Establishment and phenology of the parasitoid Microctonus hyperodae (hymenoptera : Braconidae) in New Zealand

The South American weevil Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae) is one of New Zealand's worst pasture pests causing between NZ$78-251 million damage per year. Accordingly, the South American parasitoid Microctonus hyperodae Loan (Hymenoptera: Braconidae, Euphorinae) was released at Lincoln, Canterbury, in 1991 as a biological control agent for L. bonariensis. M. hyperodae was first recovered from the field in January 1992 although noticeable population growth did not occur until a year later after which rates of parasitism increased very rapidly, although these rates declined over time. Under field conditions, M. hyperodae completed 3 generations per year after postdiapause development in September. Peak numbers of eggs occurred in late December, early March, and mid-May, respectively; the 3rd generation was much smaller than the first 2. A notable decline in percent parasitism between the 1st and 2nd generations (mid-December and mid-February each year) was related to a large influx of 1st summer generation adult weevils during an intergenerational absence of adult parasitoids. Second parasitoid generation infection rates of the weevils were very high (>90%) by early March but soon declined as protracted autumnal (April-May) emergence of weevils, combined with immigration from surrounding uncolonized areas, increased the proportion of unparasitized weevils. The small 3rd generation of M. hyperodae may be important for parasitizing these late emerging weevils. Host and parasitoid phenology conformed approximately to an existing phenological model. Analysis of field data indicated that the day-degree requirements of M. hyperodae egg and 1st, 2nd, 3rd,, and 4th larval stages above a threshold of 10.2 degree C were 14, 154, 28, 19, and 11, respectively.