The Relation between Structures and Toxicity of Oxygenated Aliphatic Compounds Homologous to the Insecticide Octanoic Acid and the Chemotaxis of Two Species of Drosophila

Octanoic acid is the main toxic compound for Drosophila melanogaster of the fruit of Morinda citrifolia, the host plant of Drosophila sechellia. It is known that octanoic acid is toxic for many insect species. The possible relation between structure and toxic activity was compared in both species. Among fatty acids with 4 to 10 carbons, a stronger toxicity of the 8-carbon chain length was observed for D. melanogaster flies. Aldehydes and ethylic esters are much less toxic than the corresponding acids. Alcohols with similar chain lengths are also highly toxic but with similar LD50 values for both species. The structure-activity studies together with species differences suggest two different toxicity mechanisms. One of these is linked to the role of octanoic acid, for which the D. melanogaster strain Indifferent, mutant for its atypical nonrepulsion by nonanol, shows resistance comparable to that of D. sechellia. Two other behaviors were shown to be modulated by octanoic acid and in a species-different way: oviposition and chemotaxis. The oviposition of fertilized D. melanogaster females is inhibited, while that of D. sechellia is stimulated and in a quantitatively similar way. Hexanoic acid induces a similar, strong, stimulating effect on the egg laying of D. sechellia but only weak inhibition in D. melanogaster. Both acids also attract D. sechellia and repel D. melanogaster females. Behavioral data are discussed together with genetic information.