Behavioral Ecology
doi:10.1093/beheco/ars172
Advance Access publication 17 October 2012
Origianl Article
Different roles for innate and learnt behavioral
responses to odors in insect host location
Ben Webster,a Erika Qvarfordt,a Ulf Olsson,b and Robert Glinwooda
aDepartment of Ecology and bDepartment of Economics, Swedish University of Agricultural Sciences,
75007 Uppsala, Sweden
Volatile chemical cues are used by herbivorous insects to locate and identify their host plants. Many species show a preference
for volatiles experienced during development in the natal habitat. The reliability of this learnt information, however, may be
limited. Many insects develop in restricted habitats, often on a single plant. Large between-plant variability in volatile emission,
due to genetic differences and different exposure to biotic and abiotic factors, means that the volatile profile of a single plant
may not be representative of the entire species. Insects must, therefore, balance the benefits of learning with the risks associated
with its reliability. This is especially important for insects for which habitat exploration is costly. We hypothesize that information
gained in the natal habitat is most likely to be utilized in situations where the cost of habitat exploration is lowest. To test this
hypothesis, the black bean aphid, Aphis fabae, was reared on artificial diet while exposed to volatiles from its host, broad bean,
and an unsuitable host, mustard. When offered the choice between bean and mustard leaves as adults, aphids showed a preference for the leaves whose odor they had experienced during development. When only exposed to volatiles from the two plants,
in the absence of cues to indicate proximity or accessibility of the odor source, aphids preferred bean volatiles, regardless of
experience. This suggests that information acquired from the natal habitat is only utilized when the perceived cost of habitat
assessment is low, with innate preferences dominating otherwise. Key words: aphid, host location, learning, olfaction, volatiles.
[Behav Ecol]
Introduction
L
earning is the acquisition and retention of neuronal representations of new information (Dukas 2008). It is a
widespread trait among insects and provides numerous selective advantages when foraging in an unpredictable environment. Many insects learn from experience acquired in the
natal habitat and this can provide several adaptive advantages
(Davis and Stamps 2004; Stamps and Davis 2006). One advantage of learning from natal experience is that it can improve
flexibility in host range and facilitate in the exploitation of
novel host plants. Many insects show strong preferences for
their preferred hosts but will attempt to settle on novel, unfamiliar plants should they be unable to locate anything more
suitable (Stamps and Davis 2006). If a phytophagous insect
successfully colonizes a novel host plant and its offspring survive to maturity, its offspring will be able to continue to take
advantage of the new host species by learning to recognize
cues associated with it. This would facilitate the expansion
of host range and could be especially adaptive in a changing
environment. Some insect species can develop physiological
adaptations to novel host plants, and learning allows insects
to best take advantage of such adaptations by allowing subsequent generations to continue exploiting the host they
become adapted to. For example, Gorur et al. (2005, 2007)
demonstrated that different clones of the black bean aphid,
Aphis fabae, may be more adapted to either bean (Vicia faba)
or nasturtium (Tropaeolum majus) showing decreased performance when transferred to the alternative host. It was
found, however, that some clones were able to adapt to the
Address correspondence to B. Webster. E-mail: [email protected].
Received 23 March 2012; revised 12 July 2012; accepted 27 August
2012.
© The Author 2012. Published by Oxford University Press on behalf
of the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
alternative host after being reared on it for a few generations
of asexual reproduction, showing increased performance
and survival on the new host. In some cases, such adaptations
may be subject to trade-offs, reducing performance on other
hosts (Tosh et al. 2004). Insects that display such adaptations
are, therefore, likely to show increased performance on the
host species they were reared on, which should favor learning
from natal experience.
The use of volatile chemical cues in host plant location is
well documented among phytophagous insects, and many
species show the ability to learn from such cues (Dukas
2008). Most herbivorous insects develop in restricted habitats,
often on a single plant. Although information gained from
the host plant may be useful, there is a risk that it may not
be representative of the entire host species. This is an idea
that has received little attention and could have important
implications for the use of learnt information in many animal
species. Plants show large variability in their volatile profiles.
Infestation with herbivores can cause dramatic differences
in volatile blends that can vary depending on the species of
herbivore and magnitude of infestation (Hare 2011). Insects
developing on an infested plant are, therefore, exposed to
volatile blends that may not be emitted by new, uninfested
plants they seek during host location. Plants of the same
species can show considerable variability in volatile emission
even when subjected to infestation with the same herbivores
due to genetic differences between individuals (Degen et al.
2004). Other environmental stress factors that may vary
spatially, including humidity, temperature, light, and nitrogen
availability, can also result in considerable variability in
volatile emission between plants (Gouinguené and Turlings
2002). Variability between plants also exists in the absence of
biotic or abiotic stress. When bean plants of the same cultivar
were grown in unstressed conditions in the absence of any
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Webster et al. • Innate and learnt responses to odors
insect herbivores, plants showed considerable variability
in the volatile blends they emitted (Webster et al. 2010b).
Insects are, therefore, faced with the task of identifying
volatile blends that likely possess differences to the blend
they experienced during development. Consistent signals
may exist within highly variable blends (Webster et al. 2010b),
but there is no way for insects to discern these without first
sampling a range of different plants. Information acquired in
the natal habitat may, therefore, not be entirely reliable.
Innate behavioral responses, on the other hand, are the
product of many generations of natural selection and are,
therefore, more likely to be tuned in to the most reliable
components of the host volatile blend. The benefits of learning from information gained in the natal habitat must be
balanced with the cost associated with its lower reliability, particularly in insects for which habitat exploration and assessment is costly.
Host location in insects can be divided into two stages,
with olfaction playing an important role in both (Bruce et al.
2005). Prior to contact with the plant, olfaction can be used
to locate and recognize potential host plants at a distance.
Once visual, tactile and/or gustatory cues indicate that the
insect is in close proximity to the plant, olfaction is used
alongside these other cues in order to make a final decision
as to whether or not to settle. Following an odor plume over
a distance subjects insects to many challenges such as the risk
of predation, dehydration, and starvation. Information used
to locate new hosts at a distance, or when the precise location
or accessibility of the odor source is unknown, must therefore
be reliable. Due to its lower reliability, we hypothesize that
information gained from the natal habitat is most likely to be
utilized when in close proximity to the plant, when the cost
of habitat exploration is low, with innate preferences playing a stronger role when the distance to the odor source is
unknown and perceived costs are higher.
Aphids are a model species in the study of insect behavior, showing strong behavioral responses to volatiles from
their host plants (Webster 2012). Due to high rates of asexual reproduction, it is possible to generate large numbers
of genetically identical aphids, allowing for the control of
genetic variation among individuals used in experiments. It
is not yet known whether behavioral responses of aphids to
plant volatiles are innate or can be modified by experience,
though evidence for learning of gustatory cues has been
found (van Emden et al. 2009). Aphids are phloem feeders
and develop as nymphs on a single plant. Winged morphs are
produced in response to overcrowding and migrate to new
hosts (Powell and Hardie 2001), but wingless aphids also
play an important role in dispersal by walking to neighboring
plants and forming new colonies (Hodgson 1991; Alyokhin
and Sewell 2003). Aphids also show a striking ability to adapt
to novel hosts (Douglas 1997; Gorur et al. 2005, 2007). This
trait should favor the ability to learn from natal experience
(Davis and Stamps 2004).
Habitat exploration is costly for aphids. Wingless aphids
play an important role in dispersal over short to medium distances, expanding to form new colonies on neighboring plants
(Hodgson 1991). Wingless aphids will also readily walk over the
soil to form colonies up to several meters away from the natal
plant (Wiktelius 1989; Alyokhin and Sewell 2003). Walking to a
new host plant, however, incurs a high mortality risk as aphids
are subject to predation, starvation, and dehydration (Wiktelius
1989; Alyokhin and Sewell 2003). Following an odor plume
to its source may be difficult due to meandering of plumes or
physical barriers encountered en-route. This makes the task of
locating a new host over any great distance difficult for walking
aphids. Although learning from natal experience would provide benefits to aphids, aiding in the expansion of host range
and allowing aphids to best take advantage of physiological
adaptations to new hosts, we predict that learnt behavior will
be restricted to when aphids are in close proximity to familiar
plants, where the cost of habitat exploration is low. This would
minimize the costs associated with misidentifying a plant odor
while still allowing aphids to take advantage of learnt information in favorable situations.
The aim of this study was to test the hypothesis that behavioral responses to plant volatiles can be modified by experience acquired during development, but this tends to manifest
only when in close proximity to a potential host with innate
preferences tending to dominate at a distance. The black
bean aphid, A. fabae, was used as a model system to test
this hypothesis. Its host selection behavior has been studied
extensively, making it an ideal study species (Nottingham
et al. 1991; Gorur et al. 2007; Webster et al. 2008, 2010a).
Wingless aphids were used because they will readily settle on
neighboring plants as well as walking longer distances to find
new host plants and so will engage in both low- and high-risk
habitat exploration. Aphids were reared on artificial diet and
exposed to the odors of a host plant, broad bean (V. faba), and
mustard (Sinapis alba), a plant on which preliminary studies
showed aphids were unable to survive to maturity and so can
be considered an unsuitable host. The aphids’ behavior was
then assessed once they had reached maturity. Aphids were
first offered the choice between leaves of the two plants, to
determine preference when in close proximity to the potential hosts to simulate low-risk habitat exploration. To simulate
higher risk habitat exploration, all other cues that might indicate close proximity to the plants were removed, with aphids
being exposed only to plant volatiles in an olfactometer.
Methods
Plants
Broad bean, V. faba (var. Sutton dwarf), and mustard, S. alba,
were grown individually in 8.5-cm pots in a glasshouse (22 °C,
16:8 h light:dark cycle). Plants were used in experiments
when they were 2–3 weeks old, just prior to inflorescence
emergence.
Insects
A colony of A. fabae was derived from aphids provided by
Rothamsted Research, UK. A colony of summer morphs
(virginoparae) were kept Harpenden, on V. faba plants in a
Perspex rearing cabinet in a room maintained at 22 °C with a
16:8 h light:dark cycle.
Aphid rearing system
The artificial diet composition and feeding system were as
described previously (van Emden 2009). Briefly, the system
consisted of approximately 1 mL of diet contained between
two layers of Parafilm M stretched over a Perspex tube
(2.5 cm tall, diameter 3 cm), into which aphids were placed.
To allow air to circulate, the diet tube was attached to a
Perspex “ventilation tube” (4 cm tall, diameter 3 cm). The
ventilation tube contained 28 holes (diameter 0.3 cm) spaced
equally around the tube and covered with mesh to prevent
aphids escaping.
Diet tubes were contained in a sealed plastic rearing chamber (18 × 18 × 18.5 cm) connected to a Perspex odor chamber (40 × 15.5 × 15.5 cm) via polytetrafluoroethylene (PTFE)
tubing (inner diameter [i.d.] 0.3 cm). White pieces of paper
were placed around the rearing chamber in order to remove
any visual cues. Charcoal-filtered air was pumped into the
368
odor chamber at a rate of 600 mL min−1 and pulled out from
the rearing chamber at a rate of 300 mL min−1, creating a
constant flow of air between the two. This allowed volatiles
to be carried from the plant to the rearing chamber; the
ventilation tubes allowing odor-laden air to reach the aphids
feeding within the diet tubes. The difference in flow rate
created a positive pressure that prevented volatiles entering
from outside. Because the odor chamber was not completely
airtight, air could escape from the system thus preventing
unnatural build up of air pressure.
Aphid exposure to volatiles during development
Four different rearing experiences were tested:
1) Diet-reared aphids exposed to volatiles from a bean
plant
2) Diet-reared aphids exposed to volatiles from a mustard plant
3) Diet-reared aphids exposed to volatiles from an empty
rearing chamber (blank)
4) Aphids reared on bean plants (control).
Twelve rearing/odor chambers were used simultaneously,
four for each of the three diet-reared aphids. At the start of
the rearing process, 10 wingless adults of A. fabae were collected from the aphid colony maintained on bean plants,
added to a diet tube, and placed inside a rearing chamber.
Aphids were left for 2 days to produce nymphs after which
adults were removed and the nymphs gently transferred to a
new diet tube using a fine paint brush where they were left
to develop to maturity. Diet tubes and plants were refreshed
twice a week. Aphids were used in experiments after approximately 10 days, shortly after they had reached maturity and
stopped feeding, indicating that they were attempting to disperse. Only wingless adults were used in experiments.
Arena bioassay
Host preference was assessed using an arena bioassay. The
arena was a flattened cylinder, 3 cm tall with a diameter of
15 cm, and covered with mesh to prevent aphids escaping. At
opposite sides of the arena were two small gaps to accommodate the stem of a leaf. Mustard and bean leaves were inserted
on opposite sides. Where possible, leaves were selected so
that they had approximately equal surface area, and both
were undamaged and still attached to the plant. Using intact
leaves was necessary because volatiles released upon mechanical damage have previously been found to cause bean leaves
to become unattractive to A. fabae (Nottingham et al. 1991).
Once leaves were positioned in the arena, 10 wingless adult
aphids were added to the centre and the number of aphids
on each leaf was recorded every 30 min, 1 h, 2 h, 3 h, and
24 h after the experiment had started. The experiment was
repeated 12 times for each rearing experience. Aphids were
used only once, with new aphids being used for each new
replicate.
Numbers of aphids on each leaf were converted to proportions for statistical analysis and generalized linear models with
a logit link were used. The data were of a repeated-measures
type, so mixed generalized linear models were employed
(Littell et al. 2006; Olsson 2002, 2011). The model included
fixed effects of treatment (rearing experience), time, and
treatment*time. Block was included as a random factor.
Correlations between observations over time were modeled
using an unstructured covariance matrix. Specific questions
on comparisons between treatments at each time period were
answered by post-hoc least squares means tests. An alpha level
of P < 0.05 was used to determine if differences were statistically significant.
Behavioral Ecology
Olfactometry
An olfactometer, identical in size and dimensions to the fourarm olfactometer described previously (Webster et al. 2010a)
but with two arms instead of four, was used to assess responses
of aphids to plant volatiles in the absence of other plant cues.
Plants used as odor sources were enclosed in polyethylene
terephthalate (PET) oven bags (35 × 43 cm; Toppits®, Klippan,
Sweden) that were sealed around the pots using a rubber
band. PTFE tubing (i.d. 3 mm) was inserted under the rubber
bands and connected to one of the olfactometer arms. Air was
drawn out of the olfactometer through a hole in the top at a
rate of 200 mL min−1, drawing in air through both of the olfactometer arms. Charcoal-filtered air was pumped into the PET
bags containing the plants at 250 mL min−1, creating a slight
positive pressure to ensure that air from the laboratory did not
enter the system.
At the start of the experiment, a single aphid was placed
in the olfactometer and allowed 10 min to acclimatize, after
which its location (left arm or right arm) was recorded every
2 min for 20 min. Between 26 and 36 replicates were completed for each experiment, depending on availability of
aphids. If an aphid remained motionless for the duration of
the experiment, it was removed from the analysis. Two experiments were carried out. In the first, aphids were offered a
choice between volatiles from a bean plant and volatiles from
a mustard plant. In the second experiment, aphids were
offered a choice between volatiles from either a bean or mustard plant and volatiles from a pot of soil used as a control.
Aphids were used only once, with new aphids being used in
each replicate.
Number of recordings in one arm was converted to proportion of total number of recordings in both arms. The analysis used generalized linear models with a logit link (Olsson
2002), and the model studied the effects of experience on
arm choice. Least significant means post-hoc analysis permitted pair-wise comparisons between treatments and tests of
the hypothesis of equal probability for selecting either arm
(P = 0.5) for the different experiences. An alpha level of
P < 0.05 was used to determine if differences were statistically
significant.
Results
Arena bioassay
The results show that experience of volatiles during development has a significant effect on host preference in adult
A. fabae when offered the choice between two leaves. Dietreared aphids with experience of mustard volatiles showed a
strong tendency to settle on mustard leaves compared with
diet-reared aphids exposed to bean volatiles, which showed
a slight tendency to settle on bean leaves. Aphids with no
prior experience of plant volatiles showed an intermediate
distribution.
Figure 1 shows number of aphids recorded on bean and
mustard leaves at different times and from different rearing
experiences. Statistical analysis revealed that there was a
significant effect of rearing experience on distribution of
aphids between the two leaves (F3,15 = 4.60, P = 0.0178).
Post-hoc least squares means analysis showed distribution
of diet-reared aphids with experience of bean volatiles and
diet-reared aphids with experience of mustard volatiles were
significant at 0.5 h (t80 = 2.41, P = 0.0184), 1 h (t80 = 2.37,
P = 0.0203), and 3 h (t80 = 2.00, P = 0.0488), with no significant
differences at 2 h (t80 = 1.86, P = 0.0662) or at 24 h (t80 = 1.54,
P = 0.1271). Diet-reared aphids with no prior experience of
plant volatiles (blank) did not show significantly different
Webster et al. • Innate and learnt responses to odors
369
Figure 1 Responses of aphids to mustard and bean leaves in an arena bioassay. A positive value indicates more aphids were present on the bean leaf;
a negative value indicates more aphids were present on the mustard leaf. Blank: diet-reared aphids reared in odorless conditions; bean: dietreared aphids exposed to bean volatiles; mustard: diet-reared aphids exposed to mustard volatiles; control: aphids reared on bean plants.
Different letters in the same time period indicate significantly different distributions (P < 0.05).
Olfactometry
Diet-reared aphids with experience of bean volatiles showed
a significant preference for bean volatiles (t115 = 2.26,
P = 0.0254), but did not show any preference for mustard
volatiles (t120 = −0.51, P = 0.6096). Similar preferences were
shown for diet-reared aphids exposed to an empty rearing chamber (bean/soil: t115 = 2.48, P = 0.0146; mustard/
soil: t120 = −0.24, P = 0.8074), and also for aphids reared on
bean plants (bean/soil: t115 = 3.83, P = 0.0002; mustard/
soil: t120 = 1.43, P = 0.1567). Diet-reared aphids exposed
to mustard volatiles showed a preference for both bean
volatiles and mustard volatiles, but neither of these were
significant (bean/soil: t31 = 1.637, P = 0.11; mustard/soil:
t30 = 1.4, P = 0.172). No significant differences were found
between different rearing experiences in their responses
to either bean or mustard volatiles (bean/soil: F3,115 = 1.03,
P = 0.3812; mustard/soil: F3,120 = 1.09, P = 0.357), suggesting that all aphids responded in a similar way to both
test odors.
The olfactometer results suggested that experience of plant
odors did not have an effect on behavioral responses to plant
volatiles alone. In the first experiment, aphids with different
rearing experiences were offered the choice between volatiles from a mustard plant and volatiles from a bean plant
(Figure 2). A significant preference for bean volatiles was displayed by diet-reared aphids with experience of bean volatiles
(t132 = 2.24, P = 0.0267), diet-reared aphids with experience of
mustard volatiles (t132 = 2.98, P = 0.0034), and aphids reared
on bean plants (t132 = 2.88, P = 0.0046). Diet-reared aphids
exposed to an empty rearing chamber (blank) showed no
preference for either odor source (t132 = −1.10, P = 0.2726).
Differences between rearing experiences were significant
(F3,132 = 3.61, P = 0.0152). Least squares means post-hoc
analysis revealed that aphids reared in the absence of plant
volatiles showed significantly different responses to each of
the other three rearing experiences (blank/bean volatiles:
t132 = 2.37, P = 0.0191; blank/mustard volatiles: t132 = −2.88,
P = 0.0047; blank/bean plants: t132 = −2.76, P = 0.0066). No
significant differences were found between other pairs of
treatments (P > 0.05).
In the second experiment, aphids were offered the
choice between volatiles from either a bean or a mustard
plant and volatiles from a pot of soil control (Figure 3).
Figure 2 Responses of aphids to volatiles from bean and mustard plants in
an olfactometer. A positive value indicates an overall preference
for bean odor; a negative value indicates an overall preference for
mustard odor. Significance *P < 0.05; **P < 0.01.
distributions to either of the other diet-reared aphids at any
time period (P > 0.05).
Plant-reared aphids (control) showed a tendency to distribute themselves on bean leaves, which increased over time.
This was significantly different to the distribution of dietreared aphids exposed to mustard volatiles at all time periods (0.5 h: t80 = 2.47, P = 0.0158; 1 h: t80 = 2.92, P = 0.0046;
2 h: t80 = 2.96, P = 0.004; 3 h: t80 = 3.51, P = 0.0007; 24 h:
t80 = 3.41, P = 0.001). No significant differences were found
between plant-reared aphids and diet-reared aphids exposed
to bean volatiles at any time period (P > 0.05) except at 24 h
(t80 = −2.22, P = 0.0295. Distribution of aphids exposed to an
empty rearing chamber was significantly different to those of
bean-reared aphids at 3 h (t80 = −2.17, P = 0.0327) and 24 h
(t80 = −2.43, P = 0.0173).
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Behavioral Ecology
Figure 3 Responses of aphids to volatiles from bean and mustard plants and a pot of soil control in an olfactometer. A positive value indicates an
overall preference for bean or mustard odor; a negative value indicates an overall preference for a pot of soil control. Significance *P < 0.05;
**P < 0.001.
Discussion
The results of the arena bioassay show that host selection
behavior in A. fabae can be modified by experience of volatiles acquired during development. This is the first study to
show evidence of learning of volatile chemical cues in aphids.
However, innate preferences for bean volatiles appeared
to dominate when not in close proximity to the plants, suggesting that learnt information is only utilized when familiar
plants are close by.
In the arena bioassay, diet-reared aphids with experience
of mustard volatiles showed a strong preference for mustard
leaves compared with aphids with previous experience of
bean volatiles. It is surprising that diet-reared aphids exposed
to bean volatiles did not show a strong preference for bean
leaves when compared with aphids reared on bean plants
(Figure 1). This result may be explained by considering the
responses of diet-reared aphids reared without any experience of plant volatiles, which showed a tendency to settle on
mustard leaves rather than bean leaves. This suggests that, in
the absence of previous experience of plant volatiles, dietreared A. fabae preferentially settle on mustard. Therefore,
experience of bean volatiles seems to have caused a shift in
preference away from mustard and toward bean compared
with the inexperienced diet-reared aphids.
The tendency for naive diet-reared aphids to settle on
mustard is surprising. One possible explanation is that the
mustard leaves offered a stronger visual stimulus to the dietreared aphids because aphids are known to respond to visual
cues (Doring and Chittka 2007). Although care was taken to
use leaves of similar size, the smallest available mustard leaves
still tended to be larger than the largest bean leaf. This means
that, in the arena bioassay, the mustard leaves tended to be
slightly larger and may have been more visually attractive.
Aphids reared on bean plants had previous experience
of visual cues associated with bean and so these may have
offered a stronger visual stimulus, despite the large size of
the mustard leaves. This raises the interesting possibility that
aphids can also learn from visual cues. Learning of such cues,
and the possible interaction with learning of olfactory cues
(Siddall and Marples 2008), warrants further investigation.
Despite the bias for diet-reared aphids to select mustard
leaves, the effect of rearing experience on host preference
is clear. Experience of either bean or mustard volatiles
caused a clear shift in preference toward the plants they had
experienced during development when compared with the
preference of aphids with no previous experience of plant
volatiles.
The effect of rearing experience diminished gradually over
time, with no significant differences found between diet treatments after 24 h. Olfaction plays an important role in the host
settling process, but final acceptance does not occur until after
sampling of the phloem, which generally does not take place
until after at least 1–2 h of sustained stylet penetration (Tosh
et al. 2003). The decline in effect of experience after 24 h is
probably because aphids had time to sample the phloem and
make a decision based on gustatory rather than olfactory cues.
It is known that host selection behavior in some aphids can
be altered by experience of gustatory cues (van Emden et al.
2009). Because none of the diet-reared aphids had previously
sampled phloem from either bean or mustard, this could
explain why aphids showed little preference for either plant
toward the end of the experiment. Aphids reared on bean
plants, on the other hand, had prior experience of gustatory
cues associated with bean, which could explain their strong
preference for bean toward the end of the experiment.
In olfactometer bioassays, aphids showed a preference for
bean volatiles over mustard volatiles regardless of whether
they had been previously exposed to mustard or bean volatiles
during development. When offered the choice between bean
volatiles and volatiles from a pot of soil control, aphids spent
more time in the olfactometer arm exposed to bean volatiles.
For diet-reared aphids with experience of mustard volatiles,
however, this preference was not significant (Figure 3). This
could suggest that experience of mustard volatiles switched
off the aphids’ responses to bean odor. Post-hoc analysis, however, failed to support this hypothesis as no significant differences in response to bean odor were found between the
different aphid treatments, suggesting they all responded in
a similar way. Although it is possible that experience exerted
some influence, this was clearly less pronounced than when
observed in the arena bioassay, where experience of mustard
odor caused a strong and significant shift in host preference.
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Webster et al. • Innate and learnt responses to odors
There is an interesting divergence between the results of
the arena bioassay and olfactometer tests, with experience
shown to have a strong effect on behavior in the former,
where the perceived cost of habitat assessment was low, but little effect in the latter, where the perceived cost was unknown
and, therefore, higher risk. This provides insight into the
different roles of innate and learnt behavioral responses to
plant volatiles. Information gained from the natal plant can
provide significant advantages in the expansion of host range
and continued exploitation of novel host plants, but the
information may be less reliable due to between-plant variability in volatile emission. Innate behavioral responses are
likely to be more reliable but do not facilitate in the expansion of host range and continued exploitation of novel host
plants in the same way that learnt information does. The use
of learnt and innate behavioral responses is, therefore, a balance of risk and reward based on the perceived cost of habitat
exploration and the reliability of learnt information. This is
an idea that has received little attention to date, but it is supported by the results of this study. This may be a widespread
trait among animals, particularly among those for which habitat exploration is costly, and warrants further investigation in
other species.
This study also highlights the fact that care should be taken
when designing experiments to assess learning behavior
to avoid drawing hasty conclusions. Failure to identify such
behavior could be the result of failing to test responses in
the proper ecological context. Conversely, too much emphasis may be placed on the importance of learning if responses
are only tested in experimental designs that simulate low-cost
habitat exploration.
An unexpected finding was that diet-reared aphids
exposed to an empty odor chamber did not show a preference for bean volatiles over mustard volatiles. This could
suggest an impaired olfactory sense and could be a parallel
to recent work showing a similar effect in Drosophila melanogaster (Hare 2011). Fruit flies reared on a synthetic medium
relatively devoid of odors showed differences in olfactory
receptor neuron firing patterns compared with flies reared
on the same medium supplemented with odorants, indicating a reduction in sensitivity and acuity in the odor-deprived
flies. This study may demonstrate a similar effect, showing
how lack of stimulation can impair behavioral responses to
odors. However, aphids reared in odorless conditions were
still able to respond to bean odor when offered the choice
between bean volatiles and a pot of soil control. It could be
that the olfactory sense was only partly diminished so that
when offered the relatively simpler choice between bean volatiles and soil, aphids were able to discriminate between the
two. Volatile blends emitted by bean and mustard likely share
similarities because many plant volatiles are found almost universally throughout the plant kingdom and different species
often emit similar blends, with key differences being in the
quantities and ratios of volatiles rather than volatiles specific
to either plant (Bruce et al. 2005). The task of discriminating
between the two blends could, therefore, be difficult for an
aphid with an even somewhat-impaired olfactory sense. The
effect of sensory deprivation on behavioral responses to odors
is interesting and warrants further investigation.
In conclusion, learning of olfactory cues was demonstrated for the first time in a species of aphid. It was shown
that learnt information plays a prominent role when in close
proximity to the plant, but innate behavior appears to dominate when cues indicating the precise location of the odor
source are unavailable and so the risk of habitat exploration
is higher. The ability to weigh the risks and rewards of using
learnt information at different stages of the host location process could confer selective advantages, particularly for insects
for which habitat exploration is costly. This trait may be widespread among other animal species and warrants further
investigation.
Funding
This work was supported by Carl Tryggers Stiftelse and by
Mistra through the PlantComMistra program.
We would like to thank Prof Jan Pettersson for his helpful comments
on the manuscript. We would also like to thank Prof Helmut van
Emden and Ms Elizabeth Wild for their advice on the aphid diet/
rearing system.
References
Alyokhin A, Sewell G. 2003. On-soil movement and plant colonization by walking wingless morphs of three aphid species
(Homoptera:Aphididae) in greenhouse arenas. Environ Entomol.
32:1393–1398.
Bruce TJ, Wadhams LJ, Woodcock CM. 2005. Insect host location: a
volatile situation. Trends Plant Sci. 10:269–274.
Davis JM, Stamps JA. 2004. The effect of natal experience on habitat
preferences. Trends Ecol Evol. 19:411–416.
Degen T, Dillmann C, Marion-Poll F, Turlings TC. 2004. High genetic
variability of herbivore-induced volatile emission within a broad
range of maize inbred lines. Plant Physiol. 135:1928–1938.
Doring TF, Chittka L. 2007. Visual ecology of aphids-a critical review on
the role of colours in host finding. Arthropod-Plant Interact. 1:3–16.
Douglas AE. 1997. Provenance, experience and plant utilisation by
the polyphagous aphid, Aphis fabae. Entomol Exp Appl. 83:161–170.
Dukas R. 2008. Evolutionary biology of insect learning. Annu Rev
Entomol. 53:145–160.
van Emden H. 2009. Artificial diet for aphids - thirty years’ experience. Redia. 92:163–167.
van Emden H, Kilbane J, Pettersson J. 2009. Changing the host selection responses of aphids through a short experience of a novel secondary plant compound. Redia. 92:183–186.
Gorur G, Lomonaco C, Mackenzie A. 2005. Phenotypic plasticity in
host-plant specialisation in Aphis fabae. Ecol Entomol. 30:657–664.
Gorur G, Lomonaco C, Mackenzie A. 2007. Phenotypic plasticity in host choice behavior in the black bean aphid, Aphis fabae
(Homoptera: Aphididae). Arthropod-Plant Interact. 1:187–194.
Gouinguené SP, Turlings TC. 2002. The effects of abiotic factors on induced volatile emissions in corn plants. Plant Physiol.
129:1296–1307.
Hare JD. 2011. Ecological role of volatiles produced by plants in
response to damage by herbivorous insects. Annu Rev Entomol.
56:161–180.
Hodgson C. 1991. Dispersal of apterous aphids (Homoptera,
Aphididae) from their host plant and its significance. B Entomol
Res. 81:417–427.
Littell R, Milliken G, Stroup W, Wolfinger R, Schabenberger O. 2006.
SAS for mixed models. 2nd ed. Cary (NC): SAS Institute Inc.
Nottingham SF, Hardie J, Dawson GW, Hick AJ, Pickett JA, Wadhams
LJ, Woodcock CM. 1991. Behavioral and electrophysiological
responses of aphids to host and nonhost plant volatiles. J Chem
Ecol. 17:1231–1242.
Olsson U. 2002. Generalized linear models: an applied approach.
Lund (Sweden): Studentlitteratur.
Olsson U. 2011. Statistics for Life Science 2. Lund (Sweden):
Studentlitteratur.
Powell G, Hardie J. 2001. The chemical ecology of aphid host alternation: how do return migrants find the primary host plant? Appl
Entomol Zool. 36:259–267.
Siddall EC, Marples NM. 2008. Better to be bimodal: the interaction of color and odor on learning and memory. Behav Ecol.
19:425–432.
Stamps JA, Davis JM. 2006. Adaptive effects of natal experience on
habitat selection by dispersers. Anim Behav. 72:1279–1289.
Tosh CR, Morgan D, Walters KFA, Douglas AE. 2004. The significance of overlapping plant range to a putative adaptive trade-off in
the black bean aphid Aphis fabae Scop. Ecol Entomol. 29:488–497.
372
Tosh CR, Powell G, Hardie J. 2003. Decision making by generalist and specialist aphids with the same genotype. J Insect Physiol.
49:659–669.
Webster B. 2012. The role of olfaction in aphid host location. Physiol
Entomol. 37:10–18.
Webster B, Bruce T, Dufour S, Birkemeyer C, Birkett M, Hardie J,
Pickett J. 2008. Identification of volatile compounds used in
host location by the black bean aphid, Aphis fabae. J Chem Ecol.
34:1153–1161.
Behavioral Ecology
Webster B, Bruce T, Pickett J, Hardie J. 2010a. Volatiles functioning as
host cues in a blend become nonhost cues when presented alone
to the black bean aphid. Anim Behav. 79:451–457.
Webster B, Gezan S, Bruce T, Hardie J, Pickett J. 2010b. Between
plant and diurnal variation in quantities and ratios of volatile compounds emitted by Vicia faba plants. Phytochemistry.
71:81–89.
Wiktelius S. 1989. Migration of apterous Rhopalosiphum padi. WPRS
Bulletin. 12:1–6.