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.
The seasonal biology and ecology of Microtonus aethiopoides, a parasitoid of the lucerne pest Sitona discoideus, were studied in laboratory and field studies in New Zealand in 1987-88. The temperature thresholds for development of the egg and and larval stages combined and the pupal stage were 9.8 and 8.2°C, resp. Additionally, these stages required 144.4 and 125.4 day-degrees C, resp., for development. M. aethiopoides could complete 2 generations a year when showing its usual aestivatory behaviour and when synchronized with that of S. discoideus. The phenology of the braconid in the field is described. Generally, there was no difference in the pattern of migration between parasitized and non-parasitized individuals of S. discoideus to their aestivation sites in the hedgerows. However, after aestivation, parasitized curculionids tended to return to the lucerne crop early. An unusually high level of parasitoid activity at all times indicated that the S. discoideus population was suppressed by the parasitoid. 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 population of S. discoideus 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 curculionid generation throughout the summer. In view of the favourable rate of thermal summation at this time, it was calculated that the parasitoid populations underwent 3 additional generations. Such atypical development prevented affected curculionids from flying out of the lucerne to aestivate. It is speculated that the mechanism for such atypical development may have been related to high juvenile hormone titres in newly eclosed curculionids. When attacked, these individuals may have had sufficient juvenile hormone to permit full development of the parasitoid. Lower concn may cause them to aestivate as 1st-instar larvae, as is usually the case.