Host selection by Microplectron fuscipennis , Zett. (Chalcididae, Hymenoptera)

Abstract

The literature of host selection among entomophagous parasites is reviewed and discussed, with the conclusion that a purely mechanistic explanation of this behavior is impossible. The following experiments were carried out to determine the factors influencing host selection by Microplectron, which parasitises the cocoon-enclosed larvae of the genus Diprion only: [female][female] offered the choice between gelatin capsules of the same shape and size as the cocoons and blunt-ended objects attempted oviposition in some of the capsules, but ceased to show interest in the other objects as soon as antennal examination encountered the acute edge. Shape therefore influences the selection. Alternation of gelatin capsules as above and larger capsules resulted in 26 attempts to pierce the larger capsules, 4 the smaller. As between opaque and translucent capsules, the opaque were preferred. Texture was of little importance. Moist smears of host blood near capsules caused orientation of the [female] in their direction at a distance of 4-5 mm. A series consisting of cocoons with larvae, capsules enclosing larvae, empty cocoons and empty capsules were accepted as hosts in the descending order given. The first 3 classes were definitely attractive to the [female][female], but only in the first 2 was the percentage of ovipositions high. In the above exps., odor of the host, present in the blood smears, the empty cocoons, the cocoons and capsules containing larvae, was shown to exert the most powerful initial attraction. Shape was of importance when the o came into contact with the object, but under conditions where other characteristics of the host were present in adjacent objects was unable to induce acceptance, which ultimately depended on the presence of a living larva. As between large and small larva-containing cocoons, the smaller in which oviposition occurred were about half the number of the larger, but since the normal total population includes only about 4-5% small cocoons, this discrimination is not important in the field. Parasitised hosts were avoided only when the larvae from the previous infection were nearly mature. Dead hosts were accepted only in 27 out of 319 tests, cocoons in which the host larvae were nearly mature in 14 out of 205 tests. This discrimination is attributed to perception of the movements of the host and parasite larvae. Until the parasite is within 5-6 mm. of the host, at which distance odor produces orientation, finding of a host depends on chance encounter. Aided by visibility, contact followed by antennal examination of shape, and possibly of texture, follows. But this combination of sense pictures of the external characteristics is normally insufficient to stimulate oviposition, final acceptance depending on the stimulus of the presence of a living larva. This response can be interpreted mechanistically as reaction to a suitable pattern of stimuli. But under experimental conditions the parasite will ultimately accept an object having no characteristic of the host except shape. Such an isolated character should not induce acceptance if this depends on response to a pattern of stimuli; and this "error of instinct" leads to the conclusion that the basis of behavior during host selection must be psychological.