Large scale demonstration of exclusion fences for management ofcabbage root maggot Delia radicum: Opportunities for IPM?

Abstract: The distribution of cabbage root maggot (CRM), Delia radicum, adults, eggs anddamage was studied in large-scale fenced and unfenced fields of rutabagas in Delta, BritishColumbia in 2009. In the field enclosed by a 1.3m high exclusion fence (2.7ha), numbers ofCRM females that were able to enter the field were highest on yellow sticky traps situated 1minto the rutabagas, relative to numbers on traps 6, 13 and 28m into the field (respectively, 65.9,72.7 and 93.2% fewer flies than at 1m). In the open field (3.2ha), CRM females entering the fieldwere also highest on sticky traps situated 1m into the rutabagas relative to numbers on traps 6, 13and 28m into the field (respectively, 19.7, 39.3 and 70% fewer flies than at 1m), but populationdecline with distance into the field was much more gradual than inside the fenced field. Thistrend was also observed with plants infested with CRM eggs, where counts dropped rapidly in thefenced field between 1.5m and >13m (92.5% drop), relative to a more gradual decline in the openfield between 1m and 13, 43 and 61m (respectively, 56.9, 77.6 and 91.4% fewer plants witheggs). The proportion of rutabagas considered culls in the fenced field at 1, 6, 13 and >28m intothe field were, respectively, 0.23, 0.23 0.01 and 0.03, whereas the proportion culls at 1, 13, 43and >61m into the unfenced field were, respectively, 0.85, 0.45, 0.25 and 0.07 at harvest. Thesedata showed that CRM females and associated egg deposition and damage to rutabagas in thefenced field were strongly aggregated at the outer perimeters of the planted field, with themajority of CRM activities being within 6 and 13 m of the fence. These trends were not observedin the unfenced field. The data suggest that exclusion fences could be used on large-scaleplantings of brassica crops (>1ha) as a first line of defence against CRM attack, combined withvarious additional control methods to reduce CRM populations accumulating 1-13m inside thefence perimeter.