Garlic IPM Newsletter No. 3 — May 14, 2026
Cool, wet weather has not slowed garlic growth at scouting locations in Buxton, Topsham, or Washington, and plants are generally healthy with about 5-7 leaves each. Some infrequent emergence issues are being investigated and are likely the result of mites or Fusarium, both of which were highlighted in last week’s newsletter on spotty emergence. What appear to be Botrytis leaf blight (Botrytis squamosa) lesions are also increasing at all sites. Samples of these lesions have been submitted to the University of Maine Plant Disease Diagnostic Lab for positive identification. Results will be shared in next week’s newsletter alongside how to use Network for Environment and Weather Applications (NEWA) to forecast foliar diseases like Botrytis leaf blight.
Forecasting Insect Pests with Growing Degree Day Models
The development of many insect pests is driven by the accumulation of temperature, which is calculated using growing degree day (GDD) models. For example, onion maggot begins to emerge at 390 GDD40°F. The subscript is the “base” temperature, referring to the lower threshold for development. In this case, no onion maggot development occurs below 40°F. The lower this temperature, the more quickly GDDs accumulate. The GDDs for each day are calculated as the average daily temperature minus the base. For example, the high in Lewiston on May 12 was 57°F, and the low was 29°F. The GDD can be calculated as follows:
(57+29)/2 – 40 = 3 GDD40°F
Most models begin Jan 1, well before any GDDs accrue. Each day’s GDD accumulates, and onion maggot adults begin to emerge from their overwintering pupae when 390 is reached. Fortunately, websites like the Maine Climate Office and NEWA track these accumulations. Remember to always refer to the correct base temperature.
The above chart shows the daily accumulation of GDDs for onion maggot in Lewiston. On May 14, 315 GDD40°F had accrued there. The 390 GDD40°F threshold for onion maggot emergence to begin is forecasted for May 19, when the black line crosses into the blue-shaded region.
For a wider range of Maine locations, visit the Maine Climate Office website and refer to the base 40°F map (top-right graph). The 390 GDD threshold has been surpassed in Kittery, and will likely be reached in Sanford within the next week. Damariscotta, Fryeburg, and Portland all are in the low 300s now, and next week’s heat wave will bump them closer to this threshold, too.
More on Onion Maggot
Despite all this talk about onion maggot, it is not typically a serious pest of garlic (though it can be in onion). Still, it is helpful to be aware of its biology to know when to look out for suspicious plant symptoms. Onion maggot flies lay their eggs at the base of allium plants after they emerge, and the maggots feed on roots and bulbs when they hatch. This causes young plants, especially newly transplanted or direct seeded onions, to suddenly yellow, wilt, and die. The maggots then move on to neighboring plants. You can read more about onion maggot in last year’s newsletter that explained this pest’s biology, impact on allium crops, and management strategies in detail.
All major insect pests of garlic in New England are listed in the chart below along with their GDD models for easy reference against the Maine Climate Office maps that update daily. GDDs for allium leafminer emergence have now been surpassed in Damariscotta, Kittery, Portland, and Sanford, and will likely be surpassed in Augusta, Bangor, Ellsworth, Lewiston, Newry, and West Rockport within the next week. Recall from the previous newsletter on allium leafminer (ALM) that this pest has not been found in Maine, but please scout your crop and reach out to Peyton Ginakes by email at peyton.ginakes@maine.edu if you see any potential ovipositioning marks.
| Pest | GDD Model |
|---|---|
| Allium Leafminer | emergence at 350 GDD1°C = 630 GDD33.8°F |
| Onion Maggot | emergence at 390 GDD40°F, peak flight at 735 GDD40°F |
| Leek Moth | emergence at 50°F (no GDD model) |
| Onion Thrips | emergence at 140.4 GDD52.7°F, each generation requires 323 GDD52.7°F |
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.
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