Foraging Behavior of Parasitoids
(The role of chemical & visual cues and learning)
The concept that the host-selection process involves responses to a composite of stimuli at several spatial scales has been addressed by a number of authors (Doutt, 1959; Flanders, 1953; Jones, 1985; #176 Lewis et al., 1975; Salt, 1935; Vinson, 1975), and the important role of chemical cues and more recently visual cues are documented (Greany et al., 1984; #145 Lewis et al., 1982; #90 Tumlinson et al., 1992; Vinson, 1976, 1981; Weseloh, 1981). Thus, the searching parasitoid will encounter a variety of cues, most of which are indirect, that vary in nature and reliability with the distance from hosts. At great distance the chemical cues may convey only the information that a habitat is available and is likely to contain suitable hosts. As the parasitoid gets closer to the host, different semiochemicals from damaged plants, feces, or other host by-products give a much more direct and reliable indication of the availability and location of the host. In fact, the searching parasitoid utilizes a dynamic continuum of semiochemicals, as well as visual cues, to locate and exploit her hosts (#102 Lewis et al., 1990; #89 Tumlinson et al., 1992; Vet et al., 1991).
Cardioceles nigriceps searching for her host on beggar weed.
Visual cues also play a very important role in foraging for host resources by parasitoids. Colors, shapes, and patterns associated with hosts are all detected and used to increase foraging efficiency (Wackers and Lewis, 1995, Wackers and Lewis, in prep).
Cardioceles nigriceps approaching it's host Heliothis virescence
on a tobacco leaf..
Moreover, learning by the parasitoid serves a crucial role in their ability to exploit these chemical and visual cues. A number of researchers (e.g., Arthur, 1971; Vet and van Opzeeland, 1984; Vet, 1985; Strand and Vinson, 1982) demonstrated remarkable plasticity in the parasitoid foraging behavior and that "experienced" parasitoids have heightened responsiveness to certain cues. We demonstrated that females of M. croceipes have the ability to associatively learn and subsequently use novel cues associated with hosts (#114 Lewis and Tumlinson, 1988). M. croceipes females reared from hosts fed on artificial diet must encounter a host feeding on a plant, or feces of a plant-fed host before they will respond to the odors of hosts feeding on plants in olfactometer and flight tunnel bioassays (#127 Drost et al., 1986; #121 Eller et al., 1988a). However, females reared from hosts feeding on plants have less need for preflight experience in order to respond significantly to the same odors (#122 Drost et al., 1988). Further, we found that M. demolitor (a species from Australia) females exhibited a host-diet influence on responsiveness and this effect was due to host-associated chemical stimuli on the surface of the parasitoid cocoons that are encountered at the time of the wasp's eclosion (#118 Herard et al., 1988).
Netalia heroica ovipositing in a Heliothis species.
Host feces is the single most important component in providing the necessary preflight experience for flight tunnel responses by M. croceipes females (#127 Drost et al., 1986; #120 Eller et al., 1988b). Further, when females encounter feces of their host they associatively learn to recognize and subsequently fly to various volatile odors (conditioned stimuli) by linking them with a host specific water extractable nonvolatile chemical (unconditioned stimulus) present in the feces (#114 Lewis and Tumlinson, 1988). This conditioned use of tracking cues via their association with host by-products without the necessity for direct contact is a valuable foraging tactic for parasitoids and predators of cryptic and evasive hosts or prey. Experienced M. croceipes and C. marginiventris females have heightened responses to the plant-produced volatiles in response to herbivore feeding (#88 McCall et al., 1993).
Learned use of chemicals and visual cues are also important in the parasitoid's location of its adult food needs, usually nectar from the plant. Further, the parasitoid has a very sophisticated ability to associatively link, separately, the occurrence of arbitrary cues with either hosts or food and subsequently use the cues based on current needs (physiological state) of hosts versus food (#103 Lewis and Takasu, 1990). M. croceipes females were able to learn novel odors when they were exposed to those odors while feeding on sucrose solution then use the odors to choose between the resources depending on their physiological state. For example, hungry wasps were more attracted to the odor associated with food, whereas a fed wasp would choose the odor linked to hosts.
Parasitoids can learn combinations of chemical and visual cues to further enhance their foraging success (Wackers and Lewis, 1995).
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Wackers, F. L. and :Lewis, W. J. In preparation.
Weseloh, R.M. 1981. pp 79-95, in D. A. Nordlund, W. J. Lewis, and R. L. Jones (eds.). Semiochemicals: Their Role in Pest Control. John Wiley & Sons, New York.
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