Outcome of Biological control of Delia sp. in cole crops with rove beetles, Aleochara sp. (Part 2)

The objective of this study was to examine the use of rove beetles, Aleochara spp., for the biological control of root maggots, Delia spp. in organic cole crops. We were interested in manipulating Aleochara numbers by either conservation strategies or by augmentative releases of mass-reared Aleochara bilineata. The results to date indicate the following

1. Mulching with straw and composted yard waste significantly reduced the root maggot population infesting individual broccoli plants. Aleochara spp. parasitism activity was higher in the mulched plots than control. Parasitism of root maggot pupae by Aleochara beetles was 37.5% (or 6 out of 16 pupae) in mulched plots and only 9.5% (or 2 out of 21 pupae) in the unmulched control plots; this increase in percent parasitism was statistically significant. Also, carabid and staphylinid beetle abundance was higher in straw mulched plots than control plots, during the early part of July. For the rest of the sampling period (May to August) there appeared to be no difference in natural enemy activity between mulched and control plots. Mulching may have also interfered with the oviposition activity of female Delia spp. flies. However, an experiment involving the introduction of D. radicum eggs to straw mulch and control plots demonstrated that D. radicum mortality (egg to larvae) was higher in mulch plots. These results further suggest that natural enemy activity, in this case predation, was higher in the mulched plots than in the control plots. From this one year of data, it appears that mulches may be an effective tool for suppressing root maggot populations, increasing parasitism by Aleochara and enhancing predator activity in general.

2. Data collected as part of the mulch survey through out the season, indicated that natural enemy levels within the field were generally low in the beginning of the spring and did not build-up until July. Our study of natural enemy populations in field margins, also conducted in the 2001 growing season, demonstrated higher densities of carabid and staphylinid beetles in grassy field margins than in weedy or bare ground field margins. These findings support the idea that natural enemies do not overwinter in the field and provides strong support for a tactic such as a beetle bank, which provides natural enemies with a grassy overwintering habitat within the field. A beetle bank was constructed on a commercial organic farm in the fall of 2001. The beetle bank, built on Snow Farms, Ladner BC, will be monitored in 2002 as part of a study that will continue to examine the impact of refuges for conserving natural enemies and enhancing biological control in agricultural fields. This study will be conducted at Washington State University and will include several organic farms in the state.

3. Augmentation trials with A. bilineata in 2001 were inconclusive. No significant reduction in root maggot populations resulted, however, these trials were conducted in the summer in dry weather and it is expected that in the cooler wetter spring conditions, results may differ. Augmentation field trials should be repeated in the spring and in irrigated fields in the summer. Mass-production of A. bilineata was also a major limiting factor for this portion of the study. A study to develop A. bilineata production protocols will be conducted in 2002.

4. Releases of marked A. bilineata to measure dispersal in the field was unsuccessful as none of the 250 individuals released were recovered in the 20 pitfall traps placed in a grid pattern in the field. It is unclear if this lack of recapture is due to rapid dispersal of beetles from field or the lack of dispersal of beetles within the field. Also dispersal of beetles may differ depending on the amount of plant cover in the field. For example, this study was conducted in the summer when plant cover was very high. Dispersal activity may be greater in the spring when plant cover is lower. Dispersal has been shown to be a major factor in limiting the efficacy of introduced biocontrol agents, therefore, it will be important to conduct a dispersal study in order to develop release protocols for A. bilineata. A more effective dispersal study design should utilize a denser grid of pitfall traps and include other types of traps, e.g. sticky traps, to capture individuals dispersing via flight.