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.