Recovery of D&C Red No. 28 from Potato Leaves¹

D&C Red No. 28 is a photoactive red dye that is insecticidal for a number of insect species. Unlike contact insecticides, D&C Red No. 28 must be consumed to be effective. Therefore, the red dye must adhere to leaves on which the insect feeds. During a field test of insecticidal activity of the red dye against the Colorado potato beetle (Leptinotarsa decimlineata L.), we measured the recovery of the red dye from potato leaves treated at rates of 70, 210, and 350 g per ha. When the dye was applied alone, we were able to recover over 91% of the red dye at 6 h post treatment and over 30% at one day. At 2 days post treatment, the limit of detection was reached (5 ng D&C Red No. 28/cm² leaf). In field studies, PEG 200 which improves dispersal, also improved adherence of red dye to leaves. In the laboratory, adjuvants such as PEG 200, Tween 80, and Gelva® also improved red dye adherence to leaves.