The effects of insect–insect interactions on the performance of three biological control agents released against water hyacinth

Insect–insect interactions can have implications for biological control programmes when multiple agent species are released. In many cases there is an increase in the efficacy when more than one species is used; however, there is a possibility that releasing an additional species into a programme could have a negative effect. The interactions between three arthropod agents of water hyacinth Eichhornia crassipes (Martius) Solms-Laubach, Eccritotarsus catarinensis (Carvalho), Neochetina bruchi Hustache and Neochetina eichhorniae Warner were investigated in an experiment to measure the impact that pairwise combinations of the insects may have on their performance. There was a significant interaction between the mirid E. catarinensis and the weevil N. eichhorniae, with significantly fewer weevil feeding scars when in combination with the mirid (approximately 0.2 scars per cm²) than when alone (approximately 0.4 scars per cm²). There were also slightly fewer petioles mined by N. eichhorniae when in combination with the mirid. Interestingly there was a negative interaction between the two weevil species when in combination, with the number of feeding scars being significantly lower per individual when the two species were in combination. None of the insects performed significantly better when in combination with another insect, however, the mirid was never negatively affected by the presence of either weevil species. The interactions observed between the insects tested were identifiable but subtle and are unlikely to have implications on establishment or performance of the insects in the introduced range, South Africa.