(Chemical signaling and food for adult parasitoids)
The role of plants in the multitrophic system in which parasitoids forage has previously been greatly underestimated. Although parasitoids can detect semiochemicals from undamaged plants and use these cues to locate the most likely habitat of their hosts, there are distinct advantages to the parasitoid to be able to detect, differentiate and respond to semiochemicals that distinguish plants damaged by their hosts from the surrounding environment. Studies show that plant chemicals induced by the host's feeding and by-products of the host derived from the plant are vital to the parasitoid's host-finding and assessment of the profitability of the habitat ( #104 Lewis and Martin, 1990; # 98 Turlings et al., 1990). Substance(s) in the oral secretions of lepidopterous larvae and other insects were found to act on wounded plant tissue to induce the plants to release the volatile signals that attract the parasitoids (Turlings et al., 1993; Dicke et al., 1993). The elicitor from the oral secretions of beet armyworms, Spodoptera exigua, has been isolated, identified, and synthesized (Alborn et al., 1997). This compound N(17-hydroxylinolenoyl) (L)-glutamine, when applied to corn seedlings induces biosynthesis and release of a volatile blend identical to that induced by beet armyworm feeding.
Furthermore, not only the damaged leaves but also undamaged leaves of a plant injured by larval feeding produce and release volatiles (Rose et al., 1996, #60 Cortesero et al., 1997). These volatiles induced by herbivore feeding are synthesized de novo by the plant just prior to release (Pare and Tumlinson, 1977a, b). However, different blends of semiochemicals are released when the same species of larvae attack different plant species or different growth stages or parts of a plant (leaves, flowers, fruit) (#81 Turlings et al., 1993a, b). In fact, there are even differences in amounts and proportions of volatile compounds released from different cultivars or varieties of the same plant species (Loughrin et al., 1995a). Perhaps, most surprisingly, we found that a plant can release different volatile blends in response to feeding by different species, even closely related species, of herbivores. Thus, cotton, corn, and tobacco plants each produce a different blend of volatiles when fed on by Heliothis virescens than when fed on by H. zea. Further, the specialist parasitoid, Cardiochiles nigriceps can use these differences to distinguish its host, H. virescens from the non-host, H. zea (DeMoraes et al., in press).
The image below shows the searching cues that are used by parasitoids in their search for hosts. The plant induced volatiles are released not only locally by the damaged leaf but also systemically by the rest of the plant. These chemicals are released in large quantities which makes detection of pests in the field easy for parasitoids. For parasitoids in particular systemically released volatiles likely serve as long range cues for the location of potential hosts (#60 Cortesero et. al. 1997).
* General plant volatiles (i.e., compounds emitted without damage)
* Green leafy volatiles (i.e., constitutive terpenes released upon mechanical damage)
* Insect related volatiles (i.e., insect or plant products directly associated with herbivore presence)
* Plant induced volatiles (i.e., inducible terpenoids emitted as a delayed response to feeding damage)
Improper fertility or other agronomic practices can limit the plants ability to provide the crucial foraging cues for the parasitoids. For example, too high a nitrogen level and/or water stress can disrupt the plants ability to emit adequate herbivore-induced volatile chemical cues (Cortesero et al., unpublished data).
Adult parasitoids must not only find hosts for reproductive purposes but also locate food to meet their short-term nutritional needs. A knowledge of how parasitoid females deal with the often competing needs for these two vital resources is essential for understanding their foraging strategies (#51 Lewis et al., 1998). Behavioral studies (#68 Takasu and Lewis, 1995; #53 Stapel et al., 1997) have provided important information on time allocation toward foraging for hosts and food and have identified some factors affecting the efficiency in finding hosts. If food was available in a plant patch, hungry females remained longer and parasitized more hosts than if food were not available. Furthermore, parasitoids appeared to be more focused on foraging for hosts as significantly more time was allocated to host damaged leaves. Parasitoid retention and parasitization in plant patches without food was significantly reduced due to their unsuccessful foraging for food.
The detectability of a food source determines its likelihood of use by parasitoids. The plant patch experiments conducted by #53 Stapel et al. (1997) also revealed that extrafloral nectar is much more detectable to M. croceipes females than sucrose and honeydew. Ninety percent of the released parasitoids were able to locate nectar versus only 45 and 40% detection of sucrose and honeydew sites, respectively.
M. croceipes parasitoid feeding from an extrafloral nectary on a cotton leaf.
Extrafloral nectaries on cotton are situated on the midribs under the leaves
and at the base of squares.
Differential detectablility of suitable food sources was also found in C. rubecula. With olfactometer experiments, Wackers and Swaans (1993) found that this parasitoid does not respond to aphid honeydew volatiles but it is strongly attracted to floral nectar volatiles. Factors influencing the detectability of food sources by parasitoids not only include volatile signals but also visual stimuli (Wackers, 1994).
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