Pesticide deposition on citrus orchard soil resulting from spray drift and runoff.

Abstract

A substantial loss of pesticide spray mixture is evident during application to citrus trees. Some losses occur as a result of the spray passing between and through target trees. Additional losses occur as the result of runoff and drip from the target trees. The total amount of spray not retained on the tree has been variously estimated to range from 25-40% of the spray gallonage used, depending upon orchard conditions and operational parameters. The waxy composition of the citrus leaf surface is not conducive to the retention of large quantities of aqueous spray mixture. In principle, the amount of runoff could be limited by maintaining the gallons of spray/tree low. The amount applied, however, is dictated by the requirements of the pestcontrol situation (CARMAN 1975). Gallonage can be minimized if pests reside on the exterior foliage (lepidopteran larvae, aphids, katydids) or the pest is active (mites, thrips) and would be likely to contact treated foliage even if not directly exposed to the spray. If the pest is sessile or may frequently reside in the interior of the tree (scales), increased gallonage is required to penetrate the exterior barrier of foliage to wet the foliage, fruit, twigs, and branches inside the tree. Runoff losses of pesticides are then countenanced in order to achieve a thorough wetting of the tree. The most frequently encountered gallonages used for scale control in mature California citrus groves range from 1500 to 3500 gal of spray/A. Aside from the economic disadvantage associated with the non-useful deposition of pesticides, the pesticide reaching the soil surface has been suggested (GUNTHER et al. 1975, SPENCER e__~t al. 1975) as a source of toxicants to workers entering organophosphorus pesticide-treated fields. Tests were therefore conducted to characterize with reasonable accuracy the magnitude and distribution of spray mixture reaching the orchard floor when dilute spray gallonages in the range generally used are applied with oscillating boom spray equipment of the type currently in use.