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As an integral part of a programme to attempt the biological control of several crop-infesting aphid pests in Australia by the introduction of specific hymenopterous parasites, a laboratory technique to try to predict their likely effectiveness in the field, was developed with the aim of improving selection of natural enemies in future. It was based on demonstrating whether the parasites attack a greater proportion of their hosts with increased host density, i.e. whether they show a Type 3 functional response (Holling, 1959). To overcome the problem of super-parasitisation at low densities when a parasite is confined with aphids on one plant (van Lenteren & Bakker, 1976) we used several plants in a large cage, all with the same density of host aphids. A parasite released into such a cage searches for and attacks the aphids on one plant, then leaves it to repeat the process on a second plant, and so on. After a standard exposure time the parasitisation rate is estimated separately for the aphids on each plant and the average performance of the parasite calculated. Using this technique for seven parasites introduced into Australia for biological control of crop aphids, only one, Aphidius salicis Halliday, attacking carrot aphid showed a Type 3 response. |
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