Investigating the Effect of Hedgerows to Enhance Natural Biological Control
Vegetable producers in the Central Coast of California are establishing hedgerows in the borders of their fields. Theoretically, adding perennial vegetation in annual crop systems can increase arthropod diversity. The goal of this project is to determine if hedgerows enhance biological control potential in annual vegetable systems by providing habitat to key insect natural enemies of crop pests. We monitored indicator insect natural enemies and pests of vegetable systems on six plant species at five hedgerow sites, tracked dispersal rates of pests and natural enemies from hedgerows into adjacent vegetable fields and tested parasitism rates in those fields and fields without hedgerows.
The objectives of this research are 1) to investigate the effectiveness of hedgerows in the Central Coast of California for biological control in mixed-vegetable systems and 2) to make practical recommendations to producers on semi-natural habitat management for pest management in mixed-vegetable systems. To meet these objectives, three main areas are addressed in this research project: 1) monitoring key arthropod natural enemies and pests attracted to hedgerow vegetation, 2) tracing the movement of these indicator arthropods into adjacent vegetable fields, and 3) measuring the rate of biological control (parasitism) in the vegetable fields. Research findings will be widely disseminated through on-farm workshops and publications to help inform on-farm habitat management decisions and conservation biological control practices.
Study 1: Monitoring key arthropod natural enemies and pests attracted to hedgerow plants.
In 2005 and 2006 we used biweekly vacuum samples to monitor indicator insect abundances at six hedgerow plants (Achillea millefolium, Baccharis pilularis, Ceanothus spp., Eriogonum giganteum, Heteromeles arbutifolia, and Rhamnus californica) within five different hedgerows. We have found that the plants, the bloom period, time of the sample and the particular hedgerow all significantly affect insect abundances for the different taxa. The strongest plant-insect association was found between parasitic Hymenoptera and Baccharis pilularis where overall mean abundance was almost four times higher than at the other plants.
Study 2: Tracing the dispersal of indicator insects from hedgerows into adjacent crop fields.
We conducted an insect tracing experiment three times during the summer of 2006 and 2007. Insects foraging on hedgerow plants were marked with a yellow fluorescent pigment, which was sprayed on hedgerow vegetation (15 L of the dye solution/~150 meters of hedgerow) twice during each trial. We did this experiment at four different vegetable fields with bordering hedgerows and placed 10 traps immediately following the second spray at 10m, 25m, 50m, and 100m from the hedgerows (40 traps/field). The mark is distinguished on insects with a UV light and microscope.
Initial analyses suggest that a higher percentage of natural enemies are using the hedgerows than pests, with the exception of Lygus hesperus. The majority of natural enemies and pests were found up to 100 meters from the hedgerow, although the rate of movement was higher at 25 meters.
Study 3: Measuring the rate of biological control in vegetable fields with and without hedgerows.
In 2006 and 2007 we set out first and second instars of Trichoplusia ni (Lepidoptera: Noctuidae) on 20 potted collard plants in each of eight vegetable fields (four with hedgerows, four without hedgerows). In each field 10 sentinel pots were placed 10-25 meters (“near”) and 50-100 meters (“far”) from the hedgerow or field margin. The crops surrounding the pots were most often a cabbage or lettuce variety, plants that host Tricholplusia ni. We conducted this experiment three times in each year.
For all the trials we found no significant differences in parasitism rates between hedgerow and non-hedgerow fields, nor between near and far distances. Across the field sites, the percent of pots where incidents of parasitism occurred ranged from 0 to 100. The lack of a clear and significant hedgerow effect on biological control is most likely due to the variability across the different sites. Although we used sites that were similar in land management (organic), cropping system (vegetable), hedgerow location on farm (upwind) and plants used in the hedgerow, we were unable to control other factors that vary: planting times of crops, acreage, microclimate, and surrounding landscape.
Next Steps in Research
Over the next six months we will be mostly focused on finishing analysis of sticky traps and vacuum samples and using these results to write two papers for a doctoral dissertation and journal publications. We plan to collaborate with other UC researchers working on conservation biological control and local outreach organizations, such as Community Alliance with Family Farmers and Natural Resources Conservation Services to disseminate results in an outreach event in summer 2008.
Impacts and Contributions/Outcomes
Overall, this research provides evidence to producers in the Central Coast region that diverse hedgerows are providing stable habitat for natural enemies important in biological control; and that these insects do not just reside within the hedgerow, but also actively move into adjacent crop fields. This is an important finding given that annual vegetable systems are highly disturbed environments (with at least three major tillage events per year), which can destabilize natural enemy communities. Another important finding is that insect pest use of hedgerow plants was relatively low in comparison to natural enemies, which should help to reassure producers that hedgerow vegetation should not exacerbate pest problems. While we were unable to make a direct link between hedgerows and improved biological control rates in the field, we feel that a more controlled experiment would better address this research component. In the meantime, our initial analyses suggest that hedgerows improve the biological control potential of vegetable systems by providing a stable habitat (floral resources and refuge) throughout the growing season. Other researchers (Idris and Grafius 1995, Costamagna and Landis 2004) have shown that parasitoid longevity and fecundity are positively associated with floral resource availability and moderated microclimates (shade or cooler temperature). We hope that by providing scientific evidence in support of habitat enhancement for the conservation of key natural enemies, grower dependence on pesticides will decline and biological diversity will increasingly be restored in the Central Coast agri-landscape.
A broader outcome of this research is our participation in the newly formed Western Region Conservation Biological Control Work Group, a consortium of over 30 researchers working on similar topics in biological control. We participated in the first meeting of the work group last March where we shared our research program in poster sessions and handouts and collaborated with the group to outline research needs within conservation biological control. Information gained at this workshop was used to secure funding to further develop the network. We will be attending the next meeting in Portland, Oregon, in February 2008.