The measurement of exposed host volume by the parasitoid wasp Trichogramma minutum and the effects of wasp size

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Abstract
The parasitoid wasp Trichogramma minutum Riley (Hymenoptera: Chalcidoidea: Trichogrammatidae) adjusts its progeny allocation to the exposed volume of its insect egg host. This paper deals with the mechanism for this size discrimination, which is not dependent upon chemical or visual cues or repeated parasitism. The wasps are also able to discriminate between hosts of the same diameter that differ in exposed surface area, and do not use curvature to determine progeny allocation under these conditions. A computer-based event recorder was used to collect information about the host-examining behaviour of the wasp. Of the examining-path parameters that change with surface area, only the duration of the wasp's initial transit across the exposed surface of the host correlated significantly with the number of eggs subsequently laid by the wasp. Artificially reducing the initial transit of the wasp by obstructing its path resulted in significantly fewer eggs per host. Both large and small wasps allocate the same number of progeny to hosts of the same exposed volume, and take the same time to complete their initial transits. We propose that Trichogramma use the absolute length of their initial transit, measured as the transit's duration, to set clutch size to absolute exposed host volume. This behaviour allows Trichogramma to lay the number of eggs into each host that ensures maximum reproductive success, regardless of the wasp's own dimensions.