Oviposition Behavior

Insects deposit their eggs in a variety of ways. Most commonly, the female is solely responsible for selection of oviposition site(s). The behaviors leading to oviposition are as complex as those leading to mating because successful ovipo-sition contributes to individual fitness and is under strong selective pressure.

A diversity of stimuli affects choice of oviposition sites by female insects. Mosquitoes are attracted to water by the presence of vegetation and reflected light, but they lay eggs only if salt content, pH, or other factors sensed through tarsal sensillae are suitable (Matthews and Matthews 1978). Grasshoppers assess the texture, salinity, and moisture of soil selected for oviposition.

Many phytophagous insects assess host suitability for development of offspring. This assessment may be on the basis of host chemistry or existing feeding pressure. Ovipositing insects tend to avoid host materials with deleterious levels of secondary chemicals. They also may avoid ovipositing on resources that are already occupied by eggs or competitors. For example, female bean weevils, Callosobruchus maculatus, assess each potential host bean by comparison to the previous bean and lay an egg only if the present bean is larger or has fewer eggs. The resulting pattern of oviposition nearly doubles larval survival compared to random oviposition (R. Mitchell 1975). Many parasitic wasps mark hosts in which eggs have been deposited and avoid ovipositing in marked hosts, thereby minimizing larval competition within a host (Godfray 1994). Parasitic wasps can minimize hyperparasitism by not ovipositing in more than one host in an aggregation. This reduces the risk that all of its offspring are found and parasitized (Bell 1990). Cannibalistic species, such as Heliconius butterflies, may avoid laying eggs near each other to minimize cannibalism and predation.

Selection also determines whether insects lay all their eggs during one period (semelparity) or produce eggs over more protracted periods (iteroparity). Most insects with short life cycles (e.g., <1 year) usually have relatively short adult life spans and lay all their eggs in a relatively brief period. Insects with longer life spans, especially social insects, reproduce continually for many years.

Some insects influence host suitability for their offspring. For example, female sawflies usually sever the resin ducts at the base of a conifer needle prior to laying eggs in slits cut distally to the severed ducts. This behavior prevents or reduces egg mortality resulting from resin flow into the oviposition slits (McCullough and Wagner 1993). Parasitic Hymenoptera often inject mutualistic viruses into the host along with their eggs. The virus inhibits cellular encapsulation of the egg or larva by the host (Tanada and Kaya 1993).

In other cases, choices of oviposition sites by adults clearly conflict with suitability of resources for offspring. Kogan (1975) and Courtney (1985,1986) reported that some species preferentially oviposit on the most conspicuous (apparent) host species that are relatively unsuitable for larval development (see Fig. 3.10). However, this behavior represents a tradeoff between the prohibitive search time required to find the most suitable hosts and the reduced larval survival on the more easily discovered hosts.

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