Can. Ent. 108: 1021-1030(1976) In Hylemya brassicae (Boucht) little oviposition occurred when access to host plant material was denied. The mustard oil, allyl isothiocyanate (AITC), stimulated the flies into greater activity and attracted them to its source. Sinigrin and four other glucosinolates (mustard oil glucosides) tested induced oviposition. AITC by itself did not induce, oviposition, but in the presence of a glucosinolate, small concentrations of AITC caused an increase in the number of eggs laid. Glucose, sucrose, casein, wheat germ oil, and a mixture of B-vitamins did not influence oviposition at the concentrations tested, but casein hydrolysate inhibited it. The role of glucosinolates and common nutrients in host selection by H. brassicae is discussed.
IntroductionThe cabbage maggot, Hylemya brassicae (BouchC) (Diptera: Anthomyiidae), is a pest of cruciferous crops. Except for a report (Traynier 1965) of egg laying near some plants of the families Resedaceae, Tropaeolaceae, and Capparidaceae, under experimental conditions, its host range is known to be restricted to the Cruciferae.The adult fly lays eggs in soil close to the host plant, and the neonate larvae bore into the roots where they feed and grow. It is obvious that although the larvae may exert some control over the choice of their host plant, it is the ovipositing adult that takes the first step in host selection. Earlier studies (Traynier 1965(Traynier , 1967bCoaker 1969) established a correlation between the characteristic 'secondary plant chemicals' produced by crucifers, viz., glucosinolates ('mustard oil glucosides') and the oviposition preferences of this insect. (In this paper, the term 'oviposition' will be used in the wider sense to cover the final act of depositing eggs and the behavioural sequences immediately preceding it.) Traynier ( 1 967b) found that in artificial test units which did not resemble plants, contact chemostimulation provided by sinigrin (a glucosinolate) and allyl isothiocyanate (AITC) (a 'mustard oil', hydrolysis product of sinigrin) stimulated oviposition. Among other possible host plant chemicals he tested, carbon disulphide and P-phenylethylamine stimulated oviposition; results with some other chemicals were inconclusive. In field tests of rape varieties (Brassica napus L.), Coaker (1969) recorded a 50% increase in the number of eggs deposited for a four-fold increase in the total glucosinolate content of leaves.The studies of Traynier ( 1 9 6 7~) and Coaker and associates (cited by Coaker and Finch 1971) also implicated the highly volatile 'mustard oils' as stimulants or attractants for gravid females of H . brassicae. Further, Eckenrode and Arn (1972) showed that AITC was as effective as fresh cruciferous plants in attracting adults (both sexes) under field conditions. Although these studies established the importance of glucosinolates as oviposition stimulants, much remains to be learned of the qualitative and quantitative relationship between glucosinolates and the ovipositional behaviour of this insect...