Potential for a Pheromone Mating Disruption Program for the Invasive Swede Midge within Complex Annual Rotational Systems

Final report for LNE18-368R

Project Type: Research Only
Funds awarded in 2018: $199,854.00
Projected End Date: 11/30/2021
Grant Recipient: University of Vermont
Region: Northeast
State: Vermont
Project Leader:
Dr. Yolanda Chen
University of Vermont
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Project Information

Summary:

Swede midge is an invasive pest that affects the production of Brassica vegetables. Since the midge is highly effective in causing marketable damage, novel approaches for managing midge infestations are needed. We previously found that the synthetic female sex pheromone can be effective in confusing male midges, potentially disrupting reproduction. However, considerable challenges exist in deploying pheromone mating disruption within annual vegetable systems, where annual rotations exist. We plan to develop an improved understanding of swede midge adult emergence, mating, and dispersal in order to develop recommendations on where and how pheromone emitters should be deployed within annual vegetable cropping systems. We found the majority of overwintering adults emerge in Year 1, with only 1.4% of individuals emerging in Year 2. Less than 0.001% of the individuals emerged in Year 3. Through a series of behavioral studies, we found that mated females positively orient towards cauliflower plants, but all other crops species did not elicit distinct preferences in males and unmated females. We found that deployment of pheromone emitters appears to act non-competitively, suggesting opportunities to reduce the density of pheromone emitters. Therefore, our research suggests that pheromone traps should be ideally placed where Brassica crops were grown the previous year.

Project Objective:

The project objective was to develop recommendations on where and how pheromone dispensers should be deployed for a swede midge pheromone mating disruption (PMD) program within annual cropping systems. The project provided a greater understanding of how PMD works mechanistically to identify options for manipulating pheromone deployment and dosages.

Introduction:

Swede Midge, Contarinia nasturtii Kieffer (Diptera: Cecidomyiidae) is a relatively new invasive pest of brassica plants such as broccoli, cabbage, kale, canola, etc.  The pest was first identified in Ontario in the early 2000s and has since spread across southeastern Canada and northeastern United States of America.  Although this pest is small in size (approx. 2 mm when mature), strikingly high rates of damage have been reported, including some reports of up to 100% losses from growers in New York and Vermont.

Swede midge larvae causes multiple damage symptoms (such as scarring, twisted leaves, multiple heads, or complete loss of head), all of which results in unmarketable produce for growers. Feeding occurs within the developing leaves at the growing tip and signs of damage are only apparent after the larvae have already left the plant.  These unique characteristics make it extremely challenging to manage swede midge with standard protocols.  One promising novel approach is pheromone mating disruption (PMD).

Pheromone mating disruption aims to prevent adult mating by releasing high doses of synthetic female pheromones that confuse males and prevent them from locating copulatory partners. We have previously found that PMD can effectively confuse male swede midge in the field; however, synthesis of the pheromones is costly and can be a significant barrier to grower adoption.  This research provided an opportunity to better understand where swede midge is mating,  how to maximize the efficiency of PMD technologies in annual rotation systems, and determined the economic and commercial viability of PMD for the management of swede midge.

 

 

Cooperators

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Research

Hypothesis:

Obj. 1.

Determine when and where swede midge adults mate

1.1 Does the distribution of adult midge emergence differ among replicated field plots?

1.2 How does adult dispersal influence their propensity to mate?

1.3 Do we see evidence of sex-biased dispersal based upon distance from emergence sites?

 

Obj. 2.

Determine if swede midge mating disruption acts competitively or non-competitively

2.1 What is the relationship between emitter density and male trap capture?

2.2 How does male exposure to high pheromone doses influence their responsiveness to mating with females?

Materials and methods:

Obj. 1. Determine when and where swede midge adults mate

 

Experiment 1.1 Does the distribution of adult midge emergence differ among replicated field plots? 

Mature lab-reared swede midge larvae, established from a wild Ontario population and reared at the University of Guelph in Guelph, Ontario, were introduced to field plots in both mid (July) and late (September) summer.   Lab-reared swede midge for the mid-summer introduction were exposed to a 16L:8D light cycle while swede midge for the late-season introduction were exposed to a 12L:12D light cycle.  The lighting schedules mimicked the daylength that developing swede midge would experience outside in the fall, which cues insects to pupate, diapause, or terminate diapause. All other rearing conditions were held constant. Mature swede midge larvae were rinsed from rearing plants and introduced to soil (50/50 mix of potting soil and soil collected from the field site) for pupation.  Each batch of infested soil contained 1000 mature swede midge larvae and was introduced to established emergence cages set up at the University of Guelph Elora Research Station in Elora, Ontario. In order to avoid possible previous infestations, we established emergence cages  in a field that had been free of brassica production for the previous five years.  Adult emergence was monitored weekly from May to October 2019, 2020, 2021. We used descriptive statistics to describe the observed emergence among mid and late season field introductions in years 1, 2, and 3.

 

Experiment 1.2 How does adult dispersal influence their propensity to mate? 

We used unmated male and female adult swede midge that were collected immediately upon emergence from our Swiss lab colony reared at the University of Vermont.  Females were placed into mating chambers (8-ounce deli containers modified to have 3 mesh windows along the side) in groups of 3. We placed moistened filter paper disk in each mating chamber to prevent desiccation. Male swede midge were individually placed into a scintillation vial (fitted with a mesh cap) for 10 minutes to simulate a period of restricted flight or into a wind tunnel (50 x 50 x 170 cm clear acrylic structure with an airspeed of 0.5 cm/s) for 10 minutes to simulate a period of unrestricted flight.  Directly after the treatment periods, males were individually placed into a mating chamber containing 3 female swede midge.  Mating groups had a 5 hour period in the mating chambers beginning within 1 hour of emergence.  This period covers the peak period of mating activity and male mate-searching activity for swede midge 1.  After 5 hours, females were removed from the mating chambers. Each female was placed in an individual mesh cage with a cauliflower host plant.  Plants were dissected 7-9 days later and checked for swede midge larvae.  The presence of larvae was used as a proxy for mating success. We used a nonparametric Mann-Whitney-Wilcoxon to test whether the distribution of mating frequency differed among treatments.

 

Experiment 1.3 Do we see evidence of host plant preference among mated female, unmated female, and male swede midge?

We tested if mated female, unmated female, and male swede midge (Swiss colony reared at the University of Vermont) differed in their host plant preference using a y-tube olfactometer.  A y-tube olfactometer is a device that allows insects to display a preference between two odor sources.  We exploded the midges to pairs of host plant, non-host plants, or fake plant odor sources.  Cauliflower was used as the host plant, potato and Swiss chard were used as the non-host plants, and a fake plant was used as a control. Each possible plant pairing was tested with a total of 120 individual swede midge (60 mated females, 60 unmated females, and 60 males).  In addition, testing occurred between the hours of 7 am - 10 am and 4 pm -7 pm in order to cover both periods of mating activity identified for swede midge.  We tested the plant pairings in a random order and made sure they were equally represented among morning and evening testing times.

Swede midge were tested individually and given 5 minutes to display plant preference once inside the y-tube.  Plant preference was defined as flying or walking at least one inch into an arm of the y-tube and staying there for a minimum of 15 seconds.  No preference was defined as staying in the main body of the y-tube.  Displays of no preference were just as important as displays of plant preference in this study as they can differentiate behavioral patterns among the subsets of swede midge tested in this experiment.  To avoid directional bias the y-tube orientation of plant treatments was switched every other swede midge. We tested plant preference among mated females, unmated females, and males using chi-square goodness of fit tests with equal expected probabilities (0.333) for all three possible outcomes; preference for plant 1, preference for plant 2, or no preference. 

 

Obj. 2. How does the density of pheromone emitters influence mating disruption?

 

Experiment 2.1 What is the relationship between emitter density and male trap capture? 

For this experiment pheromone emitters were set up in 3 different density treatments with 1 control where no pheromones were emitted.  Treatment densities included high (20 dispenser sites), medium (10 dispenser sites), and low density (1 dispenser site) of pheromone emitters.  The total amount of swede midge sex pheromone emitted was held constant across treatment densities at 1000 ug 2,9-diacetoxyundecane (2500 ug 2S,9S-diacetoxyundecane), 500 ug 2S,10S-diacetoxyundecane, and 5 ug 2S-acetoxyundecane. Emitters were set up in field cages (1.5 m x 1.5 m x 1.5 m) and placed a minimum of 50 meters apart to prevent the spread of pheromones between treatment cages. Each field cage also contains a single commercial monitoring trap.  Monitor traps are baited with a single swede midge lure that releases a much lower concentration of swede midge sex pheromone that is attractive to male swede midge.

Male swede midge were collected upon emergence from wild Ontario population reared at the University of Guelph and introduced to field cages in groups of 25 - 70 depending on daily emergence.  Monitor traps were collected after a 48 hour period and captured male swede midge were counted.  Trap counts are commonly used when assessing mechanisms behind pheromone mating disruption technology2.  Extremely low trap counts or trap shut down is a common indicator of success for pheromone mating disruption technology. We used a one-way ANOVA to test for any difference in trap counts among emitter density treatments.

 

Experiment 2.2 How does male exposure to high pheromone doses influence their propensity to mate?

For this experiment, unmated male and female adult swede midge (Vermont colony) were collected upon emergence.  Females were placed into mating chambers (8-ounce deli containers modified to have 3 mesh windows along the side) in groups of 3. One moistened disk was included in each mating chamber to prevent desiccation. Males were individually placed in modified 8-ounce deli containers and either exposed to synthesized female sex pheromones (50 ug 2,9-diacetoxyundecane (125 ug 2S,9S-diacetoxyundecane), 25 ug 2S,10S-diacetoxyundecane, and .25 ug 2S-acetoxyundecane) or not exposed to pheromones for a period of 10 minutes.  Directly after the treatment periods, within 1 hour of emergence, males were individually placed into a mating chamber containing three midge females, and held for five hours in the mating chambers beginning.  This period covers the peak period of mating activity for swede midge.  After 5 hours, females were removed from the mating chambers. Each female was placed in an individual mesh cage with a cauliflower host plant.  Plants were dissected after 7-9 days and checked for swede midge larvae.  We used the presence of larvae as a proxy for mating success. We used a nonparametric Mann-Whitney test whether the distribution of mating frequency differed among treatments.

 

Research results and discussion:

Obj. 1. Determine when and where swede midge adults mate

 

Experiment 1.1 Does the distribution of adult midge emergence differ among replicated field plots? 

In the first summer of monitoring, 2019, 21% of the 5000 total individuals from the mid-season introductions were captured in emergence traps. In the same monitoring season, we captured 7.4% of the 5000 total individuals from the late-season introduction.  Unfortunately, two data points were compromised from this group during the week when the highest rates of emergence were expected. In the second season of monitoring, less than 0.001% of the mid-season introductions were captured while 7.4% of the late-season introductions were.  In the third season of monitoring less than 0.001% were recaptured from either group.

Our data suggests that the greatest emergence of swede midge adults can be expected in year one or the same season as infestation.  Emergence can also be expected in the growing season following the initial infestation particularly for fields that experienced a swede midge infestation late in the season.  Some additional adult emergence can be expected from fields infested 2 seasons prior but numbers from these locations may be much lower than the two previous seasons.  Based on our observations pheromone dispensers should be deployed in year 1 with current brassica crops and possibly in year 2 as well in fields with previous brassicas production and swede midge infestation.  Additional monitoring trials such as field monitoring could further our understanding of adult emergence and strengthen recommendations for pheromone mating disruption deployment.

 

Experiment 1.2 How does adult dispersal influence their propensity to mate? 

We found that whether males are allowed unrestricted flight does not affect their propensity to mate (W = 942.5, p-value = 0.573, n = 42).  Regardless of the treatment, males typically mated a single time in a 5 hour period.  Treatment groups display similar distributions of mating events peaking at one mating event with a slight skew to the right. 

Our results indicate that male dispersal does not influence their propensity to mate. No difference in pre-exposure to a period of flight or restrict flight suggests that dispersal is not a defining component to the reproductive behavior or success of swede midge.  The similarity in male reproductive activity or success may imply that swede midge mating occurs at the site of adult emergence.  If mating does occur at the site of emergence then PMD will need to be deployed in multiple fields for successful management of swede midge including fields with current brassica production and previous brassica production from earlier in the season and possibly the year before as well. 

 

Experiment 1.3 Do we see evidence of host plant preference among mated female, unmated female, and male swede midge?

Due to COVID-19, we were unable to complete the proposed work under 1.3 to examine sex-biased dispersal in the field. Travel to Canada was completely restricted, so all of the remaining work had to be completed in Vermont.

Mated females were the only subset of swede midge that displayed host plant attraction through directional bias towards cauliflower host plants in our y-tube olfactometer experiments. We observed preference for cauliflower in mated females in all plant pairing combinations that included a cauliflower host plant: Cauliflower vs Swiss Chard X2(2, N = 60) = 17.205, p <.001, Cauliflower vs Potato X2(2, N = 60) = 16.952, p <.001, Cauliflower vs Fake X2(2, N = 60) = 19.931, p <.001. Mated female swede midge remained stationary or displayed no preference when exposed to all plant pairs that did not include a cauliflower host plant: Potato vs Swiss Chard  X2(2, N = 60) = 58.974, p <.001, Potato vs Fake X2(2, N = 60) = 30.251, p <.001, Swiss Chard vs Fake X2(2, N = 60) = 24.168, p <.001. 

Unmated females consistently displayed no preference among all plant pair combinations regardless of the presence or absence of a cauliflower host plant: Cauliflower vs Swiss Chard X2(2, N = 60) = 63.064, p <.001, Cauliflower vs Potato X2(2, N = 60) = 108.06, p <.001, Cauliflower vs Fake X2(2, N = 60) = 81.421, p <.001, Potato vs Swiss Chard  X2(2, N = 60) = 63.064, p <.001, Potato vs Fake X2(2, N = 60) = 58.674, p <.001, Swiss Chard vs Fake X2(2, N = 60) = 67.353, p <.001 .  Unmated females also displayed the least amount of overall activity by displaying a plant preference only 14.7 %  of the time.

Male swede midge were the only subset of swede midge to display random directionality in this experiment.  When a cauliflower host plant was present male swede midge displayed no preference for the host plant or the plant paired with it: Cauliflower vs Swiss Chard X2(2, N = 60) = 9.626, p = .008, Cauliflower vs Potato X2(2, N = 60) = 6.2403, p = .044, Cauliflower vs Fake X2(2, N = 60) = 7.5328, p = .02.  While statistically significant, directionality for males among these plant pairs was less frequent than in mated or unmated females.  Males displayed no directionality among the remaining plant pairs.

Our results indicate that only mated female swede midge display host plant preference to cauliflower host plants.  This observed behavior may be an example of the natural reproductive behavior of swede midge.  Mated female swede midge would be expected to seek out host plants as they prepare for oviposition.  In addition, we saw no evidence for host plant preference in unmated female and male swede midge.  This may indicate that host plant seeking behavior is not a natural part of mate-seeking in swede midge.  A lack of host plant preference in unmated females and males suggests that swede midge mating taking place at the site of adult emergence. Successful mating would quickly be followed by mated females seeking out host plants for egg-laying.  These results further highlight the importance of treating field sites with previous brassica production in addition to current crops for effect PMD management of swede midge.

 

Obj. 2. How does the density of pheromone emitters influence mating disruption?

 

Experiment 2.1 What is the relationship between emitter density and male trap capture? 

We found significantly higher traps counts in the control group than any other group (F(3,60) = 5.671, P =0.00173).  We found no difference in trap counts among the density treatments. While not statistically significant, we observed the lowest number of trap counts in the high-density treatment groups.  

The relationship between emitter density and male trap counts can influence the implementation of pheromone mating disruptions systems.  We observed similar trap counts among all density treatments which may indicate the need for fewer pheromone release points.  The results also illuminate the possible mechanisms behind pheromone mating disruption.  In this case, our findings suggest that pheromone mating disruptions work non competitively with swede midge.  It appears that female swede midge do not compete with pheromone emitters but instead, the technology works through non-competitive mechanisms. However, the lowest trap counts were still observed in the high-density treatment. Additional trials, including large-scale, uncaged field trials could strengthen the recommendations for efficient implementation of a successful pheromone mating disruption management of swede midge and further our understanding of optimal dispenser usage. 

Experiment 2.2 How does male exposure to high pheromone doses influence their propensity to mate?

We found that males were pre-exposure to swede midge sex pheromone did not affect their propensity to mate directly  (W = 467, p-value = 0.298, n = 33).  The median matings per male was 1 mating event in a 5 hour period for either treatment.  Treatment groups display similar distributions of mating events with a slight skew to the right. 

Our results indicate that exposure to high amounts of swede midge sex pheromone does not influence a male's propensity to mate in a 5 hour period.  This suggests that a reduction in mating through pheromone mating disruption requires the active deployment of swede midge pheromones.  We expect timed dispensers, that may ultimately reduce the cost of the technology, would need to run the duration of peak mating activity to effectively deter mating. 

Research conclusions:

Within annual cropping systems, swede midge have the potential to infest multiple field sites throughout a growing season.  Our findings suggest that swede midge mate at the site of emergence and only after mating do females display host-plant seeking behavior.  For effective PMD management of swede midge, all sites of emergence will need to be treated with pheromone dispensers. This includes sites of swede midge infestation within the current growing season and possibly from the previous growing season as well depending on the severity and timing of the infestation.  Infested sites from 2 years prior may require monitoring before PMD deployment.  Swede midge emergence from these infested sites 2 years prior and early or mid-season infestations from the previous year may be low enough not to treat at all if monitoring efforts result in low trap counts.  Additional research is needed to decipher optimal dispenser type and dosage for pheromone release.  Our findings suggest fewer dispensers may be necessary for effective reduction in mating; however, the deployment of sex pheromones during mating activity is necessary for successful PMD management of swede midge.

 

References Cited:

1. Hodgdon, E. et al. Diel patterns of emergence and reproductive behavior in the invasive swede midge (Diptera: Cecidomyyidae). The Canadian Entomologist 510-520 (2019)

2. Miller, J. et al. Mating disruption for the 21 st century: Matching technology with mechanism. Environ Entomol 1-27 (2015)

Participation Summary

Education & Outreach Activities and Participation Summary

Educational activities:

3 Journal articles
4 Published press articles, newsletters
2 Webinars / talks / presentations
4 Workshop field days
1 Other educational activities: Vteen Stem Cafe - Description below

Participation Summary:

160 Farmers
15 Number of agricultural educator or service providers reached through education and outreach activities
Outreach description:

Presentation: Managing Swede Midge - A New Vegetable Pest, was lead by Dr. Yolanda Chen at the UConn Extension's 2019 Vegetable & Small Fruit Growers' Conference held on January 7, 2019. The presentation included an introduction to swede midge, associated damage, report losses from growers, and ecologically based pest management strategies for swede midge (including pheromone mating disruption).

37th annual NOFA Vermont Winter Conference, February 16th-18th, 2019 in Burlington, VT. Dr. Yolanda Chen gave a presentation on swede midges, ecologically based pest management for the pest, and pheromone mating disruption.

Entomological Society of American Annual Conference, November 17th-20th, 2019 in St. Louis, MO. Andrea Swan gave a presentation on how the location of pest mating influences the application of pheromone mating disruption in the management of swede midge.

38th annual NOFA Vermont Winter Conference, February 14th-16th, 2020 in Burlington, VT. Dr. Yolanda Chen will give a presentation on swede midges, challenges to successful management of the pest (in both organic and commercial agricultural settings), ecologically based pest management for the pest, and pheromone mating disruption.

Other Education Activities: What's Buzzing in Entomology, a VTeens Stem Cafe in association with University of Vermont Extension 4-H.  This was a 2-hour long event that included a presentation and hands-on activities for approximately 50 middle and high school youth.  The cafe emphasized our work with swede midge, pheromone mating disruption technologies, ecologically based pest management, and IPM.  More information can be found at https://teensciencecafe.org/cool-cafes/whats-buzzing-in-entomology/ 

Press

2019    Natural plant odors may be an effect pest repellent for crops. By Chrissy Sexton. July 23. earth.com news. https://www.earth.com/news/plant-odors-repellent-crops/

2019    Essential oils, a novel way to deal with a major pest. By Fermin Koop. ZME Science. July 24. https://www.zmescience.com/science/essential-oils-pests-24072019/

2019    Garlic could be used to repel insects from crops as scientists find the smell of strange plants acts as a natural pesticide. By Ian Randall. July 24. https://www.dailymail.co.uk/sciencetech/article-7280643/GARLIC-used-repel-insects-crops-experts-plant-odours-natural-pesticides.html#comments

2019  Garlic on broccoli: A smelly approach to repel a major pest. Science Daily. July 23. https://www.sciencedaily.com/releases/2019/07/190723085955.htm

Television

2019 Across the Fence. WCAX News. “Baffling Bug – Research at UVM on the Swede midge”. https://www.youtube.com/watch?v=3JeDbV1_q0k

Radio/Podcasts

2019 University of Minnesota Extension Fruit and Vegetable News. Interviewer Natalie Hoidal. “What's Killing My Kale Episode 27: Swede Midge Management - an overview of what we know”. October 04.

 

Learning Outcomes

150 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
15 Service providers reported changes in knowledge, attitudes, skills and/or awareness as a result of project outreach
15 Educators or agricultural service providers reported changes in knowledge, skills, and/or attitudes as a result of their project outreach
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Farmers and educators gained knowledge on the reproductive ecology of swede midge.

Project Outcomes

4 Grants applied for that built upon this project
2 Grants received that built upon this project
$58,748.00 Dollar amount of grants received that built upon this project
10 New working collaborations
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.