THE IMPORTANCE OF PLANT ALLELOCHEMICALS IN HOST LOCATION BEHAVIOR OF PARASITOID
INSECTS
Mohammed Al-doghairi
Spring 1994
ABSTRACT
Parasitoid host location has received some attention, due to its
importance in the control of pest insects. There are many strategies
that parasitoids use to locate their potential host most efficiently,
and this depend on the type of semiochemicals provided by the host or
its environment.
Semiochemicals are chemicals that mediate interactions between
organisms. They are divided into pheromones and allelochemicals.
Pheromones mediate in intraspecific interactions and allelochemicals
mediate in interspecific interactions. Allelochemicals are of most
relevance to use with parasitoids, and serve important roles in the
host-searching sequence. The composition of the allelochemical signals
often differs with different plants. Parasitoids learn to respond to
the different blends of chemicals that indicate the location of their
host(s). Female leptopilina heteroma, for example, learn to respond to
the odor of the microhabitat infested by its larval Drosophila host.
Injury inflected by herbivores can induce the production of specific
chemicals in various plants. These chemicals attract parasitoids of
herbivores. Regurgitate of corn-fed beet armyworm caterpillars and other
species of caterpillars induced the release of terpenes in corn
seedlings that attract generalists and specialist parasitoids. Green
leaf volatiles (GLV) strongly attract female Microplitis croceipes wasp.
GLVs occur in nature when a caterpillar feed on a green leaf. So, GLVs
may be important cues, enabling M. crociepes to locate their host(s).
The relative role of allelochemicals originating from the plants are
reviewed with respect to host location behavior by parasitoid insects.
INTRODUCTION
Parasitoids represent a very diverse group of organisms that are best
represented in the class Insecta. There are over 100,000 species of
parasitic Hymenoptera alone (Vinson, 1984). Parasitoids attack a
variable number of host species with very different biological affords
and opportunity for the evolution of very diverse behavioral responses
and strategies for location of hosts (Vinson, 1984; Vet, 1990a;
Tumlinson et al., 1993). Female parasitoids use a wealth of chemical
information, in a complex non-random manner, when searching for hosts
(van Alphen et al., 1986; Tumlinson et al., 1993). Although, they
respond to a hierarchy of physical and/or chemical stimuli which lead
them to their potential host (van Alphen et al., 1986; Vet, 1990ab;
Lewis et al., 1991; Udayagiri et al., 1992b; Tumlinson et al., 1993).
There are many strategies that parasitoids use to locate their potential
host most efficiently, and this depend on the types of cues, or
"semiochemicals", provided by the host or its environment (Vinson, 1984;
Lewis et al., 1990; Vet, 1990ab; Tumlinson et al., 1993)
Semiochemicals are chemicals that convey interactions between organisms
(Noldus, 1989; Lewis et al., 1990; Dent, 1991; Tumlinson et al., 1993).
They are divided into two major groups, pheromones, which mediate in
intraspecific interaction and allelochemicals for interspecific
interactions. Each of these groups can be subdivided further,
allelochemicals into allomones, kairomones, synomones and apneumones
depending on weather the releaser, receiver or both benefit from the
interaction (Table 1) and pheromones into sex pheromones, alarm
pheromones and epideitic or aggregation pheromones (Noldus, 1989; Dent,
1991). Allelochemicals are of most relevance to use with parasitoids,
and serve important roles in the host-searching sequence (Lewis et al.,
1990; Papaj et al., 1990; Vet et al., 1990ab; Whitman, 1992; Tumlinson
et al., 1993; Turlings et al.,1993). The composition of the
allelochemical signals often differs with different plants (Steinberg et
al., 1993). Nevertheless, parasitoids learn to respond to the different
blends of these chemicals that indicate the location of their hosts (van
Alphen et al., 1986; Noldus, 1989; Papaj et al., 1990; Vet et al.,
1990ab; Dent, 1991; Turlings et al., 1991; Tumlinson et al., 1993).
Table 1. The different types of semiochemicals involved in insect
communication (Dent., 1991)
This report will review the relative role of allelochemicals originating
from the plants with respect to host location behavior by parasitoid
insects. We are not going to review all the literature on the subject
of host-foraging allelochemicals. Rather, our intent is to discuss
briefly recent studies on plant allelochemicals used in host location
behavior of parasitoid insects.
PARASITOIDS AND SUCCESSFUL PARASITOIDISM
Parasitoids insects are those kind of insects that have carnivorous
relationships with their hosts. They produce only one generation per
host, only the immatures are parasitic, while the adults are free
living. Immatures are solitary, each parasitoid needs the entire host
to meet its nutritional needs. As a result of this, the parasitoid grub
consumes the host larva completely and eventually kills it.
Successful parasitoidism depends on five factors. Vinson (1984) divided
these factors into (a)host habitat location, (b)host location, (c)host
acceptance, (d)host suitability and (e)host regulation. Vinson, also,
subdivide the factors a-c which make up the host selection process, into
(1)habitat preference, (2)potential host community location, (3)host
location, (4)host examination, (5)ovipositor probing, (6)ovipositor
drilling and (7)oviposition (Fig. 1).
ROLE OF ALLELOCHEMICALS IN HOST LOCATION BEHAVIOR OF PARASITOID INSECTS
Parasitoid host location consists of a series of behaviors that are
effected by information about the surroundings. Chemical stimuli are
among the main information-conveying agents available to parasitoids
(van Alphen, 1985; Vet et al., 1990b; Steinberg et al., 1993; Tumlinson
et al., 1993). They play an essential role as cues in almost all stages
of host searching and host selection (Vinson, 1984; Lewis et al., 1990;
Whitman, 1992; Tumlinson et al., 1993; Turlings et al.,1993).
The host selection process involves a series of chemical stimuli, known
as allelochemicals, to which the female parasitoid responds (Vinson,
1984; Lewis et al., 1990; Vet, 1990; Udayagiri et al., 1992b; Steinberg
et al., 1993; Tumlinson, 1993). Female parasitoid is required to search
for hosts in different plant, or in a varied plant habitat. Tehrefore,
a variety of allelochemicals will be produced, and their nature and
reliability will vary with distance from the host (Fig.2) (Lewis et al.,
1990; Vet et al., 1990a; Steinberg et al., 1993; Tumlinson et al.,
1993).
Thus, allelochemicals that reach a great distance may only convey the
information that a habitat is likely to contain suitable hosts. As the
parasitoid gets close, different allelochemicals that convey information
on the availability and location of the host will be emanated from its
host, host activities, or from host plant (Lewis et al., 1990; Vet et
al., 1990a; Udayagiri et al., 1992b; Tumlinson et al., 1993).
ORIGIN OF ALLELOCHEMICALS INVOLVED IN HOST LOCATION BEHAVIOR OF
PARASITOID INSECTS
The source of allelochemicals used by parasitoids may be expected to
vary, under the condition that the chemicals involved increase the rate
of host finding (Lewis et al., 1990; Vet et al., 1990a; Steinberg et
al., 1993; Tumlinson et al., 1993) In fact, foraging parasitoids
exploit allelochemicals from their host, host activities, or from the
host plant. and or other organisms, such as microorganisms, that are
associated with the host or its habitat, may be also involved in this
(Lewis et al., 1990; Vet et al., 1990a; Turlings et al., 1991; Whitman
et al., 1992; Steinberg et al., 1993; Tumlinson et al., 1993 ).
In this report, we will be focusing on plant allelochemicals that
mediate mutually beneficial interactions, and play an essential role as
cues in host location behavior of parasitoid insects.
PLANT ALLELOCHEMICALS AFFECTING HOST LOCATION BEHAVIOR OF PARASITOID
INSECTS
Parasitoids are hypothesized to be faced with a reliability-
deductibility difficulty (Steinberg et al., 1993). To cover this
problem, parasitoids use herbivores-induced synomones which are
chemicals that are produced by plants in response to feeding damage by
herbivores, and which attract parasitoids (Udayagiri et al., 1992ab;
Steinberg et al., 1993; Tumlinson et al., 1993).
Many parasitoids are known to discriminate, and respond to volatile
chemicals produced by uninfested plants and plants infested with a
particular herbivores species (Steinberg et al., 1993; Tumlinson et al.,
1993). Once the herbivore feeds on a plant, plant starts producing
large amount of volatile chemicals which are used as cues leading
females of parasitoid to the microhabitat of its hosts (Turlings et al.,
1991; Tumlinson et al., 1993).
Using single and dual choice tests in a flight tunnel, Turlings et al.
(1991a) showed that beet armyworm-damaged corn seedlings are the primary
source of volatile allelochemicals that attract females of Cotesia
marginiventris (Fig. 3). They also reported that C. marginiventris
females fly to and land on undamaged plants, but any minor damage would
increase responsiveness and parasitoids would fly directly to the
damaged sites. They concluded that plants are the principal source of
volatile synomones that not only direct parasitoids to the host patch,
but also to get them into the direct location of the host. Turlings et
al. (1991b) isolated and identified those allelochemicals that attract
C. marginiventris to the microhabitat of its host. The analysis of
these compounds revealed the consistent presence of 11 compounds in
significant amounts. These compound are (Z)-3-hexenal, (E)-2-hexenal,
(Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate, linalool, (3E)-4,8-dimethyl-
1,3,7-nonatriene, indol, à-trans-bergamotene, (E)- - farnesene, (E)-
nerolidol, and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. They
reported that most of allelochemicals mentioned above were released by
damaged leaves, and no detectable amount of these allelochemicals were
released by frass or larvae (Fig. 4). They concluded that damaged plants
are significantly more attractive to parasitoid wasps than frass or
larvae.
Results similar to Turlings finding were obtained by Steinberg and
coworkers (1993), who found that caterpillar-infested cabbages initiate
the release of volatile allelochemicals that play important role in
long-rang host location by Cotesia glomerata. Their findings showed
that cabbage plants are involved in the production of allelochemicals
that are emitted by Pieris- damaged plants. They also reported that
herbivore-infested plants remain attractive to C. glomerata for at least
several hours after the removal of the larvae. Their conclusion was
that allelochemicals from the second trophic level (Pieris brassicae)
seem to be less important in long-range host location by C. glomerata
than allelochemicals from the first trophic level (cabbage).
Using electroantennogram bioassay, Ramachandran and Norris (1991) found
that the antenna of parasitoid, Microplitis demolitor, is much more
sensitive to several plant volatiles than that of herbivore,
Pseudoplusia includens. They tested 5-12-carbon aliphatic compounds of
several chemical classes, and found that sensitivity of the parasitoid
antenna increased when exposed to 7-carbon compounds. They concluded
that 7-carbon compound is more important in parasitoid host-location
behavior.
Similarly, a study by Whitman and Eller (1992) tested eight different
green leaf volatile (GLV) substances in a wind tunnel to test the
ability of female Microplitis croceipes wasps to orient to different
concentrations of the eight compounds. Those eight compound tested were
hexanal, (E)-2-hexenal, (E)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (E)-2-
hexenyl acetate, (Z)-3-hexenyl acetate, (Z)-3-hexenyl propionate, and
(Z)-3-hexenyl butyrate. They found that individual GLVs varied in their
attractiveness to M.Croceipes. Eight- to 10-carbon esters were the most
attractive. They reported also that M.Croceipes wasps can orient to
extremely low doses of certain GLVs.
An elicitor in the regurgitate of herbivorous insects may induce plants
to emit volatile chemicals attractive to parasitic wasps (Tumlinson et
al., 1993; Turlings et al., 1993). Turlings et al. (1993) reported
behavioral and chemical data for a tritrophic system of the generalist
parasitoid Cotesia marginiventris and the specialized parasitoid
Microplitis croceipes, beet armyworm (BAW) caterpillars, and corn
plants. Their findings indicated that the elicitor active components
are present in relatively large concentrations in regurgitate of BAW and
that they are not related to the food source (Fig. 5). They tested,
also, regurgitate of several other species of caterpillars (Spodoptera
frugiperda, Helicoverpa zea,Trichoplusia ni, and Anticarsia gemmatalis)
and as well as of the grasshopper Schistocerca amaricana. Their results
indicated that regurgitate of those different herbivorous insects
induced the release of significant amount of terpenes in corn seedlings.
They concluded that the release of these volatiles is a general response
to attack by herbivorous insects, and is attractive to the generalist
parasitoid C. marginiventris and to the specialized parasitoid M.
croceipes.
Several studies reported that plant response to attacking herbivores,
however, is systematic and that the volatile emitted are not limited to
damaged sites (Dicke et al., 1990; Turlings et al., 1991; Tumlinson et
al., 1993; Turlings et al., 1993). This can be explained by Turlings
and Tumlinson (1991) findings. They reported that plants that were
induced to release volatile chemicals, mainly terpenoids, that are known
to be attractive to the parasitoid became less palatable to the
herbivore beet armyworm. They argued ,therefore, that induced production
of plant allelochemicals evolved first as a direct defense against
herbivorous insects, and that the attractive function probably evolved
secondarily.
CONCLUSION
Parasitoids reproductivity and survival depends, mainly, on their
ability to locate their potential host(s). From the above findings, we
can argue, therefore, that plants are clearly the main source of
volatile allelochemicals which are very important in host location
behavior of parasitoid insects. Those chemicals not only guide
parasitoids into areas that may harbor hosts, but serve to get them into
the direct location of the host. We, also, argue that those chemicals
should be regarded as plant-to-parasitoid synomones, because we have
shown that plants benefit when parasitoids, that respond to those
chemicals, destroy the attaching herbivores. Information on the active
production of these synomones, and on the chemicals basis of a
parasitoid's attraction to plant synomones should be demonstrated, and
manipulation of parasitoid host location, therefore, may be used to
increase its efficiency in the field.
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