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 was to determine if hedgerows enhance biological control potential in annual vegetable systems by providing habitat to key natural enemies of crop pests. I monitored indicator natural enemies and pests of vegetable systems on six plant species at four farm sites with hedgerows, 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.
In the Central Coast of California, growers are establishing hedgerows in the borders of their fields. Hedgerows are linear assemblages of perennial shrubs, trees, grasses and forbs. The value of hedgerows for biodiversity conservation, including beneficial insects, is well documented in Europe (van Emden 1965b, Pollard 1971, Dover 1994, Paoletti 1999), where hedgerows have a long history for their use as fences, windbreaks, erosion control and sources of firewood and timber (Baudry et al. 2000). California hedgerows are more recent additions to the landscape, being promoted for beneficial insect habitat, soil and water conservation, weed suppression, wildlife habitat and windbreaks. Hedgerows in the Central Coast of California can be quite diverse, ranging from 10 to 40 managed species (mostly native). However, farm advisers tend to recommend a core group of hedgerow species (Achillea millefolium, Baccharis pilularis, Ceanothus spp., Eriogonum spp., Heteromeles arbutifolia, and Rhamnus californica) with overlapping bloom periods with the aim of providing insect natural enemies with pollen and nectar resources throughout the year.
Widespread adoption of hedgerows for enhancing biological control services depends in large part on understanding the conditions that maximize arthropod predator and parasitoid colonization and residence within farms and being able to quantify biological control services gained from the practice. Since the quality and accessibility of food resources and shelter varies from one plant to another, and among different natural enemies, the mix of perennial vegetation used in hedgerows will determine, to some extent, the abundance and diversity of natural enemies present.
The objectives of this research were 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 were 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 have been widely disseminated through regional on-farm workshops and national and international conferences. Additionally, future publications in agriculture periodicals and academic journals will help inform both growers and researchers about the habitat quality of hedgerows for the conservation of insect natural enemies.
Key Insect Natural Enemies and Pests Attracted to Hedgerow Plants:
In 2005 and 2006 I 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 hedgerows at four different sites on the Central Coast of California. Insects were sampled at the plant individuals (N=4 or 5/hedgerow) eight times in 2005 and ten times in 2006 between the months of May and October. At each site, the average abundance of indicator insects per plant individual was calculated for each present plant species on each sampling date. The availability of floral resources was measured on each individual for each sampling period.
To determine the availability of floral resources to insects, in 2005, the proportion of each plant’s floral canopy in bloom (percent bloom) was measured by counting the number of open flowers in relation to closed buds on several flower heads and averaging the values for each plant (Andow and Risch 1985). Since percent bloom was only relative to each plant’s floral resources and not comparable among plant species, in 2006 the area of blooming flowers on the vacuumed plant individual was measured. Using a 1 m2-grid, I measured the proportion of the plant cover in flower heads and then sub-sampled four flower heads and averaged the proportion of blooming flowers. Thus, floral resource area was equal to the plant cover sampled (cm2) x percentage of plant cover with flower heads x percentage of blooming flowers within the flower heads.
The Dispersal of Indicator Insects from Hedgerows into Adjacent Crop Fields:
I 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. I did this experiment at four different vegetable fields with bordering hedgerows and placed 10 traps immediately following the second spray at 25 m and100 m from the hedgerows (20 traps/field). The mark is distinguished on insects with a UV light and microscope.
The Effect of Hedgerows on Parasitism Rates of a Sentinel Pest:
In 2006 and 2007 I set out first and second instar larvae of Trichoplusia ni (Lepidoptera: Noctuidae) on 20 potted collard plants in each of eight vegetable fields (four with hedgerows, four without hedgerows) (Figure 3). 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. I conducted this experiment three times in each year.
Key insect natural enemies and pests attracted to hedgerow plants:
Numerous natural enemy taxa important in the natural biological control of crop pests were present in Central Coast hedgerows. Lacewings, minute pirate bugs, ladybird beetles, syrphid flies and parasitic wasps were commonly found on hedgerow plants. The minute pirate bug (Orius spp.) was the most abundant predator sampled on hedgerow vegetation, followed by Coccinellidae, Geocoris spp., Chrysopidae and Hemerobiidae, respectively. Overall, wasp parasitoids were consistently more abundant on hedgerow plants than generalist predators or herbivore pests. Indeed, Chalcidoidea, a super family of minute parasitic wasps, was the most abundant insect group overall, comprising 54% of the entire indicator insects collected in 2006. Chalcidoidea is a large and diverse super family that has been most frequently and successfully used in classical biological control programs.
Key pests of vegetable and fruit crops were also present on hedgerow plants and cumulatively represented 10% and 14% of the indicator insects sampled in 2005 and 2006. The most abundant key pests found were the western spotted cucumber beetle (Diabrotica undecimpunctata undecimpuntacta), followed by the western striped cucumber beetle (Acalymma trivittatum), the western black flea beetle (Phylotretta pusilla) and the western tarnished plant bug or lygus (Lygus hesperus). The western spotted cucumber beetle represented 8.5% and 10% of the indicator insects collected across the sampling periods in 2005 and 2006 respectively. However, the majority of cucumber beetles collected originated from Site C where its highest abundances were found at H. arbutifolia and B. pilularis. Other key pests of vegetables such as the silverleaf whitefly (Bemisia argentifolli), the cabbage looper (Trichoplusia ni) and the imported cabbageworm (Pieris rapae) were either not collected or extremely rare.
Association between Floral Resource Availability and Insect Abundance
In 2005 and 2006, the abundance of Orius spp. was positively correlated with the floral resource availability of all hedgerow plants, except for C. griseus. The strongest associations were with A. millefolium (R2 = 0.52) and B. pilularis (R2 = 0.56) in 2005 and with B. pilularis (R2 = 0.53) in 2006. The mean abundance curves of Orius spp. overlap at hedgerow plants tracking the plants bloom periods, such that it’s abundance is highest on A. millefolium in the spring, on E. giganteum in the summer and B. pilularis in the late summer.
The minute pirate bug is a generalist predator of eggs and soft-bodied insects such as thrips, mites, aphids, whiteflies and small caterpillars, many of which are key pests in vegetable systems (UCANR 2008). Orius spp. have been shown to be effective biological control agents in augmentative releases in greenhouses to control thrips (Chow et al. 2008) and in natural field conditions to control aphids (Costamanga and Landis 2006, Harwood et al. 2007). Given that minute pirate bugs are the most abundant predator found in hedgerows and they appear to be utilizing hedgerow resources (pollen, nectar or alternate prey), hedgerow planting may be a particularly important practice for the management of aphids or thrips.
The abundance of Chalcidoidea was also positively associated with the floral resource availability of A. millefolium (R2 = 0.21) and only weakly associated with the floral resource availability of the rest of the plant species, except for B. pilularis with which there was no association.
Pest species also responded to floral resource availability. The abundance of D. undecimpunctata undecimpunctata was positively correlated with floral resource availability of H. arbutifolia in 2005 (R2 = 0.21) and of B. pilularis (R2 = 0.21) and E. giganteum (R2 = 0.32) in 2006. The abundances of A. trivittatum and L. hesperus were also positively correlated with the floral resource availability of B. pilularis in 2006.
Comparison of Hedgerow Plants for Harboring Indicator Insects.
In order to compare the habitat quality of different hedgerow plants for supporting indicator insect groups, I used Cohen’s d estimates, which measure the magnitude of the difference between two means, known as the “effect size.” In other words, d estimates reveal not whether the means are significantly different from each other, but by how much they are different. Cohen’s d estimates range from 0 to greater than 1, where values equal to 0.2 are considered a small difference between two means and values equal to or greater than 0.8 signifies a large effect (Cohen 1988).
I found that B. pilularis supported a sizably higher number of parasitic wasps than other plants and this pattern was evident across years and sites. For the hymenopteran families, Chalcidoidea, Braconidae and Ichneumonidae, comparisons between B. pilularis and other plants resulted in Cohen’s d estimates between 0.5 and 1.72 indicating medium to very large effect sizes. B. pilularis generally supported a higher abundance of Orius spp. in comparison to other hedgerow plants, at all sites except for site B, where B. pilularis was late in flowering. B. pilularis was also favorable habitat to Chrysopidae, Hemerobiidae, and Coccinellidae in several plant comparisons at multiple sites. At site C, B. pilularis harbored a higher abundance of cucumber beetles than other plants (d estimates = 0.41 – 0.55). In general, B. pilularis is an important insectary plant for hedgerow design.
A. millefolium was another plant that tended to host a higher abundance of indicator insects in comparison to other plants, evident by d values greater than 0.4. Geocoris spp. was almost exclusively found at A. millefolium with d values greater than 0.7 for most plant comparisons. A. millefolium also supported a higher number of Orius spp. in comparison to C. griseus at site D (d = 0.85), C. ‘Ray Hartman’ at sites A, C and D (d = 0.26, 0.53, and 0.79 respectively), H. arbutifolia at site A (d = 0.70) and R. californica at all sites (d = 0.67, 0.41, 0.52, 0.77 respectively).
Another relevant finding was that the mean abundances of Syrphidae tended to be higher on Ceanothus ‘Ray Hartman’ at the majority of sites (d estimates > 0.2). While Ceanothus griseus and ‘Ray Hartman’ tended to host fewer of the other indicator insects in comparison to the other plants. Syrphid flies selective use of C. ‘Ray Hartman’ (Rhamnaceae), a tree with blue flowers, is a new finding. Syrphid flies have primarily been shown to be attracted to yellow and white flowers (Cowgill et al. 1993, Lunau and Wacht 1994) and prefer plants from the Apiaceae family (Colley and Luna 2000). As a nitrogen fixing tree that produces pollen rich flowers, Ceanothus ‘Ray Hartman’ offers many advantages to hedgerow design – architectural complexity, microclimate mediation, windbreak, nutrient recycling, and accessible pollen for syrphid flies and other obligate pollen feeders, such as bees. Furthermore, C. ‘Ray Hartman’ has the advantage over the prostrate species, C. griseus, of having two bloom periods, one in the spring and one in the late summer, which may explain the overall higher mean abundances of syrphid flies.
There is consensus among CBC researchers that adequate pest suppression via CBC depends upon the presence of a suite of generalist natural enemies in the farmscape (Ehler and Miller 1978). Since the insects monitored in this study differentially partitioned themselves across hedgerow plants and showed some indication of plant selectivity, the current design of species rich hedgerows with A. millefolium, B. pilularis, Ceanothus spp., E. giganteum, H. arbutifolia, and R. californica as foundational plants, is effectively conserving a diversity of natural enemies in Central Coast vegetable farms. In conclusion, the establishment of perennial hedgerows in the borders of vegetable systems greatly increases the potential for habitat stability and the development of resident natural enemy communities. Based on the findings of this research, hedgerow plantings aimed at conserving insect natural enemies should have a diverse mix of plant species that include B. pilularis, which is particularly important habitat for wasp parasitoids.
The dispersal of indicator insects from hedgerows into adjacent crop fields
The fluorescent mark was found on all insect taxa at 25 m and 100 m, except for lady beetles (Coccinellidae) and flea beetles (Phylotretta pusilla), which were only found marked at 25 m (Figure 8). The mean proportion of marked individuals, however, was significantly higher at 25 m and dispersal rates varied across taxa. Less than 10% of the minute pirate bugs collected were marked at 25 m and 100 m showing low dispersal from hedgerows into adjacent crop fields. However, predators as a functional group had a higher overall mean rate of movement (17.3 ± 2.9 SE) than both parasitoids (5.4 ± 1.5 SE) and pests (6.5 ± 1.3) . In particular, the generalist predators, Syrphidae and Chrysopidae, had the highest mean percentage marked for the combined distances, followed by Hemerobiidae. The parasitic wasp family, Ichneumonidae had a combined mean percentage marked of 13.6 ± 4.4 SE, but due to the variance was not significantly different from Chalcidoidea (1.7 ± 0.4 SE) and Braconidae (2.3 ± 0.9 SE). Excluding L. hesperus, pest taxa generally had lower rates of movement into crop fields than natural enemies. Percentage values are probably inflated for L. hesperus because of their low average abundance. Because of this probable inflation, this taxa was excluded from ANOVA analyses.
Results from a two-way ANOVA with an interaction term showed that taxa, distance, and distance * taxa were significant effects for the mean percentage of marked individuals (P < 0.01 for all effects). These results support the hypothesis that natural enemies and pests move from hedgerows to adjacent crop fields, but the rate of movement varies across taxa.
The Effect of Hedgerows on Parasitism Rates of a Sentinel Pest
In 2006, parasitism was detected mostly in the June trial and at only two sites, a field with a hedgerow and a field without a hedgerow. These fields were not in the same location nor did they have the same crop matrix, thus they are not comparable. However, results from this trial suggest that the crop matrix influences parasitism rates. In the San Juan Bautista field, percentage parasitism ranged from 70% in the brassica stand to 0% where there was bare ground. Whereas in the field with an unmanaged field margin, where the entire field was planted in brassica, parasitism rates ranged from 20% at 100m to over 80% at 10m. Parasitism rates also declined with distance from the field margin in the comparison field, indicating that parasitoids may be more abundant on the edges of fields.
In 2007, parasitism was detected in all three trials, however, there was a high variation in rates within and between fields and across dates. Results from a mixed model ANOVA showed that distance nested within field margin type was a significant effect (F=4.61, P=0.0165) while field pair was weakly significant (F=2.729, P=0.0582) and distance by itself was not a significant effect (F=3.0470, P=0.09). This result is best seen in Figure 10 where the average percentage parasitism is greatest at 100m in comparison fields, although there is no difference in percentage parasitism between field margins at 25m.
Results from the 2007 trials however are not conclusive because the crop matrix was not adequately controlled within and across fields. The crop matrix affects the background population of T. ni and thus the diversity and abundance of parasitoids present in the system. Furthermore, the stage of development of the crop stand affects the apparency of the sentinel pots, which in turn affects wasp foraging behavior. Being able to place sentinel larvae in a standardized crop matrix is essential for detecting the effects of added floral resources. In complex agri-landscapes such as the Central Coast of California, on-farm CBC research may be more successful if growers were compensated for planting the same crop matrix at the same time.
Educational & Outreach Activities
Pisani Gareau, T. 2008. Farmscaping for conservation: Factors that influence growers’ conservation behavior and the potential of hedgerows for enhancing biological control services. PhD Dissertation. Department of Environmental Studies. University of California, Santa Cruz.
Letourneau, D. K., Shennan, C., Bothwell, S. G., and Pisani Gareau, T. 2008. Vegetation management for biological control in annual crops: Spatial scale and meta-communities. XXIII International Congress of Entomology. Durban, South Africa
Pisani Gareau, T., C. Shennan, and D. Letourneau. 2007. Hedgerows: Enhancing conservation biological control potential in California annual vegetable systems? 92nd Annual Meeting Ecological Society of America. San Jose, CA.
Pisani Gareau, T. and Bothwell, S. 2007. Conservation biological control in the Central Coast
of California: Investigating the influence of on-farm native hedgerows and landscape diversity
on arthropod natural enemies. (Poster) The STEPS’s Institute Central Coast Biodiversity Workshop. Santa Cruz, CA.
Pisani Gareau, T. 2007. Insect natural enemies and pests associated with hedgerow plants established in California vegetable systems (Poster) 91st Annual Meeting Pacific Branch Entomology Society of America. Portland, OR.
Outreach: This research has been disseminated to vegetable growers and researchers within and outside of California through several outreach events. I continue to collaborate and share research results with researchers and agriculture professionals at UC Santa Cruz, UC Davis, Oregon State University, and the Pennsylvania State University.
Pollinators, predators & plants: Building landscapes to attract beneficial insects. Workshop. Pennsylvania Association for Sustainable Agriculture Conference. February 7, 2009.
Beneficial insects: Habitat factors for successful pest control and pollination. Workshop at the Eco Farm Conference. Asilomar. Pacific Grove, CA. January 23, 2009
IPM workshop: Managing for insect natural enemies. Agriculture Land-based Training Association (ALBA). Salinas, CA. April 15, 2007
Habitat improvement for predators and parasitoids. Presentation and discussion with grower participants at the Center for Agroecology and Sustainable Food Systems. University of California. April 17, 2007
Investigating the influence of perennial habitat on arthropod natural enemies in the Central Coast of California. Presentation. 2nd Annual Meeting of the Western Region Functional Agricultural Biodiversity (FAB) Working Group. Portland, OR. March 2008
Insect natural enemies and pests associated with hedgerow plants established in California vegetable systems. Poster. 1st Annual Meeting of the Western Region Functional Agricultural Biodiversity (FAB) Working Group. Portland, OR. February 2007
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 I was unable to make a direct link between hedgerows and improved biological control rates in the field, I 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). I 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.
Anderson and Long (1999) estimated the total cost to establish and maintain (for two years) a 1,200’ hedgerow between $3,200 and $4,300, not including the potential loss in profit from removing land out of production. However, many farmers have off set these costs by participating in the Environmental Quality Incentives Program (EQIP), which provides financial assistance for installing conservation practices like hedgerows. Nevertheless, establishing a hedgerow is a long-term investment on the part of the farmer, because hedgerows require periodic weeding, watering and pruning. An important next step in this research would be to quantify the economic returns gained from the services hedgerows provide. When hedgerows provide multiple services, such as soil conservation, water quality protection, crop protection from dust and pesticides and conservation of insect natural enemies important in biological control, then they would provide substantial economic returns, especially in the long-term, to the farm business.
The impetus for this research was the efforts of a small group of farmers who were planting hedgerows in their field margins and wanted to know if this practice was conserving beneficial insects that could naturally suppress pest populations. In 2003, there were approximately 20 hedgerows planted in the Central Coast. By 2008 well over 100 hedgerows had been established according to the non-profit organization, Community Alliance with Family Farmers (CAFF), that has assisted farmers with this practice. This five-fold increase in hedgerows is testament to the growing interest among farmers in this practice. In 2007, I conducted a survey of conservation practices applied by farmers in the region and found that 15% of the respondents had a hedgerow on their farm. My research certainly provides evidence that hedgerows conserve natural enemies, which agencies like CAFF are able to use to help promote hedgerow development. I recommend that farmers use a diversity of plants with overlapping bloom periods to support natural enemies that require pollen and nectar. Baccharis pilularis (coyote brush) is also an important foundational plant for its insectary qualities, especially for its attractiveness to wasp parasitoids.
Areas needing additional study
The high abundance of parasitic wasps on B. pilularis early in the season may prevent pest populations from building up the in crop field. It would be prudent to determine if key parasitoids for biological control are present or if coyote bush just hosts a diverse community of insects that have little interaction with field pests. Further research is also needed to test the multiple mechanisms behind insect natural enemy attraction to native plants. Flower feeding insects were often found on plants that were not in bloom, thus other factors, such as plant structure, microclimate, or alternate prey may be driving insect abundances on plants.
Linking insect natural enemy presence and abundance in CBC habitat to attack rate of pests in adjacent crop fields is a crucial next step. Since the background noise of natural field studies can make detection of parasitism trends challenging, it will be important for future CBC research on hedgerows to evaluate multiple pest species along with their associated predators, parasitoids and pathogens, for a more holistic assessment of their function. Furthermore, since natural enemy, host prey, and alternate host populations naturally oscillate from one year to the next, I recommend long-term studies to more fully evaluate hedgerows contribution to pest management.