SOME OBSERVATIONS ON THE DOSE TRANSFER OF MYCOINSECTICIDE SPRAYS TO DESERT LOCUSTS

Three experiments investigated the dose transfer of oil-based ultra-lo w volume (ULV) sprays of Metarhizium flavoviride to the desert locust: Schistocerca gregaria. Field tests against this locust are notoriousl y difficult in non-outbreak years so the applicability of 'pre-field t rials' is discussed. In an arena test, the effect of direct contact wi th spray droplets was quantified, and large semi-permanent cages were constructed to investigate the effects of droplet size and the importa nce of secondary pick-up of spray residues. A formulation containing a pproximately 1.0x10(12) live conidia.l(-1) was applied using a Micron 'Micro-Ulva' to four groups of locusts in arenas positioned in four ro ws downwind of a single spray line. Their subsequent mortality (at day s 8-12 after application) was correlated with the number of droplets p er cm(2) on artificial targets placed in each arena. Under these exper imental conditions, at least 50 droplets.cm(-2) appear to be necessary to achieve > 90% mortality at 10 days by direct contact only, or appr oximately 30 droplets.cm(-2) to achieve a similar effect by day 12. In experiments carried out in large field cages, locusts were released o nto millet plants sprayed using various versions of an air-assisted sp inning disc sprayer: the 'Ulvafan'. Droplet size spectra of these spra yers with an oil-based formulation are illustrated. In one test, there was no significant difference between mycoinsecticide applications wi th two different droplet sizes and 50% mortality was achieved by day 8 . A second experiment indicated that although it took a shorter time t o kill 50% of the insects directly exposed to spray droplets, there wa s no significant difference in the average survival time between locus ts exposed directly and those picking up spores exclusively from the v egetation. (C) 1998 Elsevier Science Ltd. All rights reserved.