The status and potential of parasitoids of seed weevil and pod midge on winter oilseed rape

Abstract

Three parasitoid species were found to attack cabbage seed weevil (Ceutorhynchus assimilis), namely: Trichomalus perfectus and Mesopolobus morys (Hymenoptera: Pteromalidae), both ectoparasitoids of the larvae, and Microtonus sp. (Hymenoptera: Braconidae), an endoparasitoid of the adults. T. perfectus was the most important parasitoid, killing up to 71% of larvae inside pods in experimental field crops not sprayed with insecticide. T. perfectus was also widely distributed on unsprayed commercial crops in England and Wales, although levels of parasitism were usually lower. Adult females of T. perfectusexerted additional mortality on seed weevil larvae by host-feeding.

Five species of endoparasitoid were reared from cocoons of brassica pod midge (Dasineura brassicae), namely: Aphanogmus abdominalis (Hymenoptera: Ceraphonidae), Omphale clypealis (Hymenoptera: Eulophidae), Inostemma sp. and two species of Platygaster (A & B) (Hymenoptera: Platygastridae). Platygaster (species A) was the most abundant parasitoid of pod midge in winter rape, whereas O. clypealis was the predominant parasitoid of pod midge on spring rape. The multivoltine life-cycle and extended diapause strategy of pod midge presents problems for the assessment of the impact of parasitoids for their control, as do complex host/parasitoid interactions.

Platygaster (species A) was attracted to the chemical 2-phenylethyl isothiocyanate, a volatile substance released when brassicaceous plants are damaged. A novel, convenient trap, using isothiocyanates as lures, was developed to monitor parasitoids and midges.

The distribution of pests and parasitoids throughout the crop was never uniform, and no correlation was found between the spatial distributions of seed weevil and pod midge When T. perfectus were searching for hosts within an oilseed rape crop, their distribution was negatively correlated with that of adult weevils. The in-field distribution of Platygaster (species A), however, was positively correlated with that of pod midge.

A post-flowering spray of triazophos was harmful to T. perfectus, significantly reducing levels of parasitism in field-based experiments. A pyrethroid applied during flowering was less harmful to T. perfectus because it was applied before adults of this species had entered the crop. The effects of pyrethroid sprays on other parasitoids are unknown. The commercial use of triazophos has declined significantly since 1992 and numbers of the parasitoid T. perfectus on crops have increased over this period.

Farmers should encourage parasitoids on rape crops by resorting to using insecticides only where treatment thresholds for seed weevil are exceeded. When spraying is justified, a pyrethroid as recommended during flowering and not triazophos at the end of flowering should be used, to safeguard T. perfectus. The adoption of new treatment thresholds in 1992 means that now, even fewer commercial rape crops in England and Wales require treatment against seed weevil or pod midge; however, pesticide usage data confirm that many crops are still sprayed unnecessarily. Where treatment with an insecticide is justified, because seed weevil numbers exceed the threshold of two weevils per plant, crops usually exceed this thresholds by only a small margin. There is, therefore, considerable potential for naturally occurring parasitoids to exert sufficient control in such cases to obviate the need for applying a chemical treatment. This would tip the balance in favour of lower pesticide usage, and bring about both economic and environmental benefits.