The effects of water stress on the efficacy of the biological control programme against Myriophyllum aquaticum

The effects of water stress on the efficacy of the biological control programme against Myriophyllum aquaticum
Chad Keates, Philip S.R. WEYL, Martin P. Hill
Biological Control Research Group, Department of Zoology and Entomology, Rhodes University, Grahamstown

There are several factors that may influence the efficacy of a biological control programme and one that has received relatively little attention is water stress. The biological control programme against Myriophyllum aquaticum has been extremely successful under most conditions in South Africa. However, field observations suggest that the beetle Lysathia sp. is not as effective in seasonal ponds where at certain times of the year the weed grows on the banks under water stressed conditions. This study aimed to determine the effect of water-stress on the ability of Lysathia sp. to feed and oviposit. The study was conducted in two phases under controlled conditions. The first phase tested whether the females chose to oviposit on water stressed plants and secondly whether the eggs and larvae would survive under water stress conditions. The study showed  that when given the opportunity the females chose to oviposit on healthy plants as opposed to water-stressed ones (Z(1,20)= 2.803, 
P= 0.0054). However, the larvae of Lysathia sp. were able to feed and develop with no significant differences on both water stressed and non-stressed parrots feather (U(1,10)= 11.0 P= 0.834). This study suggests that Lysathia sp. is capable of developing on water stressed plants, however when given the choice adult females would rather disperse to another locality where plants are potentially under better conditions. This suggests that biological control on water bodies that dry seasonally may not be as effective as permanent water bodies.