Some observations on the dose transfer of mycoinsecticide sprays to desert locusts
1998
Bateman, R.P. | Douro-Kpindou, O.K. | Kooyman, C. | Lomer, C. | Quambama, Z.
Three experiments investigated the dose transfer of oil-based ultra-low volume (ULV) sprays of Metarhizium flavoviride to the desert locust: Schistocerca gregaria. Field tests against this locust are notoriously difficult in non-outbreak years so the applicability of 'pre-field trials' is discussed. In an arena test, the effect of direct contact with spray droplets was quantified, and large semi-permanent cages were constructed to investigate the effects of droplet size and the importance of secondary pick-up of spray residues. A formulation containing approximately 1.0 x 10(12) live conidia.l-1 was applied using a Micron 'Micro-Ulva' to four groups of locusts in arenas positioned in four rows downwind of a single spray line. Their subsequent mortality (at days 8-12 after application) was correlated with the number of droplets per cm2 on artificial targets placed in each arena. Under these experimental conditions, at least 50 droplets.cm-2 appear to be necessary to achieve > 90% mortality at 10 days by direct contact only, or approximately 30 droplets.cm-2 to achieve a similar effect by day 12. In experiments carried out in large field cages, locusts were released onto millet plants sprayed using various versions of an air-assisted spinning disc sprayer: the 'Ulvafan'. Droplet size spectra of these sprayers with an oil-based formulation are illustrated. In one test, there was no significant difference between mycoinsecticide applications with two different droplet sizes and 50% mortality was achieved by day 8. A second experiment indicated that although it took a shorter time to kill 50% of the insects directly exposed to spray droplets, there was no significant difference in the average survival time between locusts exposed directly and those picking up spores exclusively from the vegetation.
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