The effect of temperature on the development and survival of <i>Agasicles hygrophila</i> Selman &amp; Vogt (Coleoptera: Chrysomelidae), a biological control agent for alligator weed ( <i>Alternanthera philoxeroides</i> )

Abstract

Abstract Agasicles hygrophila was introduced into New Zealand in 1982 for the biological control of alligator weed, Alternanthera philoxeroides . The optimum temperature for A. hygrophila development was 27–30°C where development time was 19–20 days. Mortality was lowest (13–14%) at temperatures between 23–25°C. The estimated lower development threshold for A. hygrophila was 13.3°C. Adult beetles reared at 23–27°C were heavier than those reared at 15–20°C and 30°C, and females were heavier than males. There was significant mortality of adults during exposure to low temperatures. Mortality was 92% for adults exposed for four weeks at 10°C and 70% for adults exposed for 12 weeks at 15°C. The number of eggs laid subsequently decreased as the time adults were exposed to chilling was increased. Viability decreased as the temperature to which adults were exposed was lowered, and with increased chilling time. Females survived longer than males when chilled at 10° and 15°C. All adults died when chilled for 13 h at ‐8°C; at 2° and ‐4°C there was high adult survival, but the viability of eggs laid was low. These results suggest the overwintering capacity of A. hygrophila is much reduced when exposed to temperatures of 15°C and below.