Behavioural Aspects of the Functional Responses of a Parasite (Pseudeucoila Bochei Weld) To Its Host (Drosophila Melanogaster)

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Abstract
1. The functional response of the parasite Pseudeucoila bochei to different densities of its host (Drosophila melanogaster larvae) was studied in three series of experiments. In the first series, the host-medium sites (yeast spots) were equal in size, but the number (and hence the density) of hosts varied between 0 and 128. The parasitiza- tion period was limited to one hour. In the second series, the spots and densities were the same as in the first, but differed in the time the parasite was permitted to search. She was allowed to search the medium until she left, at which time the experiment was terminated. In the third series, the size of the yeast spot and the number of larvae in the spot differed, so that the density per spot remained constant. 2. All females found the host medium, but the smaller the yeast spot, the longer the interval between the introduction and the time before the spot was discovered. This time was not influenced by the number of host larvae present (Fig. 2). Appar- ently the wasps first located the host habitat, independently of the presence of hosts. 3. Females that failed to find a host during their first visit to a yeast spot spent a more or less fixed time searching before they left. This "giving-up" time (ca. 73 seconds at 2 cm diam) depended on the size of the yeast spot. If a host was found during this initial search, then the parasite increased the time she searched for a second host by an average of 623 seconds before leaving the spot. This behaviour is clearly adaptive when the hosts are clustered. 4. Because parasites in series 1 were permitted to return to the yeast after one or more unsuccessful visits (searching time limited to one hour), the number of visits to the host medium was initially high at the low host densities but decreased with increasing density. The hosts were more easily found at higher host densities so fewer unsuccessful visits occurred. If her first visit to the spot was unsuccessful, subsequent unsuccessful ones appeared to be of shorter duration, indicating that the wasps might recognize sites already searched. 5. Because the parasites were permitted to return to the yeast in series 1, the wasps laid more eggs in this series than in series 2, because the frequent visits to the yeast at low densities increase the likelihood of finding hosts. In series 3 once a parasite found the host medium, she discovered a host more easily than in series 2 because the hosts were at a constant high density in series 3: only the size of the yeast spot varied.

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