The role of fatty acids and soaps in aphid control on conifers.
Experiments were carried out in Scotland to determine the toxicity of various fatty acids and their potassium salts to 2 aphid pests of conifers, namely Elatobium abietinum (Wlk.), which was kept on potted Sitka spruce (Picea sitchensis) under outdoor conditions and subsequently brought into the laboratory, and Gilletteella cooleyi (Gill.) (Adelges cooleyi), brought into the laboratory on shoots of Douglas fir [Pseudotsuga menziesii]; the toxic effects on their coccinellid predator Aphidecta obliterata (L.) were also tested. Both the fatty acids and potassium soaps possessed comparable toxicity to E. abietinum, the LC50s ranging from around 0.1 to above 10%. Most fatty acids remained in suspension for a short time only, while soaps with a carbon chain greater than 14 tended to gelate. This minimised the potential of both agents under field conditions. Since larvae of A. obliterata were more susceptible than adults to several of the soaps, their application during larval development of the predator should be avoided. With the exception of potassium oleate, LC50s for G. cooleyi in the laboratory were lower than the field values because of the masking effect of foliage. The saturated soaps potassium caproate, caprylate, caprate and laurate possessed satisfactory toxic and spraying characteristics, but were marginally inferior to the unsaturated potassium oleate and lineolate in both respects.