The thermal physiology of Lysathia sp. (Coleoptera: Chrysomelidae), a biocontrol agent of parrot's feather in South Africa, supports its success

The establishment success of biocontrol agents originating from tropical regions is often limited by climate when introduced in temperate regions. However, the flea beetle, Lysathia sp. (Coleoptera: Chrysomelidae), a biocontrol agent of Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae) in South Africa, is an effective agent in regions where other biocontrol agents of tropical aquatic weeds have failed due to winter-induced mortality. The development (degree-day model) and thermal tolerance (critical thermal minimum/maximum [CTmin/max] and lower/upper lethal limits [LLT/ULT50]) of Lysathia sp. were investigated to explain this success. The model predicted that Lysathia sp. could complete 6 to 12 generations per year in the colder regions of the country. The lower threshold for development (t(0)) was 13.0 degrees C and thermal constant (K) was 222.4 days, which is considerably lower than the K values of other biocontrol agents of aquatic weeds in South Africa. This suggests that above the temperature threshold, Lysathia sp. can develop faster than those other species and complete multiple life cycles in the cooler winter months, allowing for rapid population growth and thus improving M. aquaticum control. Furthermore, the CTmin of Lysathia sp. was 2.3 +/- 0.2 degrees C and the CTmax was 49.0 +/- 0.5 degrees C. The LLT50 was calculated as similar to -7.0 degrees C and the ULT50 as similar to 43.0 degrees C. These wide tolerance ranges and survival below freezing show why Lysathia sp. has established at cool sites and suggest that it may be a suitable agent for other cold countries invaded by M. aquaticum.