Aspects of the ecology of Tyria jacobaeae (L.) : a defoliator of ragwart in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University

Abstract

Tyria jacobaeae L. (Lepidoptera: Arctiidae) was introduced to New Zealand as a biological control agent for ragwort Senecio jacobaea (L.), a poisonous pasture weed. This study investigated the diapause, pupal survival, population ecology and impact of T. jacobaeae on ragwort populations in the Wairarapa. Particular emphasis was placed on those aspects influencing the ability of T. jacobaeae to control ragwort. T. jacobaeae enters obligatory pupal diapause over the winter months. The temperature requirements of T. jacobaeae pupae were investigated under controlled and natural conditions. Diapause development was completed after approximately 105 under field conditions and 70 days at 2°C. The minimum temperature for post diapause development indicated pupae were unlikely to enter post diapause quiescence following diapause development in the Wairarapa. The production of two generations of T. jacobaeae in a single season will be of little benefit, however storage of quiescent pupae for delayed release is feasible. Pupal survival was largely determined by exposure and substrate under natural conditions. Strong density dependent mortality was detected among caged larvae, and increased larval density reduced pupal dimensions, weight and potential fecundity. No evidence of the diseases known to infect T. jacobaeae overseas was observed. Natural T. jacobaeae populations showed no controlling influence on the ragwort population studied. High larval mortality, a patchy distribution over the host population and rapid ragwort regrowth reduced the effectiveness of T. jacobaeae as a biological control agent. At the most intensively studied site the T. jacobaeae population appears to have stabilised at a level below that required for ragwort control. The presence of T. jacobaeae was well synchronised with ragwort flowering in the field, and T. jacobaeae seems to have adapted to the climatic conditions of the Wairarapa.