Ever wonder how the small parasitic wasps you release in a conservatory can find pests when they are mixed in with a wide variety of plants? Many of them simply "follow their nose," using specially adapted receptors on their antennae to smell places where pests are likely to be found.
Parasitoids who specialize on pests with a wide host range are generally believed to follow scent of the pest itself and its excretions. In contrast, specialist parasitoids of pests with a narrow host range follow a combination of smells coming from the plant, by-products of pest feeding, and the pest itself. Recent tests of this theory of pest location were conducted with two specialist parasitoid wasps, Leptomastix dactylopii and Epidinocarsis lopezi. L. dactylopii attacks the citrus mealybug, which feeds on a wide variety of host plants. E. lopezi feeds on cassava mealybug, a pest restricted to cassava. Using a device to test the response of each wasp to different scents, they found E. lopezi to be attracted to uninfested cassava, or the mealybug itself. As predicted, L. dactylopii, which attacks a pest with a wide host range, only followed the scent of the mealybug, and not the host plant used in the study.
Interestingly, both parasitoid species could also use their sense of smell to determine if mealybugs on the plant were parasitized. The odor of plants containing unparasitized mealybugs was highly attractive to both wasps, and they increased their activity. However, when exposed to the odor of parasitized mealybugs, wasps became agitated and behaved like they were ready to move on to look for unparasitized hosts. From the standpoint of the practitioner this is good news. The wasps' use of insect and plant odors make them more efficient hunters by enabling them to focus their activity on the more infested areas of the conservatory where mealybugs have yet to be parasitized.
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