Behavioural changes in Schistocerca gregaria following infection with a fungal pathogen: implications for susceptibility to predation

1. Field observations have indicated that infection of locusts and grasshop pers by the fungal entomopathogen Metarhizium anisopliae var. acridum may r esult in a substantial increase in the host's susceptibility to predation, before death is caused directly by the disease. 2. Laboratory experiments were conducted to examine how the behaviour of th e desert locust Schistocerca gregaria Forskal changes following infection b y M. anisopliae var. acridum to explore some potential mechanisms underlyin g this phenomenon. 3. In the first experiment, which involved monitoring general locust activi ty in small cages throughout the disease incubation period, infected locust s were observed to increase locomotion and bodily movement from 3 days afte r infection until death (average survival time of 11 days). There was some evidence of reduced feeding and mating behaviour following infection. 4. In a second experiment, locusts were exposed individually to a simulated predator attack and the initiation and strength of any escape responses we re measured. Infected locusts were observed to have a reduced escape capabi lity (both the propensity to escape and the strength of the response). In c ontrast to the relatively early changes in general activity observed in the first experiment, this was only apparent at the late stages of infection s hortly before death. 5. Both an increase in movement and general apparency early in the infectio n process, and reduced escape capability late on, suggest mechanisms whereb y the susceptibility of locusts and grasshoppers to predation might be enha nced following infection with M. anisopliae var. acridum.