Biological control of locusts and grasshoppers using a fungal pathogen: The importance of secondary cycling

Persistent chemical pesticides can provide an effective means of control against locusts and grasshoppers due to prolonged activity of the spray residue. However, use of these pesticides is now prohibited, and nonpersistent chemical alternatives are substantially less successful. Here we show why it is expected that biological pesticides based on the fungal pathogen Metarhizium flavoviride will be highly effective in the control of both locust and grasshopper. We demonstrate, using novel population dynamic models containing measured estimates of horizontal transmission coefficients, that secondary cycling of the pathogen after a single spray application provides a biological substitute for chemical persistence. This has significant consequences for the economics of biopesticide use in pest control. Furthermore, by identifying that secondary cycling acts in a density-dependent manner, this study also highlights fundamental differences between conventional pesticides and biopesticides and how they might be used.