Reduction of Delia radicum attack in field brassicas using a vertical barrier

Insecticide options for controlling Delia radicum (L.) (Diptera: Anthomyiidae) in brassica crops are now limited, and alternatives are needed. Vertical mesh barriers impede females reaching, and ovipositing in, small‐scale crops. We tested this pest management technique in a commercial crop to determine whether such barriers would also reduce dispersal of females at larger scales, and whether there was any detectable pattern to the spatial distributions of female D. radicum, eggs, and damage in a rutabaga crop [Brassica napabrassicae (L.) Mill (Brassicaceae)]. A mesh fence of 1.3 m high with an externally facing 30‐cm overhang was erected around the perimeter of a 2.7‐ha field. Yellow sticky traps were positioned at 12 points outside and at 104 locations along transects inside the barrier to capture adult flies. Eggs were sampled from soil around 10 plants at 62 locations and damage was assessed at harvest using a subset of 38 of these locations. There was a substantial reduction (up to 96%) in the number of female flies caught per trap inside the fence compared with outside and a change in the male:female sex ratio from 0.36 outside to 0.92 inside. Female numbers inside and outside were related and sampling date, trap location, and trap face explained significant proportions of the variance in both data sets. There was no relationship between the size of the population and the distance flies penetrated into the field or the proportion of flies recovered in the outer 10 m. Female, egg, and damage counts were largely restricted to the perimeter of the field. Although eggs and damage were related, trap counts did not reflect their spatial distributions. Damage was related to oviposition in the later weeks leading up to harvest. We conclude that yellow sticky traps are an ineffective monitoring tool compared with egg sampling, exclusion barriers can reduce numbers of females reaching a crop, and the system has the potential to deliver reductions in damage and pest control costs at a commercial scale.