Potential to enhance the efficacy of Microctonus hyperodae Loan

Abstract

The parasitoid, Microctonus hyperodae Loan (Hymenoptera: Braconidae), and its host, Argentine stem weevil (Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae)) were sampled, firstly, to quantify parasitoid mortality between the larval and adult stages during spring 1996 and, secondly, to measure the realised fecundity of two generations of parasitoid adults in summer 1996-1997. Survival of M. hyperodae larvae and pupae was low (<10%) and subsequent female density was low (<2/m 2). Pasture management practices which increase survival of M. hyperodae larvae and pupae in spring could enhance suppression of L. bonariensis. Parasitoid females from the first half of the first adult generation were estimated to have laid <15% of their eggs, while females which emerged subsequently were estimated to have laid >65% of their eggs. This variation in realised fecundity was associated with changes in host density. Habitat manipulations that enable more of the early-emerging parasitoids to survive until host densities increase in early summer could enhance suppression of L. bonariensis. INTRODUCTION Argentine stem weevil, Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae), is a major pest of pasture in New Zealand (e.g. Goldson et al. 1998a). Microctonus hyperodae Loan (Hymenoptera: Braconidae, Euphorinae) was introduced to New Zealand in 1991 for biological control of L. bonariensis. The parasitoid oviposits in the adult weevil, whereupon the parasitoid larva develops within the living host. M. hyperodae is parthenogenetic (Loan and Lloyd 1974) and proovigenic (Goldson et al. 1995; Phillips 1998). The parasitoid is now established at over 80 locations in the North and South Islands (M.R. McNeill pers. comm.). This contribution makes a preliminary assessment of the potential to enhance the efficacy of M. hyperodae using habitat manipulation by, firstly, ascertaining mortality rates of immature M. hyperodae and, secondly, measuring the reproductive rates of M. hyperodae adults in the field. MATERIALS AND METHODS Sampling was conducted in one paddock (100 m x 100 m) at the AgResearch farm, Lincoln, between September 1996 and April 1997. This was the same paddock used previously to describe the bionomics of L. bonariensis and M. hyperodae (Goldson et al. 1998a and b). Estimates of the field densities of L. bonariensis adults and M. hyperodae larvae were made using the methods of Goldson et al. (1998a and b). In brief, adult weevils were extracted from turf samples (450 mm x 40 mm x 80 mm deep) taken approximately fortnightly using a flotation method (Proffitt et al. 1993). To provide additional weevils for assessment of parasitism, 50-70 weevils were also sampled every alternate week using a modified high-powered vacuum cleaner. Weevils recovered by both methods were dissected under 70% ethanol using a binocular microscope at 25 X magnification (Goldson and Emberson 1981) and the parasitoid stages present recorded.