Thermal biology of <i>Hypogeococcus pungens</i> (Hemiptera: Pseudococcidae) explains its variable performance as a classical biological control agent for <i>Harrisia martinii</i> (Cactaceae) in Australia

The mealybug, Hypogeococcus pungens Granara de Willink (Hemiptera: Pseudococcidae), was released in Australia as a biological control agent for Harrisia martinii (Labour.) Britton (Cactaceae) in 1975. Although the mealybug successfully established in all released locations, its impact has been variable among regions, possibly as a result of climatic differences. Life-history traits (settling time, survival, development time, female reproduction, adult longevity) were compared at 6 constant temperatures (15 to 40 °C) in the laboratory. The mealybug settled on H. martinii at all temperatures tested, but at 15 °C and 40 °C, insects failed to develop and died. Temperature affected female size, fecundity, and integrated performance, all of which were highest at 25 °C. A linear model that fitted temperature to development time indicated a lower developmental threshold of 14.5 °C for both male and female mealybugs. CLIMEX models were developed for the mealybug and its host, H. martinii, and used to investigate the suitability of different regions of Australia, where H. martinii occurs for Hy. pungens. The Hy. pungens CLIMEX model suggests that cold stress limits mealybug growth in southern Queensland and that mealybug performance will vary between regions based on local temperatures. Locations with extreme low winter and extreme high summer temperatures are likely to have the most constrained populations. This may account for the observed differences in the effectiveness of Hy. pungens as a biological control agent at locations within the established range of H. martini in Australia.