Life history and host preferences of Tetrastichus julis (Walker) (Hymenoptera: Eulophidae), the principal parasitoid of the cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae)

Abstract

Tetrastichus julis is the key larval endoparasitoid of the cereal leaf beetle, Oulema melanopus. The beetle has recently invaded cereal growing regions of western Canada, and classical biological control with T. julis is a viable management option. Although the general biology of T. julis is known, several knowledge gaps on life history traits still remain particularly in relation to host preferences and their consequences for parasitoid-beetle interactions. We investigated life history parameters and host preferences of T. julis with reference to a number of developmental parameters including survivorship, clutch size, ovipositional preferences for larval stages and their influences on realized clutch size of the parasitoid. Salient features of T. julis biology observed in this study were a short developmental time, a female-biased sex ratio, which is slightly more pronounced in larger instars, extended longevity of females, sibling mating, and more importantly, the capacity of females to live longer in the absence of hosts. The seasonal trend in parasitism rate of T. julis indicated a peak in activity in mid-June. This investigation revealed that T. julis females can discriminate among host instars and can adjust clutch size accordingly and to some extent increase production of females more than males in larger instars. In laboratory and field trials, given a choice, females laid more eggs in fourth instars than in first instars. The sex ratio was female biased and was positively correlated with clutch size. At least one male emerged from most clutches (>2) and sibling mating was common. Parasitism rates were higher after an exposure period of 48 h than 24 h. Mass rearing of T. julis can be enhanced by exposing mated T. julis females for at least 48 h to fourth-instar larvae of O. melanopus. Understanding T. julis biology can enhance mass rearing to relocate parasitoids and improve sustainable management of the cereal leaf beetle as it expands its geographic range.