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Funded Project

Funding Program: IPM Enhancement Grants

Project Title: Pest Risk Assessment and IPM Tactics to Monitor and Control Wireworms for North Florida Sweet Potato Growers

Project Directors (PDs): Norm Leppla [1] Robert Hochmuth [2]

Lead State: FL Lead Organization: University of Florida

Undesignated Funding: $29,998

Start Date: Mar-01-2015 End Date: Feb-29-2016

No-Cost Extension Date: Feb-28-2017

Pests Involved: wireworm

Site/Commodity: sweet potato

Area of Emphasis: IPM

Summary: Sweet potato acreage in North Florida and the general region is increasing rapidly but some growers are experiencing such heavy losses due to wireworms that the crop is not economically viable. One 88-acre farm, for example, had 75% sweet potato damage at harvest after using cultural practices that were ineffective for suppressing wireworms in the soil prior to planting. Moreover, after repeated applications of soil insecticides and foliar sprays, significant numbers of wire worms still were present in every sample collected in the fields. A year of preliminary work during the 2014 North Florida sweet potato growing season indicated that cultural practices and wireworm damage varied widely between farms and that the highest level of control can be achieved by making two deep harrow passes through the fields to destroy the subterranean larvae and supporting crop residue. The two predominant wireworm species that survived surface tillage and damaged the sweet potatoes were different than those encountered further north in the Carolinas. Insecticide resistance has not been shown for these wireworms but currently registered insecticides have not been effective. Consequently, the growers are requesting emergency or special local need registration for additional insecticide products. More intensive evaluations are needed of IPM tactics for minimizing wireworms, such as irrigation systems that enable deep plowing, harrowing depths and passes, formulation and application of soil insecticides, effectiveness of foliar insecticides, and impact of crop rotation. These evaluations will require more efficient and accurate sampling of larvae, trapping of adults, and assessments of sweet potato damage at harvest. This project will identify the farms in North Florida where sweet potato is or will be grown in the near future and establish a network to update the number of farms, acres planted, sweet potato yields, and level of damage due to wireworms. We will determine the best sampling methods, intensity and frequency for delimiting the distribution and abundance of wireworms in the fields, asses the current risk due to wireworms for North Florida sweet potato growers to expand acreage, and recommend the best possible preventative IPM tactics. Objectives: 1. Identify the farms in North Florida where sweet potato is or will be grown in the near future. Long-term outcome: We will have a network in place to update the farms, acres planted, sweet potato yields, and level of damage due to wireworms in North Florida. The database will be maintained at the Suwannee Valley Agricultural Extension Center. Medium-term outcome: Preliminary information will be obtained from cooperating farmers in Suwannee County, including the Thomas farm (36 acres), Trey Farm (28 acres), Lee Farm (90 acres, Madison County), and Delegal Farm (88.7 acres), all planted in Covington sweet potatoes. Short-term outcome: Cooperating county Extension agents will assist in identifying farms that currently grow sweet potatoes or plan to grow them in the near future. Suwannee, Madison, Hamilton and Columbia counties will be targeted for the survey. 2. Select, map and characterize fields to be sampled for wireworms. Long-term outcome: A geographic information system (GIS) database will be created and maintained at the Suwannee Valley Agricultural Extension Center. Medium-term outcome: Characterization of the fields will include basic cultural practices, such as tillage, crop rotation, planting schedules, irrigation systems, and pesticide use. Short-term outcome: Extension agents who work directly with the sweet potato growers will assist in selecting and mapping the fields. The agents are listed as cooperators in this proposal. 3. Determine the best sampling methods, intensity and frequency for defining the distribution and abundance of wireworms in the fields. Long-term outcome: Know the minimum number and distribution of samples and maximum number of wireworms per sample required to monitor wireworm populations in North Florida sweet potato fields. Medium-term outcome: Assess the abundance and occurrence of the two most common species of pest wireworms: Conoderus rudis (black, the most common species) and C. scissus (brown, the other common species). Short-term outcome: Evaluation of methods for sampling wireworm larvae (buried corn and oats) and adults (yellow sticky, black light, hanging, and pitfall traps). In preliminary scouting for a grower, buried corn and oat traps recovered from a field at Delegal Farm yielded 657 wireworms in September 2014. 4. Determine the economic injury level of sweet potato due to wireworms and recommend preventative IPM tactics. Long-term outcome: Establish an accurate threshold for controlling wireworms in sweet potato fields and provide effective, economical and environmentally acceptable IPM tactics, e.g., crop rotation, pre-plant tillage, irrigation systems, and available and newly registered insecticides. Medium-term outcome: Provide a general threshold for controlling wireworms in sweet potato fields and recommend IPM tactics, initially pre-plant tillage with two heavy harrow passes and registered soil-applied insecticides at planting to control larvae. Short-term outcome: Assess the current risk wireworms pose for North Florida sweet potato growers to expand acreage. We estimate that 4,000 or more acres of sweet potato will be added to the Suwannee Valley area during the coming year.

Final Report:

Outputs Objective #1. Identify the farms in North Florida where sweet potato is or will be grown in the near future. Output: During the period of this project (2015-16), at least seven farms in Suwannee, Madison and Hamilton counties were growing commercial acreages of sweet potato, Ipomoea batatas (L.): Townsend Brothers, Generation Farms, Herring Farms, Blanton Farms, Coggins, Starling, and Wynn Farms. A total of about 3,000 acres of sweet potatoes were planted on these farms. In 2014, sweet potato farms sampled in this region included Thomas (36 acres), Trey (28 acres), Lee (90 acres), and Delegal (89 acres). All of the farms were planted in Covington sweet potatoes. Additional farms that grow sweet potatoes in the future will be identified and included in the informal grower network. Data will be recorded continuously on acres planted, sweet potato yields, level of pest and disease damage, and cultural practices. According to the 2016 National Agricultural Statistics Service database, sweet potatoes are grown in Suwannee, Madison and Hamilton counties but production data are “withheld to avoid disclosing data for individual operations.” Objective #2. Select, map and characterize fields to be sampled for wireworms. Output: GPS coordinates have been recorded for the farms included in 2016: 1) Townsend Brothers Farms Cliff’Big Pivot (30o24’32.10” N, 83o03’34.38” W, elev. 119 ft.) and , 2) Regan Half Circle Pivot (30o24’22.95” N, 83o17’18.47” W, elev. 87 ft.), 3) Mercer Farm (30o24’18.76” N, 83o15’36.34” W, elev. 78 ft.), 4) SVAEC (30o18’22.55” N, 82o53’58.30” W, elev. 163 ft.), and 5) Wynn Farm (30o29’41.29” N, 83o08’19.61” W, elev. 89 ft.). Each farm layout has been added to a map of the 3-county area. The soybean crops were planted after soybean, corn, pine, millet, pasture or soybean. All of the fields had center pivot irrigation that facilitated deep plowing, except one with buried irrigation drip tape that was not included in the analysis. Insecticides alone have not been effective in managing wireworms in North Florida, e.g., Mocap (ethoprop) in early season, Telone (1,3 dichloropropene), or Capture (bifenthrin). The growers have tried various insecticides, formulations, and application methods without complete success. They would like to use Regent (fipronil) but registration for sweet potatoes has been denied. Most of the wireworm damage to sweet potatoes occurs mid to late-season when it is difficult to apply insecticides and there is a 190-day pre-harvest interval. The region tends to have deep sandy soils, often in the category of loamy sands. These soils are highly leachable making pesticide and nutrient management challenging. Wireworm populations apparently go deep in the soils over the winter season and return to the root zone of the sweet potato crop when soils are warm and moist. Sweet potato transplants are typically planted in North Florida in mid-April to May and harvested in August through early November. Objective #3. Determine the best sampling methods, intensity and frequency for defining the distribution and abundance of wireworms in the fields. Output: Trapping and sampling techniques included yellow sticky traps, tap samples, baited and un-baited soil samples, and examination of tubers throughout the season. Baits were a 4-ounce mixture of corn and oats in equal parts by volume blended into half of the substrate excavated from 3 to 6 or 9 to 12-inch deep holes. After the bait was placed in a 6-inch diameter hole, the remaining soil was used to fill the hole to the surface. In 2015, wireworm sampling was conducted on four of the sweet potato fields totaling approximately 950 acres to characterize the wireworm abundance, distribution, and species complex. Larval elaterids collected from the baited soil were identified as Conoderus rudis (Brown) and C. scissus (Schaeffer). Other pests occasionally found within the fields included the beet armyworm, Spodoptera exigua Hübner; fall armyworm, S. frugiperda (J. E.Smith); cabbage looper, Trichoplusia ni (Hübner); tobacco hornworm, Manduca sexta (L.), and aphids from tap samples, and both aphids and a sweet potato weevil from the yellow sticky traps. A 5-acre grid was established in each field with 9 bait traps, 9 un-baited holes, 1 yellow sticky trap at each corner of the grid, and 4 tap samples. The bait traps and un-baited holes were placed in the center of a row halfway between two sweet potato plants. No wireworms were caught in the soil samples that were not baited. So, only the bait traps were useful for sampling wireworm larvae and depth of the traps had no significant effect on the number of wireworms trapped. The greatest number of wireworms caught in a single trap was two. For Wynn (3 fields) and Starling Farms, respectively, bait traps captured 0, 0, 0, and 9 wireworms, and the sticky traps caught 0, 50, 4, and 36 click beetles. During the previous year, at Thomas, Trey, Lee, and Delegal Farms, the respective number of click beetles captured was 26, 16, 11, and 14 during a 1-week period. No wireworm adults were collected in the tap samples. The distribution and abundance of wireworms in the fields also was evaluated by harvesting sweet potatoes and checking the tubers for damage caused by this pest. Sweet potato damage caused by wireworms was minimal and had a discontinuous distribution in the fields. For the four fields sampled in 2015, damage at the Wynn Farm was 0.8% (1/130), 4.0% (4/101), and 4.9% (6/123), and at Starling Farm 33.9% (56/165). Damage for the four fields sampled in 2014 was 6.5% (13/200), 17.5% (35/200), 11.5% (23/200), and 74.7% (657/880), respectively. This variability in damage within the same farm is typical. The sandier areas of a field often have higher wireworm damage. During the 2016 sweet potato growing season, each of the five farms listed in objective #2 were sampled by positioning 30 bait traps about 20 yards apart in a row. A 9-inch deep hole was dug for each trap and ½ cup of the bait mixture was added. The traps were left in the field from October 28 to November 14, after which they were removed, placed in individual plastic bags, and washed through a fine sieve. No wireworms were recovered from the commercial soybean fields but 30 were found in the samples from the Suwannee Valley Agricultural Extension Center. These wireworms were divided into two groups, one left in a 5-gallon bucket containing the trap substrate and, for the other, individual wireworms were placed in a 12-oz plastic cup. The cups contained a 50:50 mixture of fine sand and Miracle-Grow Plant Food (Scotts Company), and 4 corn kernels and 4 oat seeds that germinated. The substrate was kept moist and the containers were held at 80oF, 60% RH and L:D 14:10. After about 2 months, five wireworms survived with an average length of 1.8 mm. Objective #4. Determine the economic injury level of sweet potato due to wireworms and recommend preventative IPM tactics. Output: Sweet potato injury caused by wireworms is evaluated at harvest and can be severe. In 2014, one field that was not tilled due to buried irrigation drip tape had 74.7% (657/880) of the tubers damaged. The mean number of tubers per plant in 2015 was not significantly different within a given field, but the field with the greatest incidence of wireworm damage had significantly more tubers per plant (8.3, p-value 0.0011). The other three less damaged fields had means of 6.6, 5.5, and 6.1 tubers per plant. Regardless of year, the damage was greatest when the sweet potato crop followed corn or pasture and least after soybean, millet and pine. Wireworm populations seem to build-up over years, rather than reach an economic injury level in a single season. Commercial growers reported that 20% or more wireworm damage is very costly because excessively damaged sweet potatoes often must be sold for processing rather than as a fresh market product. The associated reduction in price varies from year to year, but is a significant decrease of 25-50%. Small reductions in quality (pack out %) due to wireworm damage can easily reduce profits by $500 to $1,000 per acre (http://www.lsuagcenter.com/~/media/system/4/1/f/c/41fcbe84904736b5a5d68c0acf492446/2017%20sweet%20potato%20enterprise%20budgets%20-%20finalpdf.pdf

Outcomes Objective #1. Identify the farms in North Florida where sweet potato is or will be grown in the near future. Long-term outcome: An informal network is in place to update the list of farms, acres planted, sweet potato yields, and level of damage due to wireworms in North Florida. The database is being maintained by Bob Hochmuth at the Suwannee Valley Agricultural Extension Center in cooperation with Extension agents in Suwannee, Madison and Hamilton counties. The network enables rapid and effective communication among the sweet potato growers. Objective #2. Select, map and characterize fields to be sampled for wireworms. Long-term outcome: Sweet potato production in North Florida has expanded recently. To the established smaller farms with 5 acres or less that grow sweet potatoes for local markets have been added a few much larger operations that sell to wholesale chains and processors. Most of the fields are isolated from sources of wireworms and other pests of sweet potato, such as the sweet potato weevil, Cylas formicarius (Fab.), so cultural practices and carefully timed insecticide applications should be effective in preventing local outbreaks. Cultural practices involve tillage, crop rotation, planting schedules, irrigation systems, fallowing, weed control, flooding, and incorporation of specific soil amendments. Fields maintained weed-free had significantly fewer wireworms compared to those that were weedy and biofumigants, such as partially defatted Ethiopian kale seed meal, Brassica carinata A. Braun, can lower wireworm populations. Insecticides and fumigants currently registered for wireworm in sweet potato in Florida contain thiamethoxam (IRAC group 4A), 1,3 dichloropropene & chloropicrin (IRAC group 2A & 8B), bifenthrin (IRAC group 3A), chlorpyrifos (IRAC group 1B), clothianidin (IRAC group 4A), ethoprop (IRAC group 1B), and phosmet (IRAC group 1B). The North Florida region has very diverse agriculture with many agronomic and vegetable crops being grown. As a result of this diversity, no standard crop rotation exists in the region, making predictions about wireworm populations more difficult. Sweet potato has been planted after pine trees, bahiagrass, soybeans, millet, peanut, corn, watermelon, carrot, and sweet potato Objective #3. Determine the best sampling methods, intensity and frequency for defining the distribution and abundance of wireworms in the fields. Long-term outcome: Wireworm populations in North Florida sweet potato fields can be detected using bait traps but due to their discontinuous distribution a large number of traps would be required. Post-harvest estimates of sweet potato damage more efficiently indicate the level of infestation in a field. The distribution of damaged tubers can be used to estimate wireworm population levels in the sweet potato fields over successive seasons. There was a strong correlation between the percentage of tubers damaged in a field and the number of wireworms captured per bait trap (R2 = 0.96). More attractive baits and associated methods of deployment are needed for the larval traps to be used routinely for scouting. It also is necessary to develop less time-consuming methods for extracting the larvae from the bait/soil mixture. Objective #4. Determine the economic injury level of sweet potato due to wireworms and recommend preventative IPM tactics. Long-term outcome: IPM systems can be designed to improve grower income by selling a better grade of sweet potato at harvest and reducing overall insecticide use while minimizing wireworm damage. Proper crop rotations and early season insecticide control measures are very effective for overall management of wireworm. Insecticides could be applied to larval food, such as inside or coating seed baits, due to low rates of insecticide penetration through the integument of wireworms versus ingestion. Available pre-plant and foliar applied insecticides could be incorporated. Deep pre-plant tillage would destroy larvae in the soil or expose them to desiccation and predation. Commercial fields should be tracked for cultural practices (crop rotation, tillage, etc.), insecticide use, and the abundance of wireworms prior to the sweet potato growing season. It is not necessary to register new insecticides for soil application unless wireworms develop resistance to current active ingredients. The goal is to predict the wireworm population level in a field before a sweet potato crop is planted, thus informing decisions about the risk of crop damage.

Report Appendices Sweet Potato IPM References [DOCX] [PDF] Sweet Potato Production and Wireworm Pests in North Florida [DOC] [PDF]

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