Evaluating Alternative Host Plant Use of Spotted Wing Drosophila, Drosophila suzukii

• 2016 annual report

Commodities

• Fruits: berries (blueberries)
• Additional Plants: native plants

Practices

• Crop Production: continuous cropping
• Pest Management: chemical control, cultural control, integrated pest management, physical control, sanitation, soil solarization
• Production Systems: organic agriculture
• Sustainable Communities: public policy, sustainability measures

Spotted wing drosophila (SWD), Drosophila suzukii, a native of Asia, has emerged as a devastating pest of soft fruits. Since its first detection in California in 2008, SWD has spread throughout the U.S. causing significant yield losses and increased pesticide use at an estimated cost approaching $1 billion annually. Unlike most Drosophila species, SWD females oviposit primarily in ripening fruits, presenting a major threat to U.S. fruit industries. The presence of SWD can be viewed as a “game changer” to raspberry and blueberry production, as they have historically required very few insecticides and have the highest infestations, with yield losses often reaching 100%. Currently SWD management consists of insecticide applications on a 4-5 day schedule. Increased chemical inputs add substantial new costs to growing operations and increased risks to surrounding ecosystems, leading to numerous growers abandoning these crops. A major challenge with invasive species is often our lack of fundamental knowledge about their biology, dispersal activity or phenology. Broadening our understanding of these fundamental knowledge gaps is critical to implementing and refining sufficient and effective monitoring and management practices.   To address this issue and provide alternatives to chemical controls this project will: Identify and evaluate non-crop host plants. Correlate seasonal phenology of crop and non-crop hosts with SWD populations, and pesticide application timing. Evaluate the impact of exclusion on raspberry pollinations and fruit quality.   Our ultimate goal is to provide growers with information to control SWD while maintain a high level of fruit quality without the overreliance on disruptive insecticides, improving the sustainability of crop production throughout the region, directly addressing all three NCR-SARE broad-based outcomes.

Objective 1. Alternative hosts will be determined using methods similar to what is described by Lee et. al. (2015). A minimum of three common Minnesota invasive fruiting plants that have been reported as potential alternative hosts for SWD include: exotic honeysuckles (Lonicera tartarica, L. morrowii, L. x bella), American pokeweed (Phytolacca americana L.) and buckthorn (Rhamnus and Frangula spp.). These will be identified near plots of known host crops (raspberry and/or blueberry) from a minimum of five locations separated by a minimum of 400 meters, in Minnesota. Fruits and flowers will be collected from the field sites and incubated in the laboratory to determine larval infestation rates. Fruit will be collected once a week throughout the summer for each species. All sites will be within a 50-m radius of a crop host location known to have SWD. If sampling of one host occurs at multiple sites, every effort will be made to collect those samples on the same day. Data collected from each site will include date, location, number of fruits or flowers collected per plant species, and the condition of the fruit (unripe, ripe, or overripe). Sampling of berries will involve clipping the stem of the berry to avoid breaking or damaging the fruit.

Individual fruits will be placed in 30 to 89 ml plastic cups depending on fruit size. Cups will be sealed with a screened lid to reduce fungal growth. In some cases, a small cotton swab or sand layer will be added to the bottom of the container to absorb moisture. Cups will be kept in the laboratory at 21± 1˚C. Fruit will be held for a maximum of 18 days, and will be examined daily for the presence of adults. The percent of fruit with emerging adult SWD ([number of infested fruit/total number of fruit] X 100) and number of larvae per berry will be recorded.

SWD adult populations will be monitored in the field using commercially available Pherocon SWD lures and traps (Trécé, Inc.). A minimum of three traps will be set up at each location. Traps will consist of dual lures hanging from the lid of a Pherocon trap baited with 150 ml of apple cider vinegar and 0.2 ml of unscented dish soap (Seventh Generation, Inc.). Trap catches will be collected weekly. Bait solution will be replaced and trap contents removed when traps are being serviced. Trap contents will be analyzed in the laboratory and the number of male and female SWD recorded for each trap using a stereomicroscope. Lures will be replaced every 4 weeks, according to the manufacturer recommendations (Trécé, Inc.).

Objective 2. Beginning in early spring, we will record which hosts retained fruit throughout the winter. We will collect fruit that overwintered and examine them for infestation as described in objective 1. We will monitor both crop and non-crop hosts weekly and record the phenology for each throughout the season. This will also be done with crop hosts blueberry and raspberry. Phenology will be recorded based on: presence of leaf buds and/or leaves, flowering, unripe fruit, ripe fruit, overripe fruit, and winter dormancy. Presence of adult SWD will also be monitored and recorded using the commercially available Pherocon SWD lures and traps (Trécé, Inc.) as described in objective 1. Seasonal phenology will be compared with crop, non-crop, and SWD and examined for correlation. Defining the phenology of hosts and pest is important in understanding how the pest is able to move rapidly through the landscape.

Objecive 3. Fifteen experimental plots (10’L x 17’W) have been established with two rows each of raspberries (cv. ‘Heritage’) at the UMN Outreach, Research and Extension (UMORE) Park near Rosemount, MN. Starting in May during each study year, each plot will be randomly assigned to one of three treatments (n = 5 replicates / treatment) in a randomized block design: (1) ‘open’, (2) ‘high tunnel’, and (3) ‘high tunnel + netting’. Small portable “Hanley-style” high tunnels have been constructed with 10’L x 17’W x 7.5’H frames (reduced-size of the design outlined in Hams 2009) and covered with plastic tarp. In addition, for plots in the ‘high tunnel + netting’ treatment, a layer of Reemay mesh (fine horticultural mesh that excludes SWD) will be attached to the exterior of the high tunnel and buried at the edges. Starting at the time of enclosure establishment, SWD adult population levels will be monitored using using commercially available Pherocon SWD lures and traps (Trécé, Inc.), checked weekly. Sampling for larvae will be performed weekly using fruit dunk flotation tests. Finally, given the potential for netting to affect fruit productivity through increased temperature, temperature will be recorded across the growing season using digital thermometers (LaCrosse Technology, LaCrosse, WI).