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USING HABITAT MANAGEMENT TO IMPROVE BIOLOGICAL CONTROL ON COMMERCIAL ORGANIC FARMS IN CALIFORNIA Ramy Colfer, Mission Organics, Salinas, CA 93902, U.S.A. INTRODUCTION Mission Organics grows organic vegetables on over 5500 acres per year. We do not grow any conventional crops. Like all vegetable producers, we frequently encounter disease and pest problems in many of the crops we grow. We employ many different strategies to manage these pest and disease problems. A major group of pests that we encounter are aphids. One of the key strategies that we use to manage aphids is conservation biological control. Conservation biological control entails the management of resources in the environment to enhance the survival, fecundity, longevity and behavior of natural enemies to increase their effectiveness (Landis et al., 2000). Conservation biological control has recently received greater attention in the biological control literature (Barbosa, 1998; Pickett and Bugg, 1998; Landis et al., 2000). In this manuscript, I describe how we have implemented conservation biological control through habitat manipulation to improve the biological control of aphids. In our row crop lettuces (romaine and iceberg lettuce) and cole crops (broccoli and cauliflower), we intercrop with annual flowering plants to improve our aphid biological control. The flowering plants provide a food source for aphid predators and parasitoids. The group of aphid natural enemies of primary importance is the syrphid flies or hoverflies. The use of flowering intercrops and hedgerows in organic agriculture has been criticized by some university researchers and extension personnel because the purpose of these habitat manipulations has not been clearly defined by growers. Questions such as ‘What pest are you trying do control?’ and ‘What beneficial arthropods are being attracted by the beneficial habitats to control the target pest?’ have not be clearly answered. However, in our row-crop production, we clearly identify the answers to these questions. In our cropping systems, we clearly define the pests that we are attempting to control with intercropping of flowering plants: the pests of primary importance include (i) the lettuce aphid Nasonovia ribisnigri and (ii) the cabbage aphid Brevicoryne brassicae; the pests of secondary importance include (i) the green peach aphid Myzus persicae,(ii) Potato aphid Macrosiphum euphorbiae, and (iii) foxglove aphid Aulacorthum solani. The most important group of natural enemies that we are attempting to increase with intercropping of flowering plants is the syrphid flies (Diptera: Syrphidae). A very diverse group of syrphid species are generally observed in our crops; common genera include Sphaerophoria, Syphus, Eupeodes, Allograpta, and Toxomerus. Some of the most abundant species of syrphids that we find in lettuce and cole crops include Sphaerophoria sulfuripes, Syrphus opinator, and Eupeodes fumipennis. Other important natural enemies that we observe in our crops include lady beetles (common spp.: Hippodamia convergens, Coccinella novemnotata , and Coccinella septempunctata), aphidiid parasitic wasps (Diaeretiella rapae, Aphidius matricariae, and Lysiphlebus testaceipes), brown lacewings (Hemerobius spp.), green lacewings (Chrysoperla spp.), bigeyed bugs (Geocoris spp.), and minute pirate bug (Orius tristicolor). In my presentation, I will focus on the use of intercropping of flowering plants in romaine lettuce to improve the biological control of the lettuce aphid N. ribisnigri in the central coast of California. I will begin by going through some evidence that shows that the lettuce aphid is generally controlled through the naturally- occurring biological control of syrphid flies in organically-grown romaine lettuce. This will include a description of some aspects of the ecology of syrphid populations. Then, I will present and discuss some results that support the premise that intercropping with sweet alyssum (Lobularia maritime) can improve the control of the lettuce aphid in romaine lettuce. PRELIMINARY OBSERVATION – HOW CAN WE BEST CONTROL LETTUCE APHID POPULATIONS? Early experiments testing the efficacy of organic materials to suppress lettuce aphid populations indicated that most materials had little impact on aphid populations and could interfere with aphid biological control. For example, results from an open-plot, replicated (n = 4) field trial in a romaine lettuce provided evidence that insecticidal soap could cause more harm than good for lettuce aphid control. This experiment was conducted in a 5 acre field of romaine lettuce in San Benito County during July, 2000. Plots were sprayed with insecticidal soap (2%, 100 gpa) twice on a weekly interval. Aphid and syrphid larval populations were monitored twice; five and three days after each soap application, respectively. Lettuce aphid populations tended be larger and increasing in the 55 insecticidal soap treatment but smaller and decreasing in the unsprayed plots causing aphid numbers to marginally differ between treatments (Figure 1A, Manova, Wilks’ lambda test, F = 4.41, P = 0.08). Syrphid larval populations tended to increase in the unsprayed plots but not in the soap-sprayed plots; syrphid larval populations in the second sampling tended to be higher in the unsprayed plots than the sprayed plots though this difference was not statistically significant (Figure 1B, t-test, t = 1.24, P = 0.26). These experimental results and others where the presence of syrphid populations predated the elimination of lettuce aphid populations led us to believe that syrphid populations are very important in reducing lettuce aphid populations. A large number of observational results of fields where syrphid larval populations were strongly correlated with a subsequent crash and elimination of lettuce aphid populations prior to lettuce harvest led us to the realization that the most effective strategy for controlling lettuce aphid populations in organic romaine lettuce was through conservation biological control. Also, we have observed that syrphid biological control is consistent across a wide range of regions (Monterey, Santa Cruz , and San Benito Counties) and through many seasons (spring, summer, and fall). This is in contrast to other lettuce aphid natural enemies that are important during certain times of year and/or certain locations. SYRPHID ECOLOGY - FACTORS CONTRIBUTING TO EFFECTIVE APHID CONTROL Aphid- syrphid population dynamics The interactions between lettuce aphid and syrphid populations appear to fairly simple: a Lokta-Voltera model predictions of prey and predator populations dynamics through time appears to be similar to the populations dynamics observed in a typical romaine lettuce field. Lettuce aphid populations establish and increase; this is followed by the establishment of syrphid populations. Then, aphid populations are generally eliminated once syrphid populations are sufficiently abundant. Lettuce aphid populations generally do not persist for a sufficiently long period to cause the degradation of the romaine plants due to aphid feeding (over exploitation of the prey resource). Whether syrphid activity is successful at controlling the lettuce aphid for romaine production is dependent on the timing: do syrphid larvae ‘clean up’ aphids prior to harvest? The optimum scenario occurs when both the aphid and syrphid populations are no longer present in the lettuce at the time of harvest. The goal for the organic pest management practitioner is to promote the early establishment of syrphid populations. Figure (2) graphically displays the acceptability of a romaine field in relation to the population cycling of lettuce aphid and syrphid populations and the type of harvest (romaine hearts or processed romaine). Romaine hearts go directly from the field to the consumer and cannot contain more than a low number of aphids per plant; processed romaine is cut and washed at a processing plant and can have more aphids. However, processed romaine with heavy aphid populations is not desirable because it requires more washing and is therefore more expensive to process. Aphid and syrphid spatial population dispersion An important aspect of the ecology of syrphid flies is that they are generally more widely dispersed across a romaine field than lettuce aphid populations. When lettuce aphid populations are high, syrphid eggs can generally be found on every plant within a romaine field. Here, monitoring of three romaine lettuce fields demonstrates the most common situations we encounter in our fields. The dispersion of lettuce aphid and syrphid egg populations are presented in three different types of fields. The results presented are from fields that were two-to-three weeks prior to harvest. Situation one; these results come from the Bypass ranch in Hollister (San Benito Co.) where we generally encounter heavy lettuce aphid pressure. Lettuce aphid populations were high and the majority of plants had aphid population above what would be acceptable for harvest (Figure 3A). However, syrphid egg numbers were also abundant and eggs were found on all romaine plants (Figure 3B). Under these conditions, romaine plants will become clean enough for both hearts and processed romaine harvest but the elimination of aphid populations may occur near harvest time (sometimes within days). Situation two; these results come from the Home ranch in Gilroy (San Benito Co.) where we generally encounter low-to-moderate aphid populations combined with consistently high syrphid populations. Lettuce aphids

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