Parasitoid host suitability

While sugar solutions can be a significant item in the diet of ants, parasitoids are often entirely dependent on carbohydrates as an adult food source (Jervis et al., 1993). The parasitoids longevity and fecundity are usually subject to energetic constraints (Leatemia etal., 1995 Stapel etal., 1997 Wackers, 2001), whereas the parasitoids behavior can also be strongly affected by their nutritional state (Wackers, 1994 Takasu and Lewis, 1995). There is strong evidence that the availability of suitable sugar sources can play a key role in parasitoid host dynamics (Krivan and Sirot, 1997 Wackers, 2003). 

Unlike parasitoids of other insect orders that have host-seeking larvae, most parasitic hymenoptera lay their eggs on, in, or very close to a host individual [11]. This requires the adult female to find a suitable host, often with the aid of chemical cues from host frass, pheromones, plant volatiles emitted upon host feeding or egg-deposition, silk, honeydew and other secretions. She may then chemically mark the host following oviposition to reduce superparasitism by herself or intra- and inter-specific insects [11]. 

Finally, density-dependent mortality from various enemies may be enhanced by host plant variation. Again, focusing feeding activities on a restricted set of suitable tissues should also focus the activities and abundance of pathogens, parasitoids, and predators. Sessile insects, such as gall-forming aphids (55,62),... 

Variability in induced plant volatiles complicates the reliance of natural enemies on these cues. One way of dealing with variability is through associative learning, which may allow parasitoids to learn which cues are most likely to lead them to suitable hosts at a particular time in a particular area. Moreover, recent studies suggest that plant volatile blends alone carry specific information on the herbivores by which they are attacked. For example, predatory mites can distinguish between the blends of apple trees infested by two herbivores species (20). Du et al. (21) showed that different aphid species elicit different volatile blends in bean plants and that the aphid parasitoid, Aphidius ervi, can use these differences to distinguish plants infested by its host. Aphis pisum from those infested by a nonhost. Aphis fabae... 

The suitability of arthropods as prey or as hosts of insect predators and parasitoids has been repeatedly shown to be affected by host plant chemistry. The mechanisms by which plant chemical factors mediate suitability of herbivores as hosts (prey) for their natural enemies fall into two broadly overlapping categories. The first involves host plant compounds ingested, and in some cases sequestered, by the herbivores that are toxic or distasteful to parasitoids or predators (4-6). Such compounds may be active as repellents or as acute or chronic toxins ( 5, 7-10). The second category involves the nutritional quality of the host plant mediating herbivore utilization by parasitoids and predators (11-14). Changes in herbivore size due to host plant effects, for example, have been associated with differences in size and sex ratio of parasitoids (13, 12), and differences in functional responses of parasitoids and predators (16). 

The nutritional suitability of a host was proposed by Flanders (1937) and Salt (1938) to be important to the survival of a parasitoid. House (1977) concluded... 

The suitability of a host depends on whether the parasitoid can evade or suppress the host s internal defense or immune mechanism(s). It has been suggested (Doutt, 1963 Salt, 1963) that by the careful placement of their progeny on the host (ectoparasitoids), within certain tissues (i.e., fat body, ganglion), or in certain stages (eggs) many species of parasitoids evade the host s defense. However, this assumption needs critical investigation. 

It thus appears that the suitability of a particular host stage and possibly different hosts may depend on the host s endocrine activity. However, the parasitoid also can have a profoimd effect on the host s endocrine system. 

Host recognition by parasitic Hymenoptera also can be a complex process involving a series of behaviors after initial host contact that must be completed in sequence before an egg is laid. One such sequence is illustrated in Figure 5.7, which diagrams the processes by which Aphytis melinus, a parasitoid used for biological control of California red scale, Aonidiella aurantii, first identifies a potential scale insect species as a potentially suitable host, then assesses the individual quality of that host for utilization (Luck et al. 1982). Egg deposition is the final event of a chain of other behaviors that must be completed in sequence. The series can be broken in several placed and restarted, but no steps can be skipped. 

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