Screening for virulent isolates of entomopathogenic fungi against the desert locust, Schistocerca gregaria (Forskal)

The pathogenicity of 159 isolates of Metarhizium and Beauveria spp. was screened using Schistocerca gregaria adults. The median lethal time (MLT), the average survival time and its standard deviation (SD) were used to describe the rate of kill from the cumulative observed response curve over time, following the application of conidia in bioassays. All screens included a standard strain of Metarhizium sp. (from a single spore of IMI 330189ss), which gave an average MLT value of 4.4 days (SD = 0.4) in 46 assays. Approximately 50 isolates, all belonging to the genus Metarhizium, showed virulence that was comparable with this strain. Two previously unreported preliminary tests were carried out. The first test assessed the response to the standard isolate by an established laboratory stock of desert locusts compared with a recently introduced locust stock from Niger. There were insignificant differences in MLT and LD(50) (on day 5) between the different locust strains. The second test assessed the effects of inoculation at different times after the final ecdysis. This similarly had little effect, although applications before day 10 appeared to minimize the risk of control mortality. A computer program was developed to store bioassay results in a standardized format and to produce quick estimates of these statistics.