Dynamics of Predation on Lygus hesperus (Hemiptera: Miridae) in Alfalfa Trap-Cropped Organic Strawberry.

Alfalfa (Medicago sativa L.) (Fabales: Fabaceae) can be strategically planted as a trap crop for Lygus spp. in California's organic strawberry fields. Alfalfa has been shown to attract both Lygus spp. and, in turn, a Lygus-specific parasitoid, Peristenus relictus (Ruthe) (Hymenoptera: Braconidae). However, the impact of alfalfa trap-cropped strawberries on the Lygus spp. predator complex is unknown. Here we identify key predators of Lygus spp. found in organic strawberry. First, a general survey was conducted at an organic, non-trap cropped strawberry farm, to quantify predator abundance and to qualitatively assess their feeding activity on Lygus spp. We identified the 11 most abundant predator taxa present and, by using a Lygus-specific PCR assay, determined that about 18% of the insects and spiders contained Lygus spp. remains in their guts. We then conducted a study to examine alfalfa's role in conserving the most relevant predators in trap-cropped organic strawberries. Specifically, we quantified predator abundance and qualitatively measured predator feeding activity (by gut analysis) on Lygus spp. collected in strawberry plots either lacking or containing an alfalfa trap crop. Data revealed that some predator taxa, including the numerically dominant predator, Orius tristicolor (White) (Hemiptera: Anthocoridae), aggregated in alfalfa trap crops. The gut content analyses revealed that insect and spider predators collected from the alfalfa trap crop had a significantly higher proportion of their population containing Lygus spp. remains than those collected from nearby rows of strawberries. These results suggest that alfalfa trap cropping might be a useful tactic for conserving the biological control services of generalist predators in organically grown strawberries in California.

Immigration of Lygus spp. (Hemiptera: Miridae) and predaceous natural enemies to trap-cropped organic strawberry.

Lygus spp. are polyphagous pests that overwinter in weedy vegetation. In the spring on the central coast of California, Lygus spp. emigrate from weeds into strawberry fields. Subsequent feeding on strawberry flowers causes fruit deformation that precludes sale on the fresh market. Use of alfalfa (Medicago sativa L.) (Fabales: Fabaceae) as a trap crop has been hypothesized to prevent Lygus spp. colonization in strawberries. We examined the movement of Lygus spp. and associated predators from weeds to strawberry fields with alfalfa trap crops using a protein mark-capture technique. Insects and spiders were collected from weeds, strawberry, and alfalfa 1 day, 2 days, and ~2 wk after an albumin protein mark was applied to weeds bordering strawberry fields. For marked Lygus spp. that emigrated from weeds, the majority (79%) of adults were recovered from alfalfa trap crops; however, all nymphs immigrated to strawberry. Most protein-marked predators immigrated to strawberry, rather than trap crops, resulting in a marked predator-to-Lygus spp. ratio of 5:1. Trap cropping effectively reduced the colonization of Lygus adults in strawberry. Converting weedy areas to native perennial plantings could further mitigate the risk of pest migration, while simultaneously conserving beneficial insects.

Assessing Anaerobic Soil Disinfestation as a Control Tactic for Delia radicum (Diptera: Anthomyiidae) in California Brussels Sprouts.

Fumigants are often the primary material used to manage plant disease agents, soil-borne arthropods, and weeds in California agriculture, particularly in the absence of crop rotation. However, some fumigants have come under increased regulatory scrutiny and are therefore no longer available for use. We tested two biologically based preplant crop-protection fumigant alternatives for their effectiveness in reducing populations of Delia radicum (L.), a soil-borne insect pest of cole crops. Laboratory and field tests compared pest survival in untreated control soil with survival under anaerobic soil disinfestation (ASD), which is induced by incorporating a carbon source (rice bran or grape pomace) and subsequently saturating the soil with water. We also measured and compared the effects of standard grower practices (fumigation), ASD and biofumigation (mustard seed meal incorporated before planting), and untreated control soils on fly abundance, pupal survival, and root damage in Brussels sprout fields. In both laboratory and field studies, D. radicum pupal survivorship was reduced in ASD-treated soils relative to control soils. Pupal survivorship of Musca domestica (L.) (Diptera: Muscidae), which was used as a proxy for D. radicum in some field experiments, was also reduced in ASD- and biofumigation-treated soils, when compared with untreated control soils. These results indicate that fumigant alternatives may provide useful insect pest management opportunities for compatible cropping systems.

Control of western tarnished plant bug Lygus hesperus Knight (Hemiptera: Miridae) in California organic strawberries using alfalfa trap crops and tractor-mounted vacuums.

A key economic pest of strawberries in California is the western tarnished plant bug, Lygus hesperus Knight (Hemiptera:Miridae). Alfalfa (Medicago sativa L.) is a highly attractive plant host to western tarnished plant bug, and we hypothesized that it can be successfully managed as a trap crop for pest suppression in strawberries. Completely randomized design trap cropping experiments were established on an organic strawberry farm from 2002 to 2004. Western tarnished plant bug adults and nymphs were significantly more abundant in alfalfa trap crops than in comparable edge strawberry rows. Over 3 experimental yr, twice-weekly summer vacuuming of alfalfa trap crops with a tractor-mounted vacuuming device reduced adult and nymph abundance by 72 and 90%, respectively, in trap crops. This summer vacuuming of alfalfa trap crops also significantly reduced damage caused by western tarnished plant bug in associated unvacuumed organic strawberries (June and July 2002, June 2003, and June and July 2004) compared with either an untreated control (2003) or the organic strawberry grower's standard whole field vacuuming treatment. Vacuuming of alfalfa trap crops reduces an organic grower's costs (tractor, tractor fuel, and driver time) by 78% compared with current whole field vacuuming practices. An economic analysis of a whole hectare model indicates that a positive return from the use of vacuumed trap crops could be realized in 2004. The overall potential positive net return for the 3 mo of vacuumed alfalfa trap crop treatments in 2004 was calculated at +$1,829/ha.

Spatial density and movement of the Lygus spp. parasitoid Peristenus relictus (Hymenoptera: Braconidae) in organic strawberries with alfalfa trap crops.

Alfalfa trap crops are currently used to manage Lygus spp. in organic strawberry fields on the California Central Coast. The retention of Lygus spp. in alfalfa creates aggregated distributions that provide improved opportunities for biological control by the introduced parasitoid Peristenus relictus (Ruthe). The abundance and distribution of P. relictus between two trap crops separated by 50 strawberry rows were analyzed in 2008 and 2010. Parasitism of Lygus spp. nymphs by P. relictus (measured by larval abundance and % parasitism) was greatest in alfalfa trap crops compared with strawberry rows. A significantly positive correlation between host nymphs and P. relictus larvae in and between trap crops was found. Movement of P. relictus adults from a marked alfalfa trap crop into adjacent strawberry rows or trap crops was also studied in 2008 and 2009 using a chicken egg-albumin enzyme-linked immunosorbent assay mark-capture technique. In 2008 and 2009, 85 and 49% of protein-marked wasps were captured from central trap crops, respectively, indicating that alfalfa trap crops act as a concentrated "host-density anchor" in organic strawberry fields.

Dispersion, distribution, and movement of Lygus spp. (Hemiptera: Miridae) in trap-cropped organic strawberries.

Alfalfa (Medicago sativa L.) is a highly attractive plant host to Lygus spp. and is used as a trap crop in California organic strawberries to influence the dispersion and dispersal of these pests, particularly Lygus hesperus Knight. The abundance and distribution of Lygus spp. nymphs between two trap crops separated by 50 strawberry rows was analyzed in 2008 and 2010. Nymphs demonstrated a bimodal distribution in strawberries between trap crops, where nymphs were most abundant and aggregated in alfalfa, when compared with interior strawberry rows, where nymphs were less abundant. The majority of nymphs were concentrated in trap crops and nymphal densities in interior strawberry rows were well below economic thresholds. The movement of Lygus spp. from a marked alfalfa trap crop into adjacent strawberry rows or trap crops was also studied in 2008 and 2009 using a chicken egg albumin enzyme-linked immunosorbent assay mark-capture technique. The majority of marked-captured L. hesperus adults and Lygus spp. nymphs remained in alfalfa trap crops, rather than dispersing out into strawberry rows at 24 h, 48 h, and 2 wk, postprotein application. The attenuation of Lygus spp. movement in alfalfa associated with organic strawberries is a key component of successful trap cropping. A small percentage of marked adults and nymphs were captured in neighboring alfalfa trap crops, located 62 m from the point of protein application, highlighting the dispersal capacity of this key pest.