Cabbage seedpod weevil (Ceutorhynchus assimilis, Paykull) management by trap cropping and its effect on parasitism by Trichomalus perfectus (Walker) in oilseed rape

The feasibility of managing the cabbage seedpod weevil (Ceutorhynchus assimilis, Paykull) in winter oilseed rape (Brassica napus, L.) using an early-flowering trap crop was studied during four seasons near Griffin, GA. USA. The trap crop consisted of an autumn-planted winter type variety with an autumn-planted spring type variety of B. napus which flowered two to three weeks before the main crop. The trap crop system was compared to conventional winter oilseed rape production without insecticidal control or with control of C. assimilis using esfenvalerate. The conventional control treatment greatly reduced adult numbers during flowering and larval pod infestations in all years and prevented losses of grain yield in two years and of grain quality in three of the four years. The trap crop treatment reduced adult numbers during flowering by about 60% in 1994 and 80% in 1995, and it reduced larval infestations in pods by 60% in 1995, but not in the other three years. Trap cropping also did not prevent significant losses in grain yield in any year. Greater than 96% of the larval parasitoids recovered from C. assimilis were Trichomalus perfectus (Walker). The conventional use of esfenvalerate during bloom indirectly reduced T.: perfectus numbers because of a reduction in the number of available hosts, but the parasitoid typically attacked a greater proportion of remaining host larvae. The trap crop treatment did not adversely affect parasitism of C. assimilis by T.: perfectus, but did not reliably control C. assimilis damage in autumn-sown oilseed rape. A trap crop system may be effective with lower populations of C. assimilis, but to be feasible in the USA, effective C. assimilis adulticides such as esfenvalerate must be registered for use on oilseed rape. (C) 1998 Elsevier Science Ltd. All rights reserved.