Mode of Contact of Chlorpyrifos with Russian Wheat Aphid (Homoptera: Aphididae) in Wheat

Abstract

Laboratory and greenhouse experiments were conducted to determine the mode of contact of chlorpyrifos with Russian wheat aphid, Diuraphis noxia (Mordvilko), in wheat. Triticum aestiuionm L. When infested plants were exposed to the vapor only from chlorpyrifos treatments, there was no significant aphid mortality. Apparently, the protective leaf curl surrounding the aphids shields them from toxic vapors generated during the spray operation. When aphids were exposed in vitro to the vapor only from chlorpyrifos-treated leaves, a definite, toxic effect was observed. Different pmts of infested, three-leaf stage plants were covered with plastic sleeves during spraying to determine which chlorpyrifos deposits caused a toxic effect. With the aphids located in the bottom of the third leaf, the only deposits causing significant mortality were those on the third leaf itself. Further leaf-cutting and leaf-painting experiments showed that aphid mortality was caused only by deposits on the bottom half of the third leaf. Toxicity from direct contact with dried chlorpyrifos residues on various leaf parts was generally low. A two-step process is postulated for the toxic action of chlorpyrifos against Russian wheat aphid. The initial mode of contact is as a penetrating vapor from the fresh spray deposited on the outside of the protective leaf curl. A secondary, more prolonged toxic action occurs as the dried surface residues on the leaf curl penetrate into the tissue, as a vapor or as dissolver! residue, and contact the aphids as they feed.