Reduction of feeding by the variegated grasshopper, Zonocerus variegatus, following infection by the fungal pathogen, Metarhizium flavoviride

The effect of infection by the fungal entomopathogen, Metarhizium flavoviride, on feeding by the tropical grasshopper pest, Zonocerus variegatus, was investigated in field-cage studies. A significant reduction in feeding, as indicated by faecal production, was recorded 2-3 days after inoculation for a range of spore doses (10⁴, 10⁵ and 5 x 10⁵ spores per insect). This was before any mortality was recorded due to infection. All infected individuals died by day 7. At this time, faecal production of the treated grasshoppers was equivalent to less than 2 days faecal production by grasshoppers untreated with spores. This reduction in feeding (69, 71 and 74% total reduction by day 7 in the 10⁴, 10⁵ and 5 x 10⁵ doses respectively in comparison to controls) is a substantial contribution to the overall effects of the slow acting pathogen. Furthermore, the rapid reduction in feeding indicated that this effect was not simply due to invasion of the host tissues by the pathogen or production of secondary metabolites. The possibility that reduction in feeding is associated with a behavioural response in which there is a trade-off between host defence and feeding during early stages of infection is discussed.