Insect behavior is sometimes perceived as a rigid, instinctive, inherited phenomenon not subject to change. However, there is ample evidence that many insects vary their behavior depending on circumstances, and that often learning is involved. This is also true for parasitoids. Many female parasitoids respond to host stimuli more strongly after they have parasitized a host. The heightened response, which may take the form of faster host finding and/or more intensive searching, may be considered to be a form of reward conditioning in that the female responds more avidly to host stimuli once she has been "rewarded" by being able to oviposit. Indeed, the response to host stimuli may wane if the parasitoid is prevented from oviposition. This occurs in the ichneumonid Campolitis sonorensis if it is not allowed to oviposit after contacting host frass or damaged plant material. Also, when the eucoilid parasitoid Leptopilina heterotoma is not able to oviposit, it becomes unresponsive to host cues (Drosophila larvae), but it searches more avidly if placed in a novel environment. Another ability that some parasitoids demonstrate is associative learning. In this type of learning, the parasitoid becomes able to associate a nonhost stimulus with the presence of hosts. For example, the ichneumonid parasitoid I. conquisitor can learn to distinguish between different shapes, sizes, and colors of artificial tubes holding host lepidopterous pupae, depending on which ones it has been allowed to oviposit in. Another ichneumonid, Venturia canescens, which attacks lepidopterous larvae in cereals, can learn to associate the presence of hosts with the odor of a nonhost chemical such as geraniol. Also, the braconid Bracon mellitor learned to associate with its host an antibiotic incorporated into the artificial diet of that host, the boll weevil. Thus, the sensory modalities of vision, olfaction, and contact chemoreception may be involved in the process of associative learning.
The advantage of such flexibility is probably greatest for parasitoids that are not strictly host specific. Suitably malleable behavior would help these parasitoids take advantage of changes in host and habitat composition. Learning in parasitoid searching behavior has recently generated much research interest, so it is likely that many more examples will be forthcoming.
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