A female parasitoid may find herself far from potential hosts. This could occur if the host stage she emerges from is different from the one attacked. Also, many parasitoid females have a preoviposition period before eggs are ready to be laid. During this interval of a few days to several weeks, the parasitoid may leave the vicinity of the host to mate and obtain nourishment. For example, the ichneumonid wasp Pimpla ruficollis is a parasitoid of the European pine shoot moth, Rhyaciona buoliana. Yet for the first few weeks of her adult life, she is repelled by the odor of pine, and thus avoids the forest where the host is located. As a consequence, the initial stage in host seeking in many parasitoids is to search for locations where the host is likely to occur. Parasitoids often respond to general stimuli such as light, humidity, or vegetation form, leading them to meadows, forests, swamps, ponds, soil, or different vegetation strata. These behaviors considerably narrow the areas that must be actually searched for hosts. Many parasitoids are also attracted to volatile chemicals from plants. For example, after initially being repelled by pine, P. ruficollis females that are ready to oviposit are attracted by pine odors, and thus are drawn back to the forest. The ichneumonid Itoplectis conquisitor is attracted to the odor of Scots pine but not red pine, and does not attack lepidopterous hosts on the latter. In olfactometer tests, the aphid parasitoid Diaeretiella rapae is attracted to collard leaves. Wind tunnel experiments have shown that when such a parasitoid perceives a plant volatile, she reacts by walking or flying upwind (aenemotaxis), thus often leading to the plants where her preferred host feeds. In fact, in some cases the plant attraction is so important that the parasitoid host range encompasses the often diverse herbivores that feed on that plant rather than hosts that are taxonomically closely related. Plants are not the only habitat characteristics that can produce such attractive volatiles. Parasitoids of carrion-feeding flies are attracted to fresh or decaying meat, and parasitoids of Drosophila fruit flies respond to odors from yeast in decaying fruits where their hosts are likely to be present.
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