Cold hardiness of the biological control agent, Agasicles hygrophila, and implications for its potential distribution

Abstract The physiological adaptations of biological control agents may determine where and when they will be successful. Agasicles hygrophila Selman & Vogt, has been introduced into China for biological control of an invasive weed, Alternanthera philoxeroides (Mart.) Griseb. The extent to which A. hygrophila can withstand low temperatures was tested to determine whether this physiological trait can be used to predict its potential distribution and effectiveness as a biological control agent. Our results showed that A. hygrophila eggs, larvae, pupae and adults had similar super cooling point temperatures (SCPs) which were not affected by acclimatization. Mortality increased with decreasing temperatures and with length of exposure to low temperatures. SCP was the limiting survival temperature, and chilling injury was one of the main causes of death. The upper limit of chill injury zone (ULCIZ) was 6.7 °C and the sum of injurious temperature (SIT) was about 15.0 degree days. Based on adult cold tolerance, the distribution of A. hygrophila in China is unlikely to extend beyond areas that experience temperatures at or below −8 °C, and the flea beetles will probably overwinter successfully only in areas where the annual minimum air temperature remains above 6.7 °C. These predictions were compared with known distribution data and seem to be a reliable and economical method to predict effectiveness of potential of biological control agents in regions with extreme climates.