Variations in insecticide dose received by settled locusts: a computer model for ultra-low-volume spraying

Abstract

A computer model has been devised to examine the variation in the insecticide dose received by settled locusts resulting from change in the primary application variables [volume and mass application rate, spray drop size distribution, emission height and wind speed (with logarithmic wind profile)], while taking into account formulation differences, i.e. specific gravity and volatility. A buoyancy factor can be incorporated, but has been omitted from most of the present calculations. The size and collection efficiency of the locust target are estimated, and drop collection has been computed as the sum of the contributions of the two component processes, i.e. inertial collection on the vertical projected area and sedimentary collection on the horizontal projected area. The lethal swath is shown to be narrowest at very low wind speeds, when dose is mainly dependent on the sedimentary component of deposition. There is progressive widening of swath forecast for increasing wind speed and emission height, coupled with decreasing drop size, until the increasing dilution limits the realization of the lethal dose level. In addition, fall-off in collection efficiency can place a limit to the lowest effective size. For any wind speed there is shown to be a particular volume median drop diameter which maximizes the lethal swath; however, the downwind displacement of the lethal swath is shown to increase progressively with increasing wind speed, emission height and decreasing drop size. Thus the choice of optimum size depends on a trade-off between these effects and accurate targeting in strong winds requires either (a) an appropriate upwind offset in the line of emission (or decrease in emission height), or alternatively (b) some increase in drop size to limit displacement. For aerial spraying from 10 m height at a flow rate of 151 min −1, a volume median diameter (VMD) ⩽ 120 μm is forecast to justify the recommended 100 m track spacing, but the crosswind velocity component should exceed 2 m s −1. The lethal effect is shown to be lost if the emission height is increased to 20 m, unless there is a compensatory increase in flow rate. The VMD may have to be increased to 180 μm to maintain precision in winds above 5 m s −1 if a low emission height is not feasible; however, if the target bands are diffuse, it may be preferable to retain the advantage of the wider swaths accruing from the smaller VMDs (i.e. large reduction in application rate if allied to wider track spacing), while accepting some accompanying uncertainty in placement.