OFRF funding was awarded for this project in Spring 2000, Fall 2000, and Spring 2001.

A final report describing the results of this project was submitted in October 2002.

Investigator: Renee Prasad, E.S. Cropsonsult, Vancouver, British Columbia
Project locations: British Columbia, Washington State

Summary
The production of cole crops in the lower mainland of British Columbia is expanding. As more growers make the transition from conventional to organic production there is an increased need to develop an integrated pest management (IPM) system for cole crops that emphasizes biological, cultural and other non-chemical methods for insect and disease management.

Mulch applications for vegetation management to enhance populations of rove beetles in the field. Foreground plot is composted yard waste mulch treatment followed by bare ground control, composted yard waste and finally straw mulch treatments.

Among the main insect pests for cole crop production in the Pacific Northwest are cabbage root maggots (Delia radicum, family Staphilinidae), aphids, flea beetles, thrips, and several lepidopteran pests. The cabbage root maggots (CRM) are especially harmful when cabbage seedlings are small and cannot sustain loss of root tissues.

Currently the non-chemical options available for CRM control are restricted to biological control with nematodes. As well, growers can time plantings to avoid peaks in CRM populations. Timing for plantings and nematode applications is based on monitoring the population of CRM with sticky traps. Remay row covers are used in seedling beds to protect plants from egg laying, However, physical control is not feasible once seedlings have been planted out in the field.

Given the limited number of existing non-chemical CRM controls available for organic growers it is important to explore other options. One such possibility is the rove beetle, Aleochara bilineata. This species is both a predator of the CRM eggs and a parasite of the pupae, thus contributing to mortality at two stages of the lifecycle. A. bilineata is found in southern British Columbia and the Pacific Northwest. Researchers in other areas have found up to 90% parasitism of CRM by A. bilineata in fields where insecticides have not been used for extended periods of time. However levels of parasitism of CRM by native populations of A. bilineata have not been documented for southern British Columbia.

The impact of native populations of A. bilineata for CRM control could be enhanced in two ways. Firstly through vegetation management practices that increase the habitat for the beetles. These practices may include maintaining heterogeneity at field margins, in-field use of beetle banks, or mulching. Secondly, several researchers have suggested introduction of mass-reared A. bilineata for early season control of CRM. The first generation of A. bilineata does not coincide with the laying of the first generation of CRM eggs in the early spring. It is this first generation of CRM eggs/larvae that often cause the most serious damage.

In the first year of this study (2000), project objectives were:

1. To determine the species composition and phenology of native rove beetles, Aleochara sp., in Southern British Columbia, and levels of parasitism activity on CRM;
2. To determine vegetative and field management techniques that enhance native rove beetle populations; and
3. 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. bilineata introductions 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:

1. 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;
2. To establish a demonstration beetle bank and monitor its impact on natural enemy and pest populations over time;
3. To document dispersal pattern and predation on cabbage root maggot eggs by released rove beetles, A. Bilineata in organic broccoli; and
4. 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:

1. To continue to monitor the species composition, phenology and levels of parasitism activity of native rove beetles, with particular emphasis on Aleochara sp; and
2. 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.