- Vegetables: broccoli
- Pest Management: cultural control, genetic resistance
- Production Systems: organic agriculture
Swede Midge (Contarinia nasturtii) is a gall-forming fly that feeds on the growing points of many Brassica crops, causing significant crop losses to growers. After arriving in Ontario from Eurasia around 2000, Swede Midge has now spread into several northeastern states and adjacent Canadian provinces. Most Brassica crops are vulnerable to damage, and broccoli is particularly at risk. Small-scale diversified growers and organic growers are at a particular disadvantage, as they have very few good management tools. I propose to screen broccoli (Brassica oleracea var. italica) cultivars for tolerance to Swede Midge. Intervale Community Farm (ICF) was the first farm in Vermont to identify Swede Midge in 2007. We grow about 5 acres of Brassica crops, roughly 1.5 acres of which are broccoli. I have worked to find sustainable Swede Midge management tools, partnering closely with Dr. Yolanda Chen of the University of Vermont, who has agreed to serve as the technical advisor for this project. ICF has hosted several Swede Midge research trials by Dr. Chen and her lab, evaluating organically-approved insecticide treatments, exclusion strategies, and companion planting as management approaches. My personal observations suggest some difference in damage severity between broccoli cultivars, which I believe warrants further study. This research could be of value to Brassica growers throughout the region, and I would share our results through workshops and presentations at meetings of the Vermont Vegetable & Berry Growers Association (VVBGA) and the Northeast Organic Farming Association of Vermont (NOFA-VT).
Project objectives from proposal:
I propose to screen broccoli cultivars for tolerance to Swede Midge damage. We grow 8 successions of broccoli, most of which are the cultivar ‘Gypsy.’ We also run small field trials of other cultivars. This year, I noticed what appeared to be a significant reduction in Swede Midge damage in the trial cultivar ‘Bay Meadows’, and an increased level of damage in the trial cultivar ‘Expo.’ Since the available options for Swede Midge control are limited, I propose to test 8 cultivars of broccoli and screen them for Swede Midge damage. Even a modest reduction in damage could have a significant impact: if we can drop from 30% to 20% crop loss, that would equate to an economic benefit of $500-600 per broccoli succession, or potentially $4000-4800 annually with no additional costs beyond a slight variation in seed prices. I will first work to identify promising cultivars, and also consider those in common use in the Northeast, beginning with cultivars noted in my observations and those noted in the work of others.
In conjunction with the Technical Advisor and seed companies, I will identify 8 varieties of broccoli for testing. I will look particularly for varieties with vigorous early growth and establishment, as that is Dr. Hallett’s preferred explanation why some cultivars suffered less Swede Midge damage than others. Representatives from both Johnny’s Selected Seeds and High Mowing Organic Seeds have expressed an interest in helping me to identify suitable cultivars, or find those out of common use, but still available for those with the right connections.
We have seen that late summer and early fall maturing broccoli crops suffer the most Swede Midge damage, so we will run our trial such that maturity of the average days-to-maturity corresponds with the mid-August through September harvest window.
Broccoli transplants will be seeded 5/22/16 and 6/8/16 in 150-cell plug trays using Fort-V potting soil from Vermont Compost Company. After approximately 28 days in the greenhouse, the plugs will be transplanted into the field using a waterwheel transplanter. Prior to setting of transplants, the field will fertilized in accordance with Extension recommendations for broccoli, following a university lab soil analysis.
Each of the 8 varieties will be grown in a 12’ x 12’ block, plants spaced 12” in-row and 36” between rows, providing a total of 48 plants per block. Each block will be repeated three times for each of the two planting dates. Each block will be separated from the adjacent block by a 12’ x 12’ untreated block. Crops will be managed during the season with organically-approved methods, including mechanical weed control, irrigation, and other pest controls as needed. As caterpillars are the typical pest problem we face, I expect that only Bacillus thurengensis (B.t.) would be applied, which does not have any known activity vs. Swede Midge. It would also be applied to the entire planting, so the background effect of varietal resistance should still stand out.
As heads mature beginning in mid-August, test plots will be evaluated using the standard scale for Swede midge damage developed and refined by Drs. Shelton, Hallett, Chen, et al. This scale assigns a value to plants from undamaged heads to heads completely absent due to damage, and gradations in between. We will conduct training with Dr. Chen’s lab in head damage assessment, and contract with them to work with us on the first couple of damage assessments. Each block will be evaluated immediately preceding our alternate-day harvest of the heads. I expect that each of the two plantings of broccoli will be evaluated for about 3 weeks, or roughly 10 times per planting date.
Following data collection and harvest, the plot markers and traps will be cleaned up, the broccoli plants will be fully incorporated in accordance with Swede Midge best management practices, and a fall cover crop will be sown.
In this trial we will not be able to definitively separate Swede Midge damage effects due to plant tolerance to Swede Midge damage from Swede Midge damage effects due to slight variations in maturity of the same cultivars.
Despite this, I think that the trial will still allow us to notice any significant effect. We generally see a slight, gradual change in damage rates up or down between different broccoli successions in the late summer to early fall harvest window. If we see a spike or trough in damage levels with a particular cultivar, that would be a variety worthy of further trial or ruling out. This is consistent with our observations: in our trial this year, we planted the cultivar ‘Bay Meadows’ and the cultivar ‘Gypsy’. While the days to maturity were only 2 days apart, we observed what appeared to be a significant difference in Swede Midge damage. While it could be due to the difference in days to maturity between the cultivars, such a minor difference in maturity between the two suggests that another factor as a more likely explanation.
This trial is also unable to answer whether any noted effect may be due to preferential attractiveness of different cultivars. Varieties that look better in our trials might still suffer extensive damage if grown alone in the absence of other cultivars more appealing to Swede Midge. That said, even this result could be useful in identifying varieties that could work as trap crops planted as a smaller percentage of an overall field of a less-desirable cultivar.
In mid-April, Swede Midge monitoring traps will be installed at either end of the field. Traps will be checked weekly and Swede Midge numbers noted through the anticipated end of flight in mid-October. Upon completion of the field research season, Dr. Chen’s lab will analyze the collected data.
Timeline of project
At notification of the receipt of the grant in mid-February, Andy Jones will research and source the 8 broccoli cultivars needed for the trial. In mid-April, monitoring traps will be placed by the Technical Advisor’s team at either end of the broccoli test field. ICF staff will seed broccoli for the trial plots in the greenhouse on May 22, 2016 and June 8, 2016. ICF staff will commence field preparation of the plots at the same time. We will transplant the crops around 6/22/16 and 7/8/16, depending on transplant maturity. ICF staff will begin evaluating trial plots for damage when heads appear, which I expect to be the second half of August, 2016. Data collection will be completed by the end of September, 2016, with analysis by Dr. Chen’s lab to follow. Monitoring traps will be removed from the test field in mid-late October. Results will be ready for write-up and distribution by Andy Jones by the end of December, 2016.
We will host an on-farm workshop on Swede Midge, featuring our test plots in late August or early September, 2016, when heads from multiple varieties will be visible to attendees. I will present a brief talk to the Vermont Vegetable & Berry Growers’ Association (VVBGA) Annual Meeting in December 2016 or January, 2017, and aim to present at the Northeast Organic Farming Association of Vermont (NOFA-VT) Winter Conference in February, 2017. I will also write a synopsis of our results for winter 2016-2017 distribution in the Vermont Vegetable and Berry News, a biweekly publication of the UVM Vegetable and Berry Specialist. I will publicize our on-farm workshop and conference presentations through VVBGA, NOFA-VT, and other Extension networks.