Host Selection By Asobara Tabida Nees (Braconidae; Alysiinae) a Larval Parasitoid of Fruit Inhabiting Drosophila Species

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Abstract
We studied the host selection behaviour of Asobara tabida Nees in experiments in which this parasitoid could choose between equal numbers of larvae of Drosophila melanogaster and larvae of one of the following 4 species: D. busckii, D. funebris, D. immigrans and D. subobscura. We found that: 1) the percentage parasitism in a host species increased with the survival probability of A. tabida in larvae of that species; 2) though the percentage parasitism was often the result of active rejection by the parasitoid of larvae of the less profitable species, the parasitoid often gave up before it had inserted its ovipositor in the host; this occurred: a) when a larva, after initial detection by the parasitoid, kept motionless and could not be located. b) when after detection of a larva the wasp failed to hit it with the ovipositor; the number of encounters in which the parasitoid gave up after missing stabs with the ovipositor was negatively correlated with the size of the host larvae c) when the wasp hit the larva but failed to pierce the host's skin with the ovipositor; the number of encounters in which the parasitoid gave up after it failed to pierce the host's skin was positively correlated with skin thickness d) after a combination of the events mentioned under b) and c) e) when the wasp was frightened off by vigorous wriggling of the host larva; 3) the host selection behaviour of A. tabida was affected by previous experience of the wasps with a host species; 4) A. tabida prefers D. subobscura over the 4 other species and attacks this species more efficiently than the other 4 species. D. subobscura is the only native species used in our study, D. busckii, D. funebris, D. immigrans and D. melanogaster are alien in western Europe, but may dominate the Droso- phila fauna during the second part of the summer. Oviposition by A. tabida in larvae of alien Drosophila species with a low survival probability for the parasitoid's eggs may therefore only occur when native host species are scarce and may thus help the para- sitoid to persist during the period when the preferred hosts cannot be found.

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