In the first year of this study (2000), project objectives were:
- To determine the species composition and phenology of native rove beetles, Aleochara sp., in Southern British Columbia, and levels of parasitism activity on CRM;
- To determine vegetative and field management techniques that enhance native rove beetle populations; and
- To compare the efficacy of native rove beetles vs. inoculative releases of mass reared rove beetles, species A. bilineata, in controlling cabbage maggot in the early season.
Research was conducted in four organic and one conventional field, bearing broccoli, cauliflower and potato crops, as well as in field margins.
Year 1 results: Native rove beetles were demonstrated to be present in organic and conventional cole crop fields. Of 301 Staphylinid beetles captured, 59 were of the genus Aleochara. Parasitism of overwintering cabbage maggot pupae was demonstrated. More rove beetles were found in grass margins than in tree/shrub margins or within the fields. Grass-clipping mulch around plants was shown to increase presence of rove beetles in the field. A. bilineataintroductions reduced numbers of cabbage maggots infesting plants in small field cage experiments. Peak rove beetle catches were June 15-July 6, corresponding with 2nd peak of cabbage maggot.
In the second year (2001) of this study, objectives were:
- To compare the efficacy of two mulches (barley straw and composted yard waste) for enhancing natural enemy activity for control of cabbage root maggot; including native rove beetles, Aleochara sp., and for minimizing cabbage root maggot populations and for weed control;
- To establish a demonstration beetle bank and monitor its impact on natural enemy and pest populations over time;
- To document dispersal pattern and predation on cabbage root maggot eggs by released rove beetles, A. Bilineata in organic broccoli; and
- To develop a rearing protocol for introduced rove beetles, A. bilineata.
The study took place in three organic broccoli fields.
Year 2 results. Mulch treatments had significantly fewer pupae and maggot counts than control treatments; significantly more pupae were parasitized by native rove beetles in mulched plots. Fewer weeds were found in mulched plots than in the control. Staphilinid beetles increased under straw mulch, carabids decreased. Mulch effects on predators varied over the season, with high numbers of natural enemies in July, declining to no significant difference the rest of the season. Beetle bank was established and pitfall traps setup for baseline monitoring. 400 offspring were reared from 57 collected adults. Of these 250 marked, reared beetles were released in a dispersal study, but none were recovered during a two week monitoring period. In a cage study testing releases of reared beetles on populations of CRM, there was no significant difference in broccoli root CRM populations between caged plants having augmented releases of A. bilineata, and those plants without.
Year 3 (2002) objectives were:
- To continue to monitor the species composition, phenology and levels of parasitism activity of native rove beetles, with particular emphasis onAleochara sp; and
- To collect information regarding other parasitoids of CRM, in particular cynipid wasps.
The study took place in six organic and six conventional fields.
Year 2 results: Three fields with "good" margins and three fields with "poor" margins were identified. There was no significant difference in predator activity (spider, ground and rove beetle) between margin types. Natural enemy activity was similar in good and poor margins at the beginning and end of the season, but greater in good margins mid-season, during May and June. CRM pupa parasitism increased over the season. Total number of predators caught in traps increased over the season, peaking in June. Parasitism by hymenoptera was observed during Sept.-Nov. collection dates. Organic/good margin fields had more beneficial focal species than conventional/poor margin fields.
We have determined that there is a truly complex web of insects that are potential natural enemies of CRM in southwestern BC and northwestern Washington. This community of natural enemies includes six species of beetles (ground and rove), that are predators of CRM eggs, and also severeal species of parasitoid wasps and Aleochara spp. rove beetles that attack the larval and pupal stages of this pest.
Our overall conclusions are that certain attributes of field margins (grassy perennial vegetation, high percentage of ground cover, relatively undisturbed), can provide a refuge for natural enemies, but that dispersal from margins to the field may be limited.