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The CENTER for AGROECOLOGY & SUSTAINABLE FOOD SYSTEMS UNIVersity OF CALIFORNIA, Santa CruZ Research BRIEF #13, FALL 2010 Can Hedgerows Attract Beneficial Insects and Improve Pest Control? A Study of Hedgerows on Central Coast Farms – Tara Pisani Gareau1 and Carol Shennan2 edgerows began to appear on Central importance, while not creating or exacerbat- Coast farm edges in 2000, due largely ing pest problems. It is often assumed that if to the efforts of the non-profit Com- natural enemies are attracted and retained in H a CENTER munity Alliance with Family Farmers (CAFF). farms with added habitat, then their control of On the Central Coast, hedgerows consist of pests in crop fields will also increase. However, • linear assemblages of trees, shrubs, herbs and in order for biological control to take place, grasses, many of them native species, which insect natural enemies must disperse from RESEARCH provide multiple services to farms. flowering habitat into the crop field to either When contour-planted on a sloped field, parasitize (if a parasitoid) or consume (if a • mature hedgerows with deep and fibrous root predator) their host prey. The results of studies systems can reduce soil erosion as well as that have tested the effects of added habitat on absorb mobile nutrients like nitrate (Kinama predation or parasitism rates in the field are BRIEFS et al. 2007, Bu et al. 2008). Tall, dense hedge- often quite variable (Lee and Heimpel 2005, rows act as a windbreak, which can reduce Pfiffner et al. 2009) or show no apparent effect crop field edge effects such as stunted growth (Rebek et al. 2006, Lee and Heimpel 2008). or poor germination, as well as protect crops The objectives of this study, conducted from from physical and chemical contaminants. 2005 to 2007, were (1) to assess the habitat Hedgerows with dense canopies can also quality of different hedgerow plants for insect suppress weed populations in the field margin natural enemies and pests, (2) to track the (often the source of field weeds) through com- movement of insects from hedgerows into petition for light, water, and nutrients (Boutin adjacent crop fields and (3) to test the effect 2006). These ecosystem services from hedge- of hedgerows on parasitism rates of an eco- rows have been observed and documented. nomically important pest, the cabbage looper Consequently, on California’s Central Coast, (Trichoplusia ni). hedgerows have become a recommended Many conservation biological control stud- practice for irrigated agriculture, especially ies are set up as greenhouse or field experiments for the goals of biodiversity and water quality where environmental factors, such as the host conservation (CAFF website, NRCS 2008). crop environment, herbivore populations, and Producers are also interested in hedgerows climate are controlled, and the only varying for their potential to support beneficial in- factors are the habitat or resource treatments. sect populations. However, compared with This approach is extremely useful for screening other functions they provide, the role that plants for insectary qualities and determining hedgerows play in enhancing biological con- fundamental behavior response of particular trol services is less understood. Generally, increasing the diversity of plants and providing undisturbed habitat in the field margin increases both the diversity and abundance of insects. Reducing tillage decreases insect mortality rates, allowing surviving insects to reproduce and contribute new individuals to the future generation. Furthermore, untilled land with permanent vegetation can increase the fitness of insects by providing them with critical resources such as pollen, nectar, host prey, and shelter. From an invertebrate conservation standpoint, an increase in insect diversity is a positive result of hedgerows. However, from a biological control standpoint, we are also interested in increasing numbers of specific Figure 1. Tara Pisani Gareau collects insects from a Research natural enemies that attack pests of economic hedgerow shrub in San Juan Bautisita, CA. Brief #13 1Boston College (formerly at UC Santa Cruz); 2Environmental Studies Department, UC Santa Cruz Can Hedgerows Attract Beneficial Insects and Improve Pest Control? A Study of Hedgerows on Central Coast Farms insect species. However, actual agro- butifolia), and coffeeberry (Rhamnus The effect of hedgerows on parasitism ecosystems—where an array of factors californica). We chose to monitor these rates of a sentinel pest influence insect interactions with plants native California plants because they In 2006 and 2007 we set out first and other insects—can be complex. are commonly used as foundational and second instar cabbage looper larvae The question of whether or not plants in hedgerow design (Earnshaw on 20 potted collard plants in eight hedgerows can enhance biological 2004). vegetable fields. We used the vegetable control services in Central Coast farm At each hedgerow site, we sampled fields adjacent to the hedgerows where systems is best addressed with on-farm insects from four to five replicates we monitored indicator insects and studies. Thus we assessed the hedge- of each plant species eight times in paired those fields with a nearby veg- rows that growers planted in their 2005 and ten times in 2006 between etable field that lacked a hedgerow in fields, rather than manipulate hedge- the months of May and October. We the field margin. row and crop field plantings. While this also measured the availability of floral In 2006, five sentinel collard pots approach can add more variability or resources on each individual plant for were placed in each field at 10, 25, “noise” to results, detected patterns each sampling period. 50 and 100 m from the field margin. also better reflect the ecological real- Since the rotations of the vegetables The dispersal of indicator insects from ity of hedgerows planted in Central often led to sentinel pots being placed hedgerows into adjacent crop fields Coast farms. in different crop environments, in We conducted an insect tracing 2007 we tried to better standardize METHODS experiment three times during the the surrounding crop by placing ten The attractiveness of hedgerow summer of 2006. Insects foraging on sentinel pots within a near distance of plants to key insect natural enemies hedgerow plants were marked with a 10–25 m and far distance (50–100 m) and pests yellow fluorescent pigment, which was in crops known to host cabbage looper This study took place at four farms sprayed on hedgerow vegetation twice or if not available, on bare ground. with hedgerows on the Central Coast during each trial according to methods Larvae were collected from the potted of California. Using a vacuum sampler described by Schellhorn et al (2004). collard plants after seven days and (Figure 1), we monitored key insect We did this experiment at four dif- brought back to the laboratory where natural enemies and pests at six hedge- ferent vegetable fields with bordering they were raised on an artificial diet in row plants: common yarrow (Achillea hedgerows and placed 10 traps imme- a growth chamber. Percentage parasit- millefolium), coyote brush (Baccharis diately following the second spray at ism was calculated for each distance by 60 pilularis), California lilac (Ceano- 25 m and 100 m from the hedgerows totaling the number of collected larvae thus griseus and C. ‘Ray Hartman’), (20 traps/field). The mark is distin- from the pots at the distance, dividing perennial buckwheat (Eriogonum guished on insects under magnification that number by the total number of 50 giganteum), toyon (Heteromeles ar- with a UV lamp. parasitized larvae and multiplying by 100. This experiment was conducted three times in each year. 40 60 Figure 2. Mean number of wasp parasitoids, generalist predators Wasp Parasitoids and herbivores collected in 1 minute vacuum samples from May through October 2006. 30 50 Generalist Predators Herbivores Wasp parasitoids included hymenopterans from the families: Ichneumonidae, Braconidae, and Chalcidoidea. Generalist predators 20 40 include the following insect taxa: lady beetles (Coleoptera: Coccinellidae), syrphid flies (Diptera: Syrphidae), green lacewings (Neuroptera: Chrysopidae), brown lacewings (Neuroptera: Wasp Parasitoids Hemerobiidae), minute pirate bugs (Orius spp., Hemiptera: 30 Generalist Predators Mean abundance/1 min vacuum sample Mean abundance/1 min vacuum sample Anthocoridae), big-eyed bugs (Geocoris spp., Hemiptera: Lygaeidae), 10 Herbivores and damsel bugs (Nabis spp., Hemiptera: Nabidae). Herbivores included the western spotted cucumber beetle (Diabrotica 20 undecimpunctata undecimpunctata, Coleoptera: Chrysomelidae), the western striped cucumber beetle (Acalymma trivittatum, Coleoptera: 0 Chrysomelidae), flea beetles (Phylotretta spp., Coleoptera: A. millefolium Mean abundance/1 min vacuum sample 10 R. californica Chrysomelidae), the western tarnished plant bug (Lygus hesperus, C. griseus E. giganteum Hemiptera: Miridae) and aphids (Homoptera: Aphidae). 0 A. millefolium R. californica C. griseus E. giganteum B. pilularis C. griseus A. millefolium E. giganteumH. arbutifoliaR. californica C. ‘Ray Hartman’ 2 CENTER RESEARCH BRIEF #13 | FALL 2010 Center for Agroecology & Sustainable Food Systems Results bugs (Geocoris spp.), green lacewings 14% of the indicator insects sampled in Key insect natural enemies and pests (Chrysopidae) and brown lacewings 2005 and 2006. The most abundant key attracted to hedgerow plants (Hemerobiidae), respectively.

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