
5
0
A a
0-5
A
5-10
A ab
10-15
A
A
ab
a
*
15-20
20-25
25-30
Time Period (min)
Fig. 2. Frequency (mean percent of recordings) of observed stippling behavior of female pea leafminer (L. huidobrensis)
in no-choice experiments with cucumber (C. sativus) plants treated with Agral 90 (0.30 ml/liter) (treated) or with water
(control). Within a treatment, frequencies with the same letter are not signiÞcantly different; time periods marked with an
asterisk indicate a signiÞcant difference in stippling frequency between treated and control plants (TukeyÕs studentized range
test, ␣ ⫽ 0.05).
cause phytotoxicity. Petiole death was observed, but
not quantiÞed, in Sylgard-treated celery in initial experiments, and was likely caused by the enclosed conditions in which plants were housed for 5 d rather than
to the surfactant alone. Phytotoxic effects of Sylgard
were observed on 8-mo-old celery plants, but these
plants were both overmature and became infested
with thrips. Thrips vector many plant diseases that
cause chlorosis and necrosis of foliar tissue (Jones
2005), possibly confounding foliar ratings. Symptoms of phytotoxicity observed on cucumber leaves
treated with low doses of Agral were likely caused
by leaf age and senescence rather than to the surfactant, as similar symptoms were observed on control plants.
The results of this study indicate that certain host
plant selection behaviors of female L. huidobrensis
may be altered by the presence of a surfactant on the
foliage of a potential host plant. From this study, it
seems the normal hierarchy of behaviors during hostÐ
plant interactions by L. huidobrensis, as indicated by
behaviors on control cucumber plants, is stippling
followed by sitting, walking, grooming, and Þnally
push-ups. On treated plants, females exhibited this
hierarchy of behaviors until the Þnal two 5-min observation periods where females continued to engage
more frequently in sitting, and less frequently, in stippling. In addition, the presence of a surfactant significantly altered the time taken for L. huidobrensis to
make initial plant contact, the number of landings on
a plant, the amount of time spent on a plant, stippling
rates in both choice and no choice trials, as well as the
frequency of stippling behavior. Because behavioral
data were standardized based on the amount of time
a female spent in contact with a plant, it is unlikely that
differences in behavioral frequencies between control and treated cucumber plants across time is an
artifact of the increased time taken for females to
make initial plant contact or the reduced amount of
time spent on treated plants. However, the low
incidence of recorded behaviors in the initial 10 min
of observation is likely caused by delayed plant
contact by females exposed to either plant treatment. Thus, reductions in observed stippling and
changes to host selection behaviors were apparently
caused by an antixenotic effect of the surfactant on
L. huidobrensis rather than a toxic effect of the
surfactant on the plant.
Although somewhat more frequent on treated
plants, the push-up behavior appears to be an irregular
activity across time on both treated and control plants
and is unaffected by the presence of a surfactant. It
was noted that ßight and walking behaviors were often
preceded by a series of push-ups that lasted 1 or 2 s.
In a stationary ßy, push-ups may be a reaction to
increasing neurological activity as part of a physiological and/or neurological sequence that is initiated before locomotion (Dethier 1976).
Chemical detection and recognition allows insects
to locate optimal feeding areas (Desouhant et al. 2005,
Kendrick and Raffa 2006, Hjalten et al. 2007), seek
mating opportunities (Carazo et al. 2004, Bengtsson et
al. 2006, Segura et al. 2007, Witzgall et al. 2008), avoid
natural enemies (Ferris and Rudolf 2007), and select
suitable oviposition sites (Bengtsson et al. 2006, Tasin
et al. 2006, Steiner et al. 2007). Chemical cues present
in plants are a major determinant for host plant identiÞcation in many insects, and plants that do not have
the correct chemical signature may be ignored as
oviposition sites (Wallin and Raffa 2000, Clausen et al.
2002, Cunningham and Floyd 2004). Foliar applications of plant extracts have resulted in reduced damage to crops because of decreased oviposition by L.
huidobrensis (Banchio et al. 2003). The surfactants
used here did not seem to repel L. huidobrensis from
the plant because there was no difference in the time
spent per landing on control and treated plants. In
addition, ßight activity in both treatment and control
contexts was at its maximum during the 15- to 20-min
period (data not shown); however, the time to Þrst
plant contact occurred nearly 14 min later on treated
than control plants, suggesting that the surfactant may
October 2009
MCKEE ET AL.: SURFACTANTS AND HOST SELECTION BY PEA LEAFMINER
interfere with perception of the host plant by L. huidobrensis. Given the low volatility of Agral 90 (Syngenta
Crop Protection Canada 2006), it is unlikely that Agral
90 produces volatiles that block or disrupt the normal
functioning of olfactory chemoreceptors of L. huidobrensis from a distance. However, the normal release
of plant volatiles from the leaf surface may be prevented or inhibited by a coating of surfactant on the
leaf surface, resulting in lack of perception of the plant
by olfactory and gustatory chemosensillae.
Successful feeding and oviposition can be dependent on the successful execution of a series of host
acceptance activities (Mowry et al. 1989, Henderson
et al. 2004). Martin et al. (2005) have suggested that
L. huidobrensis feeding and oviposition cues are overlapped or that oviposition is dependent on positive
feeding cues. Dethier (1976) states that ßies that are
in no immediate need of nutrition and are not concerned with reproduction are “going nowhere, doing
nothing.” However, the females selected for use in this
study were all gravid and therefore should have had an
urge to oviposit on plants deemed to be acceptable. On
control plants, stippling was the most frequent behavior of female L. huidobrensis as time progressed; however, on treated plants, females engaged more frequently in sitting. This result suggests that they were
unable to identify the plant as an acceptable host
because of the coating of surfactant. An inability of L.
huidobrensis to properly detect hostÐplant chemistry
could deter the initiation of feeding and also ovipositional behavior. If these behaviors are interrupted, L.
huidobrensis may revert to an activity that is most
energetically favorable, such as sitting, with a consequent reduction in stippling damage.
The results from this study agree with those of
Hallett et al. (2004) and support the potential use of
surfactants in pest management programs directed at
protecting certain vegetable crops from L. huidobrensis damage. In the laboratory setting, surfactants were
shown to deter L. huidobrensis stippling over a 2-h
period on cucumber and over a 5-d laboratory period
on celery. Preliminary Þeld evidence suggests that
surfactants may help to reduce L. huidobrensis stippling for periods of up to 7 d (Hallett et al. 2004).
However, considerable further study is needed to examine the interactions between L. huidobrensis and
surfactant-treated crops in Þeld settings, as well as the
effect of environmental conditions on surfactant performance. Surfactants may have the highest potential
to be of use for crop protection in the hydroponic
greenhouse vegetable industry. Typically, the growing
environment within these greenhouses is highly regulated with no natural precipitation that may wash-off
surfactants applied to crops, possibly extending the
effectiveness of the surfactantÕs protective properties.
Although further studies are needed to test the applicability of crop protection by surfactants, available
evidence suggests that potential exists for surfactants,
or similar products, to be integrated into a pest management regimen for L. huidobrensis.
1393
Acknowledgments
We thank D. Stanley-Horn for advice on pea leafminer
maintenance and several anonymous reviewers for constructive comments on an earlier version of this manuscript. This
work was supported in part by a Canadian Natural Sciences
& Engineering Research Council Discovery grant to R.H.H.
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Received 2 January 2009; accepted 26 June 2009.