Intraspecific variation in Microctonus hyperdoae and M. aethipopides (Hymenoptera: Braconidae); significance for their use of biological control agents

Abstract

Microctonus aethiopoides Loan and M. hyperodae Loan (Hymenoptera: Braconidae; Euphorinae) are parasitoids introduced to New Zealand to assist suppression of the lucerne pest Sitona discoideus Gyllenhal and the pasture pest Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae) respectively.

Morphological variation in adults of M. aethiopoides was examined as a means of measuring genetic variation. The extent of genotypic variation in M. aethiopoides could not be readily ascertained using morphology because morphological variation in adults reared from different host species was not due to genetic differences. Rather, morphological variation in M. aethiopoides was accounted for by effects of factors such as host size and nutritive quality on adult parasitoid morphology. Some previously accepted evidence for the existence of genetic differences between M. aethiopoides populations is, therefore, questionable.

Morphological variation in M. hyperodae adults derived from eight diverse South American locations was examined. Morphological differences between M. hyperodae reared from one host species under controlled conditions indicated genetic variation between some of the geographic populations. However, no between-population variation was found during examination of larval morphology, enzyme/non-specific protein electrophoretic banding patterns, and effects of antibiotics on M. hyperodae asexuality. The morphological variation between M. hyperodae adults of known South American origins was exploited using discriminatory function analysis to develop a method of classifying M. hyperodae of unknown South American origins recovered from the field in New Zealand. Rigorous crossvalidation tests indicated that more than 80% of specimens could be classified correctly using this method.

In New Zealand, the release in 1991 of equal numbers of M. hyperodae from each South American geographic population led to the hypothesis that, in the absence of between population performance differences, each South American population should be equally represented in samples taken from the field. Analysis of M. hyperodae adults collected in New Zealand during 1992-1994 did not support this hypothesis. Parasitoids derived from east of the South American Andes were significantly more prevalent than expected.

A rigorous and quantitative method of analysing Microctonus spp. oviposition behaviour was developed and a definitive description of M. hyperodae and M. aethiopoides oviposition behaviour provided. Variation in M. hyperodae oviposition behaviour was examined as a possible reason for the differential performances of the South American geographic populations in New Zealand. No clear behavioural differences between the geographic populations were identified, but two preliminary possibilities were recognised. First, there is an indication that some M. hyperodae geographic populations derived from Argentina and Uruguay may be better adapted to searching in New Zealand's pastoral habitats. Second, some evidence supports the hypothesis that, in Chile, M. hyperodae parasitises species other than L. bonariensis and is not as specialised to attack L. bonariensis as M. hyperodae from Uruguay, Argentina and Brazil.

As shown by morphometric analysis, M. hyperodae genetic variation has been important to its use as a biological control agent. However, reasons why such variation has been important, and to what extent, were not clearly established. The implications of these results for classical biological control practice are discussed and an experimental approach to biological control is advocated.