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

Agasicles hygrophila was introduced into New Zealand in 1982 for the biolog ical control of alligator weed, Alternanthera philoxeroides. The optimum te mperature for A. hygrophila development was 27-30 degrees C where developme nt time was 19-20 days. Mortality was lowest (13-14%) at temperatures betwe en 23-25 degrees C. The estimated lower development threshold for A, hygrop hila was 13.3 degrees C. Adult beetles reared at 23-27 degrees C were heavi er than those reared at 15-20 degrees C and 30 degrees C, and females were heavier than males. There was significant mortality of adults during exposu re to low temperatures. Mortality was 92% for adults exposed for four weeks at 10 degrees C and 70% for adults exposed for 12 weeks at 15 degrees C. T he number of eggs laid subsequently decreased as the time adults were expos ed to chilling was increased. Viability decreased as the temperature to whi ch adults were exposed was lowered, and with increased chilling time. Femal es survived longer than males when chilled at 10 degrees and 15 degrees C. All adults died when chilled for 13 h at -8 degrees C; at 2 degrees and -4 degrees 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 degrees C and below.