Cultivated and wild host plants supporting populations of the cabbage root fly

SUMMARY To determine the relative importance of different potential host plants for supporting populations of the cabbage root fly, wild and cultivated crucifers were sampled for pupae at four locations during 1971‐3. In addition, eighty‐three species of Cruciferae native to or naturalized in the British Isles were inoculated with cabbage root fly eggs in a glasshouse to determine which species could support the larvae. In the field, most pupae (28‐7/plant) were collected from untreated cauliflowers. Fewer pupae (i‐7–8‐6/plant) were obtained from untreated crops of Brussels sprout, cabbage and swede. Applications of chlorfenvinphos reduced populations to two or less pupae per plant on all crops. Of five common weed species sampled, only Raphanus raphanistrum produced as many pupae as certain of the untreated brassica crops. Pupae did not occur in samples from Capsella bursa‐pastoris but Sisymbrium officinale, Thlaspi arvense and Sinapis arvensis usually supported low numbers. In the glasshouse, only forty‐four of the eighty‐three cruciferous species tested supported larval development. Most pupae were obtained from 12‐wk‐old plants of Barbarea intermedia. B. stricta, Brassica napus, Cochlearia officinalis and R. raphanistrum and from 24‐wk‐old plants of Brassica rapa, Erysimum aureum, Cochlearia anglica and C. officinalis. Plant age considerably affected pupal production. Plants within a genus often gave similar results, pupae not being recovered from any of the Diplotaxis or Arabis species tested, or from young plants of Erysimum spp. In other families, Reseda lutea and R. luteola supported larval development, but the widely‐separated Plantago major did not. Arguments for and against the removal of cruciferous weeds from the vicinity of cruciferous crops are discussed.