Entomopathogenic nematodes (EPNs) represent an important alternative to chemical controls for the protection against various agricultural insect pests (Morris 1985; Georgis et al. 2006). Research exploring the application of EPNs for insect pest control is increasing rapidly as EPNs become more accessible and commercially available (Georgis et al. 2006). However, despite several decades of EPN research there remains significant gaps in knowledge regarding the efficacy and cost-effectiveness of EPNs. Management of insect pests using EPNs is often unpredictable as multiple abiotic (e.g. temperature, moisture, etc.) and biotic (e.g. EPN strains, host behavior/biology, etc.) factors must be considered to maximize EPN activity (Chen et al. 2003; Georgis et al. 2006; Shapiro-Ilan et al. 2006). Optimizing EPN infection rates requires focused research and experiments to develop appropriate methods for specific ecological settings. Furthermore, researchers must consider the evolutionary context for deploying EPNs. Lab-reared commercially available EPNs, though convenient, are often genetically depauperate and/or maladapted for the field conditions where they are being deployed (Stuart and Gaugler 1996). The use of non-adapted EPN strains (i.e. commerical) also precludes them from persisting during or over multiple growing seasons (Shields pers. comm.).
Previous studies testing EPNs for CRM management provide important insights for their use. For example, Bracken et al. (1990), Schroeder et al. (1996), and Nielsen (2003), all report improved infection rates in CRM with the EPN, Steinernema feltiae, as compared to other EPN species. In addition, field application of commercially available S. feltiae in cabbage crops displayed reduced CRM damage in EPN treated plots (Schroeder et al. 1996; Beck et al. 2014). Yet, despite these promising results, success may be limited to specific crops or strains, as Simser (1992) showed limited control of CRM in collards plants. Other possible reasons for this variance, may be attributed to environmental mismatches between EPN strains and local ecologies, improper application timing, inhospitable environmental factors and genetic founder effects (Stuart and Gaugler 1996; Fenton et al. 2001; Chen et al. 2003).
Our proposal looks to build upon the current research of EPNs for CRM management in two ways: 1) by testing the efficacy of EPNs in root crops and 2) by exploring the feasibility and utility of small-scale regionally adapted EPN strains. Root crops such as radishes and turnips are particularly sensitive to CRM, as any root damage may lead to reduced market value. However, there is scant research documenting the use of EPNs in these crops. Moreover, the vast majority of contemporary studies utilizing EPNs for CRM primarily use commercially cultured nematodes. Considering the high cost and low genetic variability associated with commercially reared EPNs, our study will aid in the development of more affordable long-term EPN solutions.
The goal of this project is to add to the available IPM toolbox for vegetable growers to decrease the incidence of damage and reduce yield loss associated with CRM. To accomplish this, we will test the field efficacy of two types of entomopathogenic nematode (EPN) soil applications, commercially available (CA) and regionally adapted (RA), for the management of CRM. Specifically, we will determine if soil applications of CA and/or RA will decrease the number of CRM in the radish rhizosphere and increase the market quality (as measured by root damage) and/or yield of radish crops when compared to untreated controls (UC). We will also determine if the EPN infection rate varies temporally (i.e. at seeding, post-EPN application and post-harvest) for each treatment.
We will look to answer are the following: Do soil applications of ENPs decrease the amount and severity of damage leading to increases in market quality and yield in radish crops? Do soil applications of each method, (CA, RA and UC) differ in their ability to reduce the amount and severity of damage in radish crops? Do regionally adapted nematodes persist longer as biologically active control agents when compared with commercially available applications?
After consideration and discussion with our farm partners, we decided to replicate our experiment in two locations, Bear Roots Farm in Barre, VT and the UVM Horticultural Research and Education Center (HREC) in South Burlington, VT, as opposed to a single location with temporal replicates (i.e. spring and summer). This alteration to our original experimental design allowed us to assess the performance of EPNs during the most damaging/vulnerable time of the season for CRM infestation. In addition, this setup allowed us to explore the persistence of EPNs in two different soil types (Bear Roots – silty; HREC -sandy). Pink beauty radishes was grown during the spring brassica growing season at each location. On each farm, a field that was cover cropped, plowed, disked, and fertilized using Pro Grow and compost was then formed into 46in wide beds. Radishes were direct seeded with an Earthway seeder with 1” spacing, ½” depth, and 3 rows/bed on June 1st at UVM HREC and June 13th at Bear Roots Farm.
We had five treatments replicated ten times at Bear Roots Farm and four times at UVM HREC, thus we had a total of 70 experimental units, 50 at Bear Roots Farm and 20 at UVM HREC. The plots at Bear Roots Farm were 10ft long with 10ft buffers and 9ft long with 9ft buffers at UVM HREC. Plots in adjacent beds at both sites were staggered such that no plot was immediately adjacent to a plot in an adjacent bed. The five treatments were 1) commercially available H. bacteriaphora and S. feltiae (CA:Hb+Sf); 2) commercially available S. carpocapsae and S. feltiae (CA:Sc+Sf); 3) locally adapted H. bacteriaphora and S. feltiae (LA:Hb+Sf); 4) locally adapted S. carpocapsae and S. feltiae (LA:Sc+Sf); and 5) an untreated control (UC). The treatments were applied immediately after seeding, on June 1st at UVM HREC and June 14th at Bear Roots Farm.
Locally adapted EPNs, S. feltiae (Sf), S. carpocapsae (Sc), and H. bacteriaphora (Hb), were sent from the Shields lab at Cornell. Approximately 2 weeks after inoculation, EPNs were separated from rearing materials by washing through a 20 mesh wire screen (841 μm openings) and then washed through 40 mesh (400 μm opening) screen with a large volume of non–chlorinated water following the protocol developed by the Shields lab. Locally adapted EPNs were applied as a soil drench at a dosage of approximately 23,200 IJs per ft2. Commercially available (CA) EPNs, NemAttack Pro™ – Sf, NemAttack Pro™ – Sc and NemaSeek Pro ™ – Hb, were purchased through Arbico Organics (www.arbico-organics.com) and applied at the labelled dosage of 3,125 IJs per ft2. The untreated controls received no nematode treatments, simply an equal amount of non-cholonated water that was used in the application of the nematode treatments.
Approximately 40 days after seeding (7/12/17 at UVM HREC and 7/26/17 at Bear Roots) ten plants per plot were chosen randomly and evaluated using a root damage index. Individual root damage scores were 0, no visible injury present; 1, superficial feeding scars present (no wounds reaching the root cortex); 2, deep scars or wounds present but tap root intact; 3, tap root severed or girdled but plant was alive; and 4, plant was dead.
A waxworm bioassay was used to measure EPN activity in the field at seeding, prior to EPN application, and at harvest. At each time interval, three soil samples were taken with a soil core (2.5 cm diameter by 30 cm deep) per treatment per block. The samples were then split into 0-10cm and 10-30cm sections. Each soil sample was thoroughly mixed and placed into a 0.5liter plastic cup. Ten waxworms (Grubco) were placed in each cup of soil and incubated in darkness for approximately 6 days at room temperature at UVM. After the incubation period, each cup was examined for nematode-infected waxworms (showing typical symptoms of nematode infection coincident with the presence of infective juvenile nematodes) and the infected waxworms counted. We are planning to determine the persistence of EPNs through the winter by conducting the same bioassays in April 2018.
We measured incidence of CRM damage (Figure 1) and EPN presence (Figure 2) in all tests plots at each location. Site effects factored significantly in relation to damage incidence. UVM HREC displayed little to no pest pressure (Mean RDI < 5) in all experimental plots. As a result, we did not see any trends associated with our EPN treatments at the HREC. Conversely, plots located at Bear Roots farm did indicate high levels of CRM damage with both commercially available and locally adapted Sf+Hb treatments showing the lowest mean damage (CA:Sf+HB = 18.5; LA:Sf+Hb = 17.3; CA:Sf+Sc = 21.8; LA:Sf+Sc = 23; UC = 18.8), though differences were not statistically significant (p > 0.05).
Our post-harvest bioassays indicated the presence of EPNs in all treatments excluding the untreated control at the HREC. Untreated controls at Bear Roots Farm did indicate the presence of EPNs within the soil despite negative bioassay results prior to EPN application. According to our post-harvest bioassays, EPN treatment plots showed high levels of persistence (> 50%) at both locations and treatment types. Only CA_Sf-Hb plots at Bear Roots displayed a lower than 50% infection rate (2/10 plots) for the EPN bioassays.
According to our results, evidence for the effective use of EPNs as biological control agents for CRM remains inconclusive. A couple of factors likely played a role in these suboptimal results. First, in the HREC plots, the lack of pest pressure precluded the collection of useful data for the assessment of CRM control. Second, due to early season flooding at the Bear Roots test plots, our EPN application did not coincide with the best application timing. We were racing against the clock and would have ideally applied the EPNs earlier in the season to better establish in the field. Finally, differences between the commercially available and locally adapted EPNs cannot fully be realized in a single season. The locally adapted strains of EPNs generally take longer to establish as their population dynamics need to equilibrate under new environmental conditions. This establishment can take up to two years before seeing widespread control. This is a stark contrast from the commercially available EPNs. These lab reared populations are presumably unable to overwinter in temperate soil and cannot survive without high concentrations of hosts. We believe that the lack of divergence between the two EPN results is the result of this short-term data collection rather than an indication of similarity in utility.
Although both locally adapted and commercially available strains of EPNs persisted from application to harvest in plots at both locations, the long-term persistence of the two types of EPNs remains to be evaluated. We will be sampling soil from both locations this coming spring to assess the overwintering capacity of the CA and LA strains of EPNs. This will provide a more comprehensive assessment of the persistence of the two different types. The positive identification of EPNs within select untreated control bioassays is likely the result of contamination during the soil sampling process or false positives in the scoring process and not the result of poor EPN application.
Education & Outreach Activities and Participation Summary
We will be hosting a farm partner banquet this coming January to discuss the successes, challenges and opportunities associated with this project. Attendees to this banquet will include up to seven farms in the region. During this banquet we also plan to pilot our farmer survey that will be used at our upcoming outreach events (e.g. Winter NOFA-VT, VVBGAC, etc.). These results will be reported in our final project report and will provide a clearer picture of the learning outcomes. We can validate, through our collaborations, that the owners/managers of Bear Roots Farm (Barre, VT), Alchemy Gardens (Shrewsbury, VT), Kettle Song Farm (Worcester, VT), Saint Michael’s College Farm (Colchester, VT), Intervale Community Farm (Burlington, VT) and UVM’s HREC (Burlington, VT) all have gained significant knowledge regarding the use of EPNs. These growers have also shown positive attitudes toward expanding the use of EPNs on their respective farms and are interested in future collaborations in using EPNs.
Project outcomes cannot be fully realized prior to receiving formal feedback from our farm partners and local grower community. We will complete this section following our winter outreach events and include it in the final SARE report
This assessment cannot be fully realized prior to receiving formal feedback from our farm partners and local grower community. We will complete this section following our winter outreach events and include it in the final SARE report.