Garlic IPM Newsletter No. 7 — June 11, 2026
Scapes are beginning to emerge in southern Maine as high temperatures pushed plant development this week. Those same high temperatures paired with recent precipitation and lingering humidity favor the development of purple blotch and Stemphylium leaf blight, which we have seen evidence of at all scouting locations. Onion thrips were also observed in low numbers at each site this week, and most of Maine has now surpassed the threshold for emergence.
Scouting for and Managing Onion Thrips
Onion thrips are small, shy insect pests that can cause outsized injury to allium crops. They overwinter in previous allium fields as well as in grass and legume fields and weedy borders, and emerge at 140 GDD52.7°F. Thrips puncture and suck sap from leaves, creating scratchy or streaky white scars. Severely infested fields may take on a silvery cast from afar. Economic damage occurs when scarring impacts photosynthetic capacity to the extent that bulb size is reduced. In onions, an additional concern is that the scarring can result in leaves that are too light and brittle to properly lodge, increasing the chance of bacterial rots. Thrips feeding can also spread disease.
The action threshold for thrips in onions is 1-3 thrips per leaf. By the time thrips damage is noticeable in passing, this threshold is typically far surpassed – so scouting is critical for good control. The lower threshold is recommended for organic management systems, when the forecast calls for hot and dry weather, and when plants are bulbing. To scout for thrips, inspect ten random groups of three plants each (30 plants total). Be sure to include areas along field edges, from which adults often migrate. Count the number of leaves and then closely inspect leaf axils (where leaves meet the growing point), necks of plants, and along leaf midveins to count thrips. Thrips are small (1-1.5 mm), range from light yellow to dark brown, and elongated. While they’re often hidden, they can be active and are likely to move when detected. Divide the total number of thrips observed by the total number of leaves inspected for the average thrips per leaf count.
Cultural management strategies of onions thrips are generally geared toward reducing infestations in future seasons, as opposed to in-season control:
- Use clean planting stock free of thrips
- Destroy culls and crop residue
- Avoid planting alliums near grains, hay fields, clovers, and brassicas, and rotate crops well
Thrips generally reproduce asexually, so populations grow rapidly. An important implication of this is that thrips populations readily develop resistance to insecticides, so products must be carefully chosen and rotated. While there are a fair number of OMRI-listed insecticides for onion thrips in garlic, most are suppression/repellant only. The following spray recommendations apply regardless of what type of insecticide is used:
- Once threshold of 1-3 thrips per leaf is reached, use a 7-10 day spray interval
- Do not exceed more than two consecutive applications within a resistance group
- Insecticides must be applied with enough water for full coverage at the growing point (100 gallons per acre)
- Adjuvants may improve coverage
Refer to the New England Vegetable Management Guide for a starting list of thrips control materials, and ensure that any product used is labeled for garlic in Maine (or your state) against onion thrips.
Disease Update
Botrytis leaf blight lesions have remained level for several weeks now at all scouting locations. However, the NEWA disease forecast for onions shows that conditions in all Maine and Maine-adjacent New Hampshire locations are favorable for this and/or other diseases to develop, to varying degrees. It is advisable to check the nearest NEWA weather station near you to see what diseases to be on the lookout for and when.
Where fungicides have already been applied, do not exceed a 10-day spray interval. You can assume 7 days of protection from a fungicide application, but this protection is gone where more than one inch of rain has since fallen. Using the NEWA forecast, ensure that protective fungicides are applied before very favorable weather conditions occur, where the disease is forecast to be over threshold for more than three days.
Where fungicides have not been applied, use the NEWA forecast to scout your crop carefully when conditions become favorable.
Peyton Ginakes, PhD
Research Associate
University of Maine Cooperative Extension
Highmoor Farm
52 US-202
Monmouth, ME 04259
(207) 933-2100
This project is funded by a Specialty Crop Block Grant through the Maine Department of Agriculture, Conservation, and Forestry. Funding for the Maine 2024 Specialty Crop Block Grant Program was made possible by a grant/cooperative agreement from the USDA Agricultural Marketing Service. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA.
Where brand names or company names are used, it is for the reader’s information. No endorsement is implied nor is any discrimination intended against other products with similar ingredients. Always consult product labels for rates, application instructions and safety precautions. Users of these products assume all associated risks.
University of Maine Cooperative Extension is an equal opportunity institution and provider committed to nondiscrimination. For more information, visit extension.umaine.edu/nondiscrimination.