LABORATORY SUSCEPTIBILITY OF LOCUSTA-MIGRATORIA (L), AUSTRACRIS-GUTTULOSA (WALKER) AND VALANGA-IRREGULARIS (WALKER) (ORTHOPTERA, ACRIDIDAE)TO AN OIL FORMULATION OF METARHIZIUM FLAVOVIRIDE GAMS AND ROZSYPAL (DEUTEROMYCOTINA, HYPHOMYCETES)

Conidia of Metarhizium spp. formulated in mineral oil (Propar 12) were bioassayed against adults of three acridids which are sporadic pests of pasture and crops in northern Australia. Each insect was treated by placing a drop of a suspension of conidia in oil under the pronotum a nd was then incubated at 29 degrees C and observed daily. Eight isolat es were screened against Locusta migratoria and those found to be the most virulent were all M. flavoviride ''group 3''. One of these ''grou p 3'' isolates, FI985 originally from Austracris guttulosa, was the mo st virulent, while three other genetically similar isolates from Afric a also were highly virulent. With these four isolates, a dose of 50,00 0 conidia/insect caused 100% mortality in 8 d. Subsequent experiments were all with FI985. The LD(50) for adult L. migratoria was 4,363 coni dia (95% confidence limits: 987-16,460) on a slope of 1.50 for the 9-d mortality data and was 387 (68-1,602) conidia on a slope of 1.11 for the 12-d data. The spur-throated locust, Austracris guttulosa, was mor e susceptible than L. migratoria-the LD(50) for 9-d mortality was 413 conidia (227-867) and, for the 12-d mortality data, it was 92 conidia (43-164) on a common slope of 1.59. The larger females died on average 1.5 d later than the males at each dose. A more limited test of the s usceptibility of the giant grasshopper, Valanga irregularis showed tha t this species was also susceptible to FI985. Taken together with prev ious data, this shows that FI985 could be used as a mycoinsecticide ag ainst all five of Australia's main acridid pests.