Final report for GNE21-271
Project Information
Swede midge is an invasive pest of Brassica crops and causes severe plant damage where populations establish. Since swede midge feeding results in marketable damage, novel approaches are needed for managing the pest. We previously found that the synthetic female sex pheromone can impair male midges and disrupt mating when released at high doses. Additionally, the sex pheromone can attract male swede midge when released at lower doses and can be used in a mass trapping management approach for swede midge. However, mated female midges are not impacted by the sex pheromone and can undermine both mating disruption and mass trapping efforts. In this grant we assess the potential of plant-based attractants for mated female swede midge. Using steam distillations of swede midge host plants in a series of behavioral assays, we found that mated females positively orient toward broccoli distillate. All other host plant distillates did not elicit a distinct preference in mated females. In subsequent trials we found that broccoli distillate did not attract mated female swede midge in cage trials among live host and non-host plants. Our research suggests that mated female swede midge are attracted to host plant steam distillates in laboratory settings, however attraction to steam distillates diminished when tested among live plants. We suggest further research to determine the specific host plant chemical attractants of mated female swede midge and assessment of their ability to attract mated female swede midge among live host and non-host plants.
Objective 1: Determine the relative attractiveness of host plant distillates on mated female swede midge.
1.1 How do mated female swede midge respond to individual host plant extracts?
We tested whether mated female swede midge are attracted to individual host plant distillates (Red Russian Kale, White Russian Kale, broccoli, cauliflower, collard greens) using a two choice y-tube olfactometer system. We used paired choice tests along with a water control to determine attractiveness among the distillates. A minimum of 50 female midges were tested per host plant distillate. All insects were tested individually for clear observation of behavioral response.
Objective 2: Test the attractiveness of host plant distillates among live host and non-host plants.
2.1 Do steam distillations attract mated female swede midge in the presence of live host plants and non-host plants?
We tested the most attractive host plant distillates from Objective 1 in baited Jackson traps to determine their efficacy as lures among Brassica host plants and non-host plants. We used cage trials for greater experimental control on the number of insects released. These trials gave us an accurate measure of what percentage of the released females are trapped to measure the efficacy of an attractant.
The purpose of this project was to assess the potential of host plant distillates as attractants for mated female swede midge. Swede midge is an invasive insect pest that attacks all major Brassica crops. Swede midge larvae secrete digestive fluids that severely damage plant tissue and cause scarring, multiple shoots, deformed plant growth, and the complete loss of marketable plant parts (Hallet 2007). Feeding by even a single swede midge larva can lead to unmarketable produce (Stratton et al. 2018). Given that only systemic neonicotinoids are effective for crop production, the lack of products for organic management of the midge and heavy losses due to swede midge have forced some organic growers to abandon Brassica production in the Northeast. More recently, swede midge has increased its distribution and is now reported throughout Eastern Canada and North Eastern United States (Chen et al. 2009, Hodgdon unpublished).
The biology of the midge makes it an extremely challenging pest to manage. Swede midge belongs to the family Cecidomyiidae; a group of flies known for their ability to strongly influence plant growth and plant defenses (Stuart et al. 2012). Swede midge larvae secrete salivary fluids at or near the growing tips of plants essentially targeting the most economically significant plant parts. Larvae feeding at the growing tips are shielded from foliar insecticides (Wu et al. 2006, Hallet et al. 2009). In addition, damage symptoms are not apparent until mature larvae have left the plants for the protective cover of the surrounding soil during pupation (Stratton et al. 2018). Furthermore, adult swede midge are tiny (2 mm) and have a life span of 2-3 days, making them extremely challenging to observe in the field and preventing successful management with traditional scout and spray tactics. Due to the severity of crop damage and the elusive nature of this pest, there is a strong need for innovative pest management techniques.
One promising solution is the use of pheromone mating disruption (PMD) as a potential management strategy. PMD technology releases high levels of synthetic female sex pheromones to prevent males from finding females and reduce mating. The female swede midge sex pheromone has been identified and can be synthesized commercially (Hillbur et al. 2005). PMD can disrupt males from finding females in the lab and in the field (Hodgdon et al. 2019). However, females can still mate outside of treated areas and migrate into Brassica fields to lay eggs. An attractive lure for mated females can capture individuals that temporally or spatially evade PMD. Furthermore, combining the two technologies could result in a mass trapping system that has the potential to provide high management success. Mass trapping aims to reduce pest pressure by luring large numbers of the target pest with baited traps. Developing a female attractant and combining male and female attractants has the potential to trap large numbers of swede midge. A successful mass trapping system for swede midge will reduce severe economic losses and align with organic management practices to increase the ability to manage pests in a sustainable agriculture framework.
Research
Objective 1: Determine the attractiveness of host plant distillates among mated female swede midge.
Objective 1.1 Do steam distillations of host plants attract mated female swede midge in y-tube assays? Using y-tube behavioral assays we tested for mated female behavioral attraction to steam distillation of host plants. We placed individual insects into the y-tube and exposed them to a single distillate and a water control. We observed behavioral responses to multiple host plant distillates.
Test Insects – Previous trials aimed at establishing host plant preference among male, unmated female, and mated female swede midge resulted in mated female swede midge displaying host plant preference while unmated female and male swede midge did not (Campbell et al, in prep.). For this reason, we omitted unmated females from trials and only tested mated females. We randomly selected female swede midge from our Vermont based colony reared at the University of Vermont. We previously established that approximately 60% of randomly selected females were mated in the oviposition (OP) cages. Females were held in glass vials with mesh tops for a period of 15 – 60 minutes before being used in experiments.
Treatments – Treatments include exposure to steam distillations from 5 brassica host plants (De Cicco Broccoli, Red Russian Kale, White Russian Kale, Champion Collard Greens, and Snow Crown F1 Hybrid White Cauliflower).
Methods – We used a 1000ml distillation apparatus with Graham condenser (YUXun YX, Amazon, Seattle, WA U.S.A.) to make brassica steam distillations. De Cicco Broccoli, Red Russian Kale, White Russian Kale, Champion Collard Greens (High Mowing Organic Seeds Hyde Park, VT U.S.A) and Snow Crown F1 Hybrid White Cauliflower (Brassica oleracea var. botrytis, Johnny’s Selected Seeds Winslow, ME U.S.A). We processed the plants at 10-12 weeks and used all leaves, petioles, and meristem tissue in steam distillations. Plant tissue was pulsed in a food processor for 10 seconds before being transferred to the distillation flask. We placed the flask on top of the two-neck flask containing 500-mL of water once the water came to a boil. Distillate collection began when the first drop of distillate fell into the glass collection vial and ran for a total of 15 minutes. Each 5-minute period of the total 15-minute collection period was collected into separate collection vials. Vials were capped and stored in a cool dry place out of direct sunlight before use in experiments.
We used a y-tube olfactometer (Sigma, Scientific, Macanopy, FL U.S.A.) set up with an air compressor delivering air through activated carbon filters then Teflon tubing. The tubing attaches two 10-cm long glass odor adaptors that fit into both arms of a y-tube. The inner diameter of the y-tube was 1.8-cm. The arms of the y-tube were 8-cm long and the stem measured 14.5-cm to the junction. The airflow was set to 0.3 L/min through each of the y-tubes as established in previous swede midge research (Hodgdon et al. 2019a).
We observed mated female behavioral response to five different host plant steam distillations using a y-tube olfactometer. We tested a single steam distillate at a time by pipetting 1 µl of distillate onto a 1 cm2 square of white filter paper and placing it into a glass odor adaptor on one side on the y-tube. Each distillate was tested against a 1 µl water control also applied to a 1 cm2 square of white filter paper and placed in the odor adaptor for the opposing y-tube arm. Individual female swede midge were introduced at the y-tube stem and given five minutes to move towards either arm of the y-tube and display a behavioral choice. When individuals failed to make a choice, we removed them from the y-tube and did not test them again. We switched the direction of the y-tube after each replicate to remove directional bias. We tested the distillate treatments in random order, where every five midges were one block, for a total of n=50 replicates for each treatment. All glassware was cleaned with ethanol and allowed to air dry between each block.
Data Collection and Analysis – We defined a behavioral choice as moving at least 2.5 cm into one arm of the y-tube and staying there for a minimum of 15 seconds. We recorded female responses with a binary scoring system: flight toward host plant steam distillate (1), or flight toward the water control (0). We used a series of chi-square tests to test whether the number of female swede midge flying towards the distillate differed significantly from 50%.
Objective 2: Test the attractiveness of host plant distillates among live host and non-host plants.
2.1 Do steam distillations attract mated female swede midge in the presence of live host plants and non-host plants? Using cage trials we tested for female swede midge attraction of host plant distillates among host and non-host plants. Groups of female swede midge were introduced to pop-up cages containing live host or non-host plants and one sticky trap baited with host plant distillate. We used trap counts to test for differences in host plant distillate attraction among live host or non-host plants.
Test Insects – Female swede midge were randomly selected from our Vermont based colony reared at the University of Vermont. We previously established that approximately 60% of randomly selected females were mated in the OP cages. Females were held in groups of 20 in glass vials with mesh tops for a maximum of 30 min before being used in experiments.
Treatments – Plant treatments include red Russian kale (host plant), chives (non-host plant), fake plant, or no plants.
Methods – We placed groups of 20 females into 1 ft3 mesh cages with one of the plant treatments Each cage contained four plants ranging from 6-8 weeks of age, of similar height, and grown in 4-inch pots. The fake plants were placed in 4-inch pots with moist potting media and the no plant control consisted of 4-inch pots with moist potting media and no live or fake plant material. Additionally, each cage had a single trap (2.5cm2 square of white poster board covered in sticky trap with a dental wick lure placed through the center). The traps were baited with 3 ml of host-plant steam distillate, except for in the no plant treatment where the traps were baited with water control, and positioned in the center of the 4 plants or pots and halfway between the top of the pots and the top of the mesh cage. Cages were placed outside between May 20 and June 2, 2023, and were shaded from direct sunlight with a minimum of 20 m between them. Experiments were conducted when nighttime temperatures stayed above 10oC. Female swede midge were introduced to the cages once the cages were placed outside. Cages were collected 24 hours later.
Data Collection and Analysis – Trap counts were recorded immediately following cage collection. We used a one-way ANOVA to test whether the total number of female swede midge caught in the baited trap differs among the treatments.
Objective 1.1 Do steam distillations of host plants attract mated female swede midge in y-tube assays?
Of the five host plant distillates we tested, we found that mated female swede midge traveled towards the broccoli distillate X2 (1, N = 50) = 5.12, p = 0.024 and into the treatment arm of the y-tube significantly more than towards the control arm. Broccoli distillate was the only distillate to elicit behavioral responses in mated female swede midge towards the host plant distillate. Among cauliflower, collard green, and both kale distillates mated female swede midge exhibited no behavior attraction.
Our results indicate that mated female swede midge display attraction to broccoli distillate. This observed behavior may be an example of the natural reproductive behavior of swede midge as we expect mated female swede midge to seek host plants in preparation for oviposition. Interestingly, we did not observe attraction to any of the other four host-plant distillates. Attempts to identify plant compounds captured in the steam distillates failed due to mechanical equipment failure. Future steps include identifying which compounds are present in the steam distillates and which compounds may be eliciting behavioral responses from mated female swede midge.
2.1 Do steam distillations attract mated female swede midge in the presence of live host plants and non-host plants?
Despite capturing the highest number of mated female swede midge among non-host plant treatments (chive and fake plant respectively), we found no statistical difference in trap counts among treatment groups. We had our lowest trap counts among the water control treatment followed by the kale host plant treatment. We recommend similar trials if a more attractive host plant extraction can be identified.
Our results suggest the possibility of host plant extractions being useful in baiting mated female swede midge. Additional research is needed to identify specific plant compounds that elicit mated female behavior and which may act as an attractant in field applications. Female attractants may be especially successful in fields with active swede midge emergence and without a current brassica crop. Mated female attractants in these settings could limit the flow of mated female swede midge to new brassica plantings.
Education & Outreach Activities and Participation Summary
Participation Summary:
To aid in outreach efforts, we created a infographic that outlines the swede midge life cycles and also highlights the inflow of mated female swede midge to new brassica plantings. The swede midge life cycle infographic has been used in one presentation on swede midge at Canola Week 2023 hosted by the Canola Council of Canada. Here we presented information on best practices for identification and management, management challenges, and new research directions. We anticipating using the infographic and research results in future presentations such as the Entomological Society of America's annual meeting in November, 2024, as well as one publication.
Project Outcomes
Swede midge continues to be an extremely challenging pest to manage where it establishes. Additionally, growers have expressed the need and interest for alternative swede midge management strategies. Semiochemical management strategies, such as mass trapping, are desirable because they can reduce the use of pesticides and have much lower impact on non-target species. Our results add to a body of knowledge around semiochemical based management strategies for swede midge that may someday result in the development of additional management tools for this invasive pest.
This project highlighted the challenges and complexity inherent in sustainable agriculture and swede midge management. There seems to be consistent hurdles when scaling up research from laboratory trials to cage and field trials. Every step along the way we continue to build our knowledge base to inform future research directions. We look forward to future opportunities to further the development of semiochemical management strategy for swede midge.