Host Finding

Unless random search is important, parasitoids usually find their hosts as a result of cues derived directly or indirectly from the host itself, often after they have entered the host habitat. Often host cues are perceived at close range. For example, the braconid Cardiochiles nigriceps, increases its searching when it contacts secretions produced by the mandibular glands of its host, Heliothis spp. caterpillars, as the latter feed on plants. The braconid Microplitis croceipes searches areas contaminated by chemicals contained in the feces of Heliothis caterpillars, whereas the braconid Cotesia melanoscela intensely searches leaf areas where host gypsy moth (Lymantria dispar) caterpillars have deposited silk strands. Once these materials have been perceived, stereotyped searching behaviors occur that typically consist of intense examination of the area with the antennae or tarsi. Parasitoids often also decrease their walking speed (orthokinesis) and/or increase turning rates (klinokinesis).

These behaviors, which serve to keep the parasitoid in the area having the host products, often lead to host discovery.

Other parasitoids are attracted from longer distances directly to hosts. Some parasitoid females respond to pheromones produced by their host. Parasitoids of the European elm bark beetle, Scolytus multistriatus, are attracted to "multilure," the aggregation pheromone of adult beetles. Aphytis spp. (Hymenoptera: Aphelinidae) are attracted to the sex pheromone produced by their host, California red scale (Aonidiella aurantii). A number of true bugs produce sex pheromones that are attractive to a variety of fly (Tachinidae) and to hymenopteran parasitoids. Some parasitoids are drawn from a distance to chemicals produced by plants in response to damage caused by host herbivores. For example, the braconid larval parasitoid Cotesia marginiventris responds to volatile terpenoids released from corn seedlings as a result of eating damage caused by host Spodoptera caterpillars. These chemicals may be components of the induced resistance that plants have developed against pathogens and herbivores. Even leaves not directly damaged by a herbivore may produce such materials. The induced plant chemicals may, along with materials directly produced by herbivores, serve to attract parasitoids. For example, parasitoids of bark beetles are attracted to a combination of plant chemicals produced by trees as a reaction to the mass attack of the beetles as well as to the aggregation pheromone produced by these beetles. Simultaneous responses of parasitoids to longrange host and plant cues illustrate that the division between the categories of habitat and host finding is often arbitrary.

Bee Keeping

Bee Keeping

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