Seasonal Biology and Ecology in New Zealand of Microctonus aethiopoides (Hymenoptera: Braconidae), a Parasitoid of Sitona Spp. (Coleoptera: Curculionidae), with Special Emphasis on Atypical Behaviour

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Abstract
(1) Laboratory studies on Microctonus aethiopoides, a parasitoid of Sitona discoideus, established that the development temperature thresholds of its egg and larval stages combined and pupal stage were 9.8 and 8.2.degree. C, respectively. Additionally, the egg and larval stages combined and pupae required 144.4 and 125.4 day degrees (.degree.D), respectively, for development. This species can complete two generations a year, when showing its usual aestivatory behaviour and when synchronized with that of S. discoideus. (2) After arriving in an uncolonized district, populations of M. aethiopoides required c. three seasons to stabilize, after which the patterns of S. discoideus parasitism in different locations showed common features. Moderate April-July parasitism (c. 50%) in lucerne declined as the late instar larvae emerged from their weevil hosts and pupated during the winter. The near absence of parasitoid oviposition during the winter resulted in minimal parasitism in August-September. Emergence of new wasps and the onset of their ovipositional activity produced a marked peak in percentage parasitism (85-100%) in October-November amongst the few remaining weevils of the year''s generation. Emergence of the new weevil generation reduced parasitism to a second minimum in December. Parasitism, mainly in the form of first instar larvae, then increased in the pre-reproductive weevils in the lucerne crop in January-March. (3) Generally, there was no difference between parasitized and non-parasitized weevils in their pattern of migration to their aestivation sites in the hedgerows. However, after aestivation, the parasitized weevils tended to return to the lucerne crop early. No active parasitism occurred in the hedgerows, and weevils parasitized before aestivation were subject to no greater mortality than unparasitized ones. (4) The unusually high level of parasitoid activity at all times indicated that the S. discoideus population was suppressed by the parasitoid. Moreover, the improved understanding of the parasitoid''s phenology has greatly increased the opportunity for integrated pest management. (5) The unexpectedly high levels of parasitism observed were attributed to the uncoupling of the strictly sympathetic aestivatory behaviour of the parasitoid. A mean of 3% of the infected pre-aestivatory S. discoideus population sustained atypical full continuous parasitoid development and these remained in the lucerne throughout. This change in behaviour provided a basis for continued parasitism of the emerging new weevil generation throughout the summer. In view of the favourable rate of thermal summation at this time, it was calculated that the parasitoid populations underwent three additional generations. Such atypical development prevented affected weevils from flying out of the lucerne to aestivate. This served to maintain the infective wasp population in the field. It is seculated that the mechanism for such atypical development may have been related to high juvenile hormone titres in newly eclosed weevils. When attacked, these individuals may have had sufficient juvenile hormone to permit full development of the parasitoid. Lower concentrations may cause them to aestivate as the first instars, as is usually the case.