Effect of secondary plant substances on host-plant selection by the cabbage root fly

Abstract

A list is provided of 9 isothiocyanates and 1 nitrile detected in cruciferous plants that are potentially attractive to insect pests. The chemicals from this list that were detected in 8 cruciferous plants are tabulated in relation to the extent of oviposition on and around the plants by adult females of Delia brassicae (Wied.) (Erioischia brassicae). In laboratory tests with 30 isothiocyanates as attractants for larvae of D. brassicae, 2 compounds that did not occur in nature (tert-butyl isothiocyanate and 2-methoxyphenyl isothiocyanate) were more attractive than allyl isothiocyanate, the compound that is at present in use in field traps, and 5 naturally occurring compounds were at least as attractive as this compound. Both larvae and adults were attracted to low concentrations of allyl and tert-butyl isothiocyanate, and repelled by higher concentrations. A method for testing adult attractants in field cages is described. The use of attractants in the field is discussed, with special reference to captures of Ceutorhynchus assimilis (Payk.) in traps baited with allyl isothiocyanate in the United Kingdom in 1976. It is suggested that an attractant that was 5-10 times as potent as the compound now used would probably be sufficient to control D. brassicae in the field if used in traps placed at 20/ha.