Parasitoids of cabbage seed weevil and brassica pod midge in oilseed rape.
Oilseed rape (Brassica napus (L. )), the second most important arable crop in the UK after
cereals, is attacked during the summer by cabbage seed weevil Ceutorhynchus assimilis
(Paykull) and brassica pod midge Dasineura =brassicae (Winnertz). In mainland Europe,
these two 'pests are known to be attacked by a range of hymenopteran parasitoids but
virtually nothing is known about their parasitoids in the UK. This study found that the
ectoparasitic Trichomalus perfectus (Walker) (Pteromalidae) was the most abundant species
attacking C. assimilis, achieving, in unsprayed crops, levels of parasitism up to 73%. The
egg-larval Platygaster subuliformis Kieffer (Platygastridae) was reared from D. brassicae
for the first time and is probably a new species record for the UK; it was the most
abundant species attacking the midge in winter rape. The endoparasitic Omphale clypealis
(Thomson) (Eulophidae) was the most abundant species attacking D. brassicae in spring
rape. The complexity of assessing parasitism in the multivoltine D. brassicae is discussed.
In-field temporal and spatial distributions of hosts and parasitoids were studied using a
novel randomization test and the results related to known biological characteristics of
species. Using a new design of insect flight trap, P. subuliformis was shown to respond to
2-phenylethyl isothiocyanate and O. clypealis to 2-propenyl isothiocyanate, both volatile
secondary plant compounds, produced by brassicaceous plants on damage. Field trials
demonstrated that the post-flowering application of the organophosphate insecticide
triazophos, has a detrimental effect on T. perfectus, killing adults and larvae, whereas the
synthetic pyrethroid alphacypermethrin was less harmful because it was applied before the
main migration of T. perfectus into the crop. The importance of the parasitoids ' of rape
pests to integrated protection strategies for the crop are discussed.