Optimizing management of a new invasive species, swede midge, on small-scale organic farms: Part II

Optimizing management of a new invasive species, swede midge, on small-scale organic farms: Part II

Summary

Swede midge (SM) is an invasive insect pest threatening the viability of organic production of Brassica crops in the Northeastern US. SM attacks all Brassica crops, including broccoli, cauliflower, cabbage, kale, kohlrabi, etc., as well as canola and mustard-type weeds. SM is a fly that lays eggs in the meristems of these crops, and secretions of the feeding midges/larva cause scarring and distortion of plant tissues, including lack of head formation, resulting in unmarketable crops. Larvae pupate in the soil and emerge as adults with 4-5 overlapping generations per year. Once SM becomes established, it can cause complete loss of marketable crops. SM is quite small (< 2 mm) and its damage difficult to identify, so it is commonly misdiagnosed. There is an urgent need to develop effective pest management tools and to conduct outreach/education to familiarize at-risk small-scale organic growers with SM diagnosis and best management practices to protect them from devastating SM outbreaks.

                This is the second year of a project that seeks to advance our understanding of SM population dynamics in relation to management practices, to identify and optimize effective management strategies, and to increase awareness of SM and knowledge of its management among at-risk small-scale organic brassica growers.  We monitored SM populations on five organic farms in five counties in New York including the Cornell Organic Research Farm.  We conducted ten on-farm replicated small-plot research trials including evaluation of insect exclusion netting in combination with different mulch types, garlic oil as a repellent, SM plant preference, effect of plastic mulch on SM pupation and emergence, and effect of post-harvest practices, tillage and tarping on SM emergence. 

                It has become clear that effective and economical management of SM is very farm-specific. Five of the seven farms that we have worked with are no longer suffering from economic losses from SM; three are using far and wide crop rotation, one has adopted insect exclusion netting (IEN), and the fifth is using row cover, crop rotation and abandonment of broccoli production. Unfortunately, two farms continue to suffer economical losses from SM and we plan to continue our work on these farms.  We have promising leads on crop preference and plastic mulch/tarp barrier that need to be further developed to become additional effective management strategies.  We are currently working on developing educational materials for organic management of SM including an organic section for the SM information website, a fact sheet, newsletter articles and a SM diagnosis training video, all to be launched within the next year.  Our goal is that through our research and education, organic brassica growers will not suffer from economic losses due to SM.

Objectives/Performance Targets

Our objectives were to:

1. Advance understanding of swede midge (SM) population dynamics on small-scale organic Brassica farms related to management practices.
2. Optimize management strategies including newly developed disruption tactics (insect exclusion netting and garlic oil repellent), crop selection and rotation strategies.
3. Increase awareness of SM and knowledge of its management among at-risk small-scale organic Brassica growers.

This is the second year of this project. In 2016, we partnered with five at-risk small-scale swede midge (SM)-infested organic farms located in five NY counties, all of whom participated in our project in 2015.  These included Canticle Farm in Allegany (Cattaraugus Co.), Quest Produce in Almond (Allegany Co.), Fellenz Family Farm in Phelps (Ontario Co.), Muddy Fingers in Hector (Schuyler Co.) and Blue Heron in Lodi (Seneca Co.).  In addition, we monitored SM populations and conducted a trial at the Cornell University Organic Research Farm in Freeville (Tompkins Co.) for a total of six farms in six counties.

SM monitoring: For monitoring SM populations, we set up SM pheromone traps in a total of 50 sites across the six farms, which included 16 spring emergence sites (where SM-infested Brassicas grew previous fall 2015), nine spring, 12 summer and eight fall Brassica plantings, five high tunnels where Brassicas were produced, and two transplant production/hardening off sites.  Monitoring occurred from early-May until mid-November and traps remained at each site for an average of 13 weeks.  Number of SM adults per trap were reported to growers via email on a weekly basis.  At crop maturity, SM damage of the different brassica crops associated with each trap were each rated separately on a 5-point (0-4) scale.  Trap catch and damage information was used to help the grower cooperators make real-time management decisions to the best of our current knowledge.  For example, to indicate when spring emergence had subsided in order to determine when it was safe to plant a new Brassica crop.  Trap catch data from the same farms over multiple years will allow us to test our predictions, monitor annual variability and evaluate efficacy of implemented management strategies over time.  Additionally, to accurately determine completion of spring emergence, at four of the spring emergence trap sites (where SM-infested brassicas were grown during previous fall), we placed the SM traps under each of three hand-made 2 ft x 2 ft inclusion cages that were made of insect netting to capture only the SM that emerged from the soil while excluding those that originated elsewhere.  Similarly, we set up these cages in the spring at four sites where summer-harvested Brassicas were grown the previous year. 

Optimizing use of insect exclusion netting (IEN): We continued our efforts to understand the benefits and detriments of using IEN in combination with different mulch types for spring and summer plantings of broccoli.  We also trialed another type of IEN that was more durable and green in color with claims of creating a cooler microclimate (Filet Anti-Insect AF4040, 70 gram, 16 ft width, HarvestTech) compared to our standard white IEN (ProtekNet, 25 gram, 14 ft width, Dubois Agrinovation).  At Muddy Fingers, a trial was set up to compare hay mulch to landscape fabric under IEN in a spring planting of broccoli.  Unfortunately, a rodent (rabbit or woodchuck) broke into the IEN and destroyed most of the broccoli.  However, we did have an Onset Hobo pendant temperature data-logger installed inside the tunnels so that we can compare differences in temperatures between the two mulch types under IEN.  Another trial was set up in a summer broccoli planting at Muddy Fingers with the following treatments: 1) white IEN over hay mulch; 2) white IEN over reflective silver plastic mulch; 3) white IEN over white plastic mulch; and 4) green IEN over hay mulch).  At Canticle, a trial was set up to compare white IEN over black plastic biodegradable mulch and bare ground.

                Due to the small-scale production of Brassicas on organic farms, randomized complete block designed (RCBD) trials with several replications were not be feasible.  All treatments in each trial were replicated twice and ranged in length of bed (3 ft wide with 2 rows of broccoli) from 20 feet to 50 feet.  Treatments will include a minimum 20 plants/treatment-replicate and 3 replications.  A SM pheromone trap and Hobo temperature sensor were placed under the IEN in each replicate of each treatment.  At Canticle, SM traps and temperature sensors were also set up in open air over both bare ground and black plastic mulch.  Traps were serviced as per the monitoring project.  At crop maturity, SM damage was rated using the 5-point scale.  Other differences among treatments including plant size, maturity and other pest pressure were quantified, when relevant. 

Garlic oil repellant trials: Canticle hosted three trials evaluating essential garlic oil 1% (Bulk Apothecary), which was trialed with surfactant, Nufilm-P (Miller).  Trials were set up in a spring planting of Red Russian kale, summer planting of kohlrabi and fall planting of Red Russian kale.  Trials were arranged as randomized complete block designs with 3-5 replicates and 10 plants/treatment-replicate.  Garlic oil solution was applied using a spray bottle to individual plants until run-off.  Applications started within 1 week of transplanting and continued weekly until 2 weeks pre-harvest.  At harvest, SM damage was rated.  Another trial was set up at Quest Produce in a spring broccoli planting, but the broccoli plants were very weak and not enough of them survived to give meaningful results.     

Post-harvest SM monitoring: We planned to study SM population dynamics in relation to crop stage of both harvested crop and nearby crops in order to help us to understand the risk associated with not destroying a crop post-harvest.  Each week after harvest, the stage of the harvested crop with particular attention to development of secondary side-shoots was to be recorded, and side-shoots inspected for SM-infestation.  The same data was to be collected from nearby Brassica crops as well as SM damage rated.  Traps were to be located in the harvested and nearby Brassica crops.  We did not conduct this study as planned due to lack of appropriate planting to use.  Instead, we ceased opportunities to set up a couple more trials: an evaluation of post-harvest practices on SM emergence at Canticle and an evaluation of the effect of plastic mulch on SM pupation at Quest Produce. 

Evaluation of spring emergence following different mulch type. At the Cornell HTC Vegetable Research Farm, an intensive systems study was underway in cabbage that suffered SM-infestation in 2015.  The project is studying six tillage systems overlaid with three mulch types.  We took the opportunity to compare the effect of fall shallow tillage to no tillage and to no tillage + spring tarping on the effects of spring emergence on SM.  Our hypothesis was that fall tillage would disrupt the SM pupae within the top 1 inch of soil and reduce SM emergence in the spring.  We hypothesized that tarping would prevent SM from emerging.  We set up emergence cages as previously described in early May in each of three replicates per treatment, and continued to monitor SM until their emergence was complete on Jul-27. 

Post-harvest practices on SM emergence study (add-on): At Canticle, the grower direct seeded a bed of the seemingly SM-preferred Red Russian kale to see if it would serve as a “trap crop” for SM on his farm.  Although we were not able to evaluate whether this planting drew in SM that would have otherwise infested other brassica plantings, it certainly became heavily infested with SM.  We took advantage of this SM-infested crop to set up a trial to evaluate post-harvest practices on SM emergence.  We set the trial up just after SM larvae dropped to the soil to pupate (as SM larvae could not be found in damaged plants or new growing tips).  Treatments included, 1) Untreated where Red Russian kale was left to grow; 2) shallow tillage (grower practice); and 3) black plastic mulch, which was placed over top of living RR kale plants (so as not to disturb the soil).  Each treatment plot was a section of 4 ft wide bed by 6 ft long with two replicates.  IEN was placed over top of each plot and a SM pheromone trap placed inside to ensure that trap catches came only from the 4 ft x 6 ft area.  SM trap catches were collected from Jul-7 until Oct-19.

Crop preference study (add on): In our first year of study we found that Red Russian kale and Asian brassicas were the most and least preferred brassica crops, respectively, while SM caused the most economical damage to broccoli, because the least damage deemed the crop unmarketable.  To understand the dynamics of SM between preferred and less preferred crops, we decided to set up replicated small-plot trials at Quest Produce at a site where severely SM-infested broccoli occurred the previous fall.  Treatments included broccoli, Red Russian kale and Bok Choy (Asian brassica), which were planted alone and in two-crop combinations for a total of six treatments, which were replicated 4 times; reps 1 and 2 were planted on May-6 and reps 3 and 4 were planted on Jun-3.  Organic transplants were produced by Quest Produce.  The trial was planted on a 3 ft wide bed with black plastic mulch and drip irrigation.  Each treatment plot was a bed width ( 3 ft) by 12 feet long with about 20 plants of broccoli and Bok Choy and 40 plants of Red Russian kale (narrower plant spacing).  In the combination treatments, different crop types were grown side-by-side with each row being a different crop.  At maturity, SM damage was rated and incidence quantified.

Effect of plastic mulch on SM pupation and emergence (add-on): After the preference study at Quest Produce was complete, we took advantage of the severely SM-infested Red Russian kale to evaluate the effect of black plastic mulch compared to bare ground on SM pupation and emergence.  Growers question whether plastic mulch may serve as a barrier to SM larvae pupating in the soil, and if so, could this be used as a strategy to reduce SM population.  Each Red Russian kale plot was divided in half and the plastic mulch removed from one-half of the plot.  Emergence cages were set up over top of the infested kale on both the bare ground and black plastic mulch sides of each plot in each of the four replicates.  SM trap catches within each cage were monitored from Aug-12 to Oct-19.

Analysis: Data collected from replicated trials was/will be analyzed using General Analysis of Variance and means will be separated using Fisher’s Protected LSD test with 5% significance.  Monitoring data was/will be summarized and related to the unique circumstances of each farm.

Economic analysis: Only differences in inputs relating to SM management will be considered including cost of IEN, garlic repellant, and labor requirements associated with treatments.  Yield data from the trials will be used, and growers will be asked to provide sale prices and input costs.

Education and Outreach: Our goal is to have our educational efforts reach the majority of at-risk organic small-scale Brassica growers in Northeastern US where broccoli, cabbage and cauliflower are grown on 364 farms.

Hoepting was to write an informational article designed to alert Brassica growers to be aware and on the lookout for swede midge (SM).  The article was to include details about diagnosing and scouting for SM as well as preventative management tactics and was to be distributed in the Northeast Organic Farming Association (NOFA) newspaper, The Natural Farmer and Cornell’s Small Farm Quarterly Summer 2016 issues, which should blanket Brassica growers in the Northeast US.  An article to alert growers of SM damage was published in the September 21 issue of Cornell Cooperative Extension newsletter, Veg Edge.  During the growing season, grower cooperators did not host twilight meetings where SM management were demonstrated.  Rather, we participated in the Cornell Small Farms Program and Reduced Tillage in Organic Vegetables Field Day at the Organic Research Farm in Freeville, NY on August 17, 2016 where 47 participants learned how to diagnose SM damage and the latest research underway to manage this pest. 

Updates and recommendations for managing SM generated from this project will be summarized in another article that will be distributed in The Natural Farmer and Small Farm Quarterly in Spring 2017 issues, as well as other similar Extension newsletters/websites.  Hoepting and Chen did not present latest research results/recommendations at the NOFA-NY and -VT winter conferences, as originally planned, but will pursue again in 2018, if relevant. Growers hosting on-farm trials and monitoring serve as resources to other interested growers.

In 2016, we began work towards developing a new section for the “Swede midge information site for the US” website (http://web.entomology.cornell.edu/shelton/swede-midge/), originally developed by Cornell University, to include organic management.  This is in collaboration with Elisabeth Hodgedon, Ph. D. candidate under direction of Dr. Yolanda Chen, University of Vermont.  It will review all of the strategies that have been tested and failed in order to prevent blind implementation of failed methods as well as the new information and recommendations derived from this project.  Hodgedon is working on sections including intercropping, mating disruption, essential oils and the economics of SM damage and management strategies, while we are summarizing crop type differences, insect exclusion netting, crop rotation and separation, SM monitoring, natural enemies, biological controls, plant resistance and organic insecticides.  Plans include to also develop a fact sheet for organic management of SM in 2017.  In fall 2016, footage was shot to produce a “how to diagnose SM” video, which should be available by the 2017 growing season.

Accomplishments/Milestones

• Meet with grower cooperators to devise their individual case studies and on-farm trials: where monitoring traps will be placed, how exclusion netting and garlic repellency trials will be set up (crop type, which planting, plot size, whether exclusion netting will be with or without mulch, type of mulch, etc.) Hoepting and grower cooperators. April 2016. Successfully executed.

• Write and submit “Be on the lookout for SM” article to NOFA newspaper, The Natural Farmer and Small Farms Quarterly. Hoepting. April 2016. Did not do. Instead, an article, “Do you have swede midge? New pest of brassicas often misdiagnosed” and “Swede midge diagnosis in kohlrabi” were published in September 21^st issue of Veg Edge, Cornell Cooperative Extension newsletter.

• Order field supplies (pheromone traps supplies, etc.). Hoepting. April 2016. Successfully executed.

• Deploy first monitoring traps. Hoepting and Celentano. May 2016. Successfully executed.

• Set up SM traps in mulch study at Freeville Organic Research Farm. Hoepting, Celentano, Maher. May 2016. Successfully executed.

• Monitor traps, assess SM damage, quantify trap captures and disseminate information to grower cooperators. Spray garlic repellency trials. Celentano, Taleb and Hoepting. May to mid-November 2016, weekly or bi-weekly visits. 16 hours per week for 20 weeks. Successfully executed.

• Harvest evaluations of trials. Hoepting, Celentano and Taleb. June to October, 2016. Successfully executed.

• Conduct Twilight Meetings, as appropriate. Hoepting, Celentano and Taleb, Maher. August and September, 2016. Participated in Cornell Small Farms Program; Reduced Tillage in Organic Vegetables Field Day at Organic Research Farm, Freeville, NY on August 17, 2016.

• Add-on: shot footage for use in “how to diagnose SM” video. October 2016. Successfully executed.

• Collect grower feedback and economic info from growers. Hoepting, Hall and Taleb. November 2016. To be completed in winter 2017.

• Data entry, analysis and summary. Taleb, Hoepting. May-December 2016. In-progress.

• Write and submit annual report. Technician, Hoepting. December 2016. Successfully executed.

• Present results at NOFA-NY winter conference. Hoepting. January 2017. Will not do in 2017. Presented a workshop in 2016 NOFA-NY conference.

• Write and distribute project results in newsletter article, update SM website with latest information. Hoepting, Chen. February 2017. On-track. Working on content for SM website.

• Present results at NOFA-VT winter conference. Chen. February 2017. Will not do in 2017. Presented a workshop in 2016 NOFA-NY conference.

• Write and submit final report to NESARE. Hoepting. May 2017. In-progress.

Impacts and Contributions/Outcomes

Two years into this project, it has become clear that effective and economical management of swede midge (SM) is very farm-specific. Five of the seven farms that we have worked with are no longer suffering from economic losses from SM; two are using far and wide crop rotation, one has adopted insect exclusion netting (IEN), and the third is using row cover, crop rotation and abandonment of broccoli production. Unfortunately, Blue Heron and Canticle farms continue to suffer economical losses from SM and we plan to continue our work on these farms.

Blue Heron grows 12 acres of mixed vegetables in three main fields (No. 1, 4 & 6) that are separated by wooded areas and tree lines. They also have 6 high tunnels where several brassicas crops and transplants are produced. Although strides were made towards developing an SM management plan in 2015, the farm transitioned to new owners during 2016 season and no SM management strategies were implemented. It appears that SM pressure increased even more at Blue Heron since 2015. In 2016 in field 1, SM populations built slowly and a summer planting of broccoli was harvested with only minor SM damage. Unfortunately, the crop was not destructed post-harvest and the SM population eventually built up in late September and remained high until late-October.  It is expected that there will be a large spring emergence of overwintering SM at this site in spring 2017. In field 4, brassicas were planted throughout the season from mid-April until late August and were exposed to high spring emergence pressure. Although the summer planting of red cabbage produced mostly marketable heads, with SM pressure continuing to build into the fall, fall plantings of broccoli and Romanesco had 55% and 33% unmarketable heads, respectively. It is expected that this field will have very high emergence of SM in spring 2017.  Field 6 was planted to fall cabbage after the majority of spring emergence was complete and suffered no losses from SM.  Spring SM emergence is expected to be lowest in field 6 compared to fields 1 and 4 in 2017.  SM was captured and damage observed in brassica plantings in the high tunnels and transplant hardening off area With such a serious and potentially devastating infestation of SM at Blue Heron and the new owners from Pennsylvania having no experience with SM, it is critical that we continue to work on this farm to develop and implement economical management strategies.

Canticle Farm grows over 40 different kinds of vegetables year round on about 8 acres and in three high tunnels, of which about 1 acre is cropped to brassicas from mid-April until mid-December. In 2015, the farm suffered from 69% unmarketable heads in fall broccoli in the back field and the summer planting of Red Russian kale (RR kale) was 75% infested with SM.  Using trap catch data as an indicator, it appears that the SM population on this farm has increased from 2015 to 2016. In 2016, spring kohlrabi had 60% SM incidence and 23% unmarketable bulbs. The SM population moved to the front corner field where it caused 53% of the broccoli to be unmarketable.  In the middle field, the late summer brassica plantings suffered from 40%, 20% and 13% unmarketable crop loss in broccoli, cauliflower and turnip, respectively. The challenge with managing SM at Canticle is the small land base and high proportion of susceptible brassicas for which crop rotation/separation are ineffective. It is important that we continue to work on this farm to develop an effective SM management plan.

A critical component of developing a SM management plan for a particular farm is to understand the SM population dynamics on the farm. Our farm monitoring showed variability of SM populations between the very different growing seasons of 2015 and 2016. Generally, spring emergence of SM was slower and to a lesser extent in 2016 than it was in 2015, which was likely a consequence of the very dry weather in spring 2016 compared to the wet weather of spring 2015, as SM require moisture to trigger emergence. The milder fall weather of 2016 allowed SM populations to remain very strong throughout the month of October, whereas they dropped off in October 2015. 

Farm monitoring of SM populations demonstrated that far and wide crop rotation can be an effective management strategy on certain farms. For example, at Blue Heron farm in 2015, red cabbage that was planted in mid-July just as spring emergence of SM was nearing completion (in field 1) resulted in only 2% unmarketable heads, compared to when it was planted in mid-April and exposed to SM season long, which resulted in 100% SM incidence and 72% unmarketable heads.

In our 2015 and 2016 trials, insect exclusion netting (EIN) was highly effective in protecting brassica crops from SM damage, especially on farms that were too small for crop rotation to be effective. Four on-farm trials showed that broccoli could be grown under IEN with no losses from SM, compared to 50-85% unmarketable plants grown in open air. As long as the ground had not recently been cropped to brassicas, IEN was effective. Use of various mulches in combination with IEN proved important for effective weed management. Differences in plant development and quality occurred between IEN and open air and among mulch types under IEN that resulted in both increased and decreased yield. For example, compared to open air, broccoli under IEN was advanced in spring, but suffered heat stress in fall. Similarly, under IEN, fall broccoli suffered more heat stress with black plastic than with straw mulch or bare ground. IEN also led to a beneficial exclusion of flea beetles and a detrimental inclusion of cabbage worms (from infested transplants) and slugs. In 2016, we worked towards optimizing the effects of IEN in combination with different mulches on their effects on plant development and the entire pest complex.  Muddy Fingers farm have readily adopted use of IEN to control SM in their broccoli, while Canticle is still deliberating on the economic feasibility of this expensive and labor-intensive strategy.

Differences in SM damage among different types of brassica crops exposed to the same SM population occurred. At one site, unmarketable heads at harvest due to SM damage was 72%, 15% and 7% in red-, green- and pointed cabbage, respectively. In another planting, broccoli, cauliflower, Romanesco and red cabbage had 55%, 0%, 33% and 0% unmarketable heads, respectively. At another site, winterbor and purple kale had 20% and 30% plants with minor SM damage, respectively, while 75% of RR kale plants were infested with minor/moderate damage. Other brassica crops that appeared to be disfavored by SM in our case studies were turnips/radishes and Bok Choy and other Asian brassicas. These observations begged the question whether differences in plant type could be incorporated into SM management strategies, particularly on farms like Canticle that are too small for crop rotation to be effective and on farms like Blue Heron that are too big for IEN to be economically feasible.

To address this question, we conducted a preference study in spring 2016 at Quest Produce at a site that had a high SM population in fall 2015 in order to capture SM pressure from the expected high spring emergence. Treatments included homogenous plantings of each broccoli, RR kale and Bok Choy, and heterogeneous plantings of broccoli + RR kale, broccoli + Bok Choy and Bok Choy + RR kale each in two 14 ft long rows with 4 ft between plots. SM infestation occurred naturally. SM Incidence ranged from 85-99% for RR kale, 35-62% for broccoli and 0% for Bok Choy, clearly demonstrating the preference of SM for RR kale over broccoli and the extreme preference for either of these crops over Bok Choy. Homogeneous plots of broccoli averaged 60% SM incidence of which 44% were unmarketable heads, which was higher than the broccoli grown in heterogeneous plots with RR kale (42% incidence; 37% unmarketable) and Bok Choy (35% incidence; 19% unmarketable). Our finding that SM damage was less in broccoli when grown in heterogeneous plots with either a more or less preferred crop begs the question whether such crops could be used as trap crops to alleviate SM damage in broccoli. It would also be worthwhile to investigate what effect crop type has on buildup of SM population, and importance and timing of crop destruct of SM trap crop.

                Although our data has not been analyzed completely, a preliminary look at our findings indicated that there were no differences between fall tillage and no tillage with regards to spring emergence of SM. Especially interesting was that SM emerged underneath a tarp in a no tillage treatment. If SM can emerge underneath a tarp and then presumably perish, perhaps this could be a management strategy to destroy an over wintering population that could be readily implemented on small farms that do not have enough land base for far and wide crop rotations to be effective; an area that certainly warrants further study. Our preliminary data also indicated that plastic mulch resulted in lower SM trap catches where SM-infested plants were grown on plastic compared to bare ground and where SM infested ground was covered with black plastic compared to where SM infested bare ground was tilled. These important leads require further investigation under controlled conditions.

Collaborators: