Garlic IPM Newsletter No. 8 — June 5, 2025
Onion thrips (Thrips tabaci) are fairly reliable pests of allium crops in Maine, unlike the last several insect pests highlighted in this newsletter. Despite their name, they actually feed on a wide range of plants but can be very damaging to alliums.
Life Cycle
Thrips overwinter in culls and residues, weedy patches, and nearby fields containing grains and legumes. When adults emerge, they lay eggs singly in epidermal leaf layers, which then hatch at 140 GDD52.7 (which has been surpassed in much of western, southern, central, and midcoast Maine). Nymphs molt several times before pupating in the soil. Each generation requires about 323 GDD52.7, and at least two generations per year occur in northern New England. Populations develop most rapidly in hot, dry conditions.
Scouting
Onion thrips are very small and shy, but scouting is crucial for early detection and control. Inspect leaf folds and along the base of leaves/plants closely. Adults are brown and 1-1.5 mm long; nymphs are smaller and paler yellow, sometimes even whitish. Scouting for thrips can be done at the same time as weekly scouting for diseases. Count the thrips and numbers of leaves on each plant (ten groups of three plants each = 30 plants total). The action threshold is 1-3 thrips per leaf. Do not wait to see feeding damage before taking action. Erring toward the lower threshold is appropriate when plants are bulbing, conditions are hot and dry, or when insecticide efficacy is expected to be low.
To find images of an adult onion thrip and nymph onion thrips, go to the UC IPM Onion and Garlic Pest Management Guidelines webpage.
Damage
Thrips feed on leaves with their rasping mouthparts that they use to consume sap from plant cells, leaving scars that reduce plants’ photosynthetic capability and openings for secondary pathogens. This damage can give fields a silvery cast from a distance, sometimes called “blast”. Feeding wounds will have a stippled or streaky appearance at closer range, and will be less defined than botrytis lesions. The threat to allium crops is greatest at early bulbing stages, because reduced energy will inevitably reduce yield. Thrips can also vector viruses from plant to plant.
To find an image of thrip scarring, go to the UC IPM Onion and Garlic Pest Management Guidelines webpage and onions with subtle thrips streaking, visit the Cornell Cooperative Extension webpage for Scouting Tips for Onion Thrips.
Management
Cultural controls can greatly impact thrips damage. Remove and destroy all culls, and practice good crop rotation practices. Avoid planting near fields containing grains or clovers, as thrips will migrate from those plants to nearby alliums when cut. Brassicas are also preferred vegetable crop hosts, so avoid planting near these when possible. Inspect seed garlic and onion sets and seedlings before planting them out. Straw mulch can help deter thrips.
Controls should be applied on a 7-10 day interval. Other considerations include:
- Insecticides must be applied with ample water (100 gal / acre), so that material reaches the growing points where thrips primarily reside.
- Because thrips reproduce asexually, insecticide resistance builds quickly within populations. Rotate insecticide groups every other application.
- Use a shorter spray interval in hot weather.
- Including a spreader-sticker will improve coverage.
See the New England Vegetable Management Guide for control options. Note that many organic controls are suppression only, so use the lower action threshold and/or rotate with higher efficacy materials. If using a repellant (kaolin), full plant coverage is essential.
These are excellent resources on this pest for further reference:
- UMass Extension Vegetable Program
- Cornell Cooperative Extension Vegetable Program
- Penn State Extension
Pest and Disease Situation
Much of the state is seeing warmer weather, but with high humidity and rain in the forecast, fungal diseases are likely to continue developing and spreading. Staying on a 7-10 day spray interval will help manage spread of diseases where present. Check NEWA for your local disease forecast.
If you are using row cover to protect your alliums from onion maggots, be sure to mark your calendars to remove row covers about 4 weeks after peak flight (735 GDD40) near you.
Please remember to contact Peyton Ginakes with any signs of leek moth or allium leafminer.
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.
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