Alligator weed, Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae) is a perennial, stoloniferous herb which grows primarily as an emergent aquatic plant. It was first recorded in New Zealand in 1906 and is distributed from the Waikato Region northwards. Agasicles hygrophila Selman & Vogt, a biological control agent for alligator weed, was first introduced into New Zealand in 1981. However, since its release it has been suggested that temperatures in New Zealand may be too low for optimum development and over-wintering of this species, and this may influence the effectiveness of A. hygrophila as a biological control agent. The aim of this study was to investigate the effect of temperature on the biology and population ecology of A. hygrophila and its distribution in New Zealand. Field studies were conducted at two sites in Northland from 1992-1995. Aquatic alligator weed was present throughout the year at the Whatatiri site. Alligator weed dry weights peaked at the Whatatiri site in late January 1994 and late December 1994 and declined as A. Hygrophila populations began to build up. Peak A. hygrophila populations occurred at the Whatatiri site during February in 1994 and 1995. Alligator weed dry weights peaked at the Matarau site in late March 1993, early February 1994 and late February 1995. Few A. hygrophila were observed at the Matarau site during the three seasons studied. Agasicles hygrophila were reared in the laboratory at temperatures of 10-30°C. Optimum temperatures for A. hygrophila development were 23-27°C. Development rate was the highest at 27-30°C where it took 19-20 days to develop from egg to adult. However, mortality was lowest (13-14%) between 23-25°C. The lower temperature development threshold for A. hygrophila was estimated as 13.3°C. Female lifetime egg production was measured at four temperatures from 15-30°C and optimum oviposition and viability occurred at 25°C. Adult female longevity decreased as the temperature was increased from to 15°C to 30°C. Experiments were conducted to investigate the effect of low temperatures on the survival and fertility of adult A. hygrophila. Adults were chilled at constant temperatures of 10 and 15°C for up to 12 weeks and egg production was measured after the chilling period ended. Adults survived up to 6.5 weeks when chilled at 10°C and 16 weeks (estimated) at 15°C. However, the subsequent number of eggs laid and their viability decreased as adult exposure to chilling was increased or temperature was decreased. Females survived longer than males, when chilled at 10 and 15°C. Adults were chilled for 13 hours to simulate frost conditions. Chilling at -8°C for 13 hours killed all adults tested. At 2 and -4°C there was high adult survival; however, viability of subsequently laid eggs was low. Agasicles hygrophila was estimated to have a degree day (DD) requirement of 277 (egg to ovipositional adult) above the 13.3°C lower temperature development threshold. Temperature was measured at both field sites in the 1994-95 season and it was estimated that there were enough heat units for four generations of A. hygrophila to occur at both sites. The biological fix point was estimated to occur between 15 September to 24 November 1994 at the Whatatiri site and 15 September to 20 February 1995 at the Matarau site. The climate matching model, CLIMEX, was used to estimate the potential distribution of alligator weed and A. hygrophila in New Zealand. The CLIMEX model predicted that northern regions of the North Island were most suitable for A. hygrophila growth and development in New Zealand. A. Hygrophila establishment was predicted to be unlikely in the South Island due to the higher number of frosts and low DD accumulation. Alligator weed has not yet reached its full potential geographical range within New Zealand and is predicted to have a wider potential range than A. hygrophila. Observations from this study indicate that the low spring and summer temperatures experienced in New Zealand may inhibit population build-up and low winter temperatures may cause high over-wintering mortality. Therefore the present strain of A. hygrophila is unlikely to be a suitable biological control agent for widespread control of alligator weed in New Zealand. Consequently, additional biological control agents and integrated control strategies are required for alligator weed in New Zealand. The next step in this biological control programme should be to assess the impact of Vogtia malloi Pastrana, the other surviving biological control agent released in New Zealand to control alligator weed. If results show that this species is also limited in its ability to control alligator weed, then new cool-adapted agents, including additional strains of A. hygrophila, should be sought in South America. This will be especially important if alligator weed continues to spread further south.