Cooperative Extension at Highmoor Farm

Garlic IPM Newsletter No. 5 — May 28, 2026

Good growing conditions have continued, with moderate rainfall and warming temperatures encouraging the start of garlic bulb development. No major new diseases, or progressions in foliar diseases, have been observed at any scouting locations, and plants are generally healthy.

Insect Pests

With several recent warm spells, growing degree days (GDDs) that drive insect development are rapidly accruing. All of the state except Katahdin and the westernmost part of the county have warmed enough for allium leafminer (ALM) adults to emerge, where they have overwintered. As a reminder, ALM has not been found in Maine, but since it has been found in Canada and other New England states, it is good to keep an eye out for its damage. The GDD emergence threshold for onion maggot has also been met in most of the state. In addition to these two fly pests, the southernmost parts of Maine have now warmed enough for onion thrips to emerge. The chart below shows GDD accumulation for each of these three pests in locations represented by the Maine Climate Office GDD maps.

The table lists GDDs from Maine Climate Office website and numbers marked with an asterisk (*) indicate that sufficient GDDs have been met in that location for pests to emerge, and regular scouting as outlined below is warranted.

Scouting for Insect Pests

Just because enough GDDs have accrued in an area for pests to develop does not mean that they are present. These numbers are simply a timely reminder to scout for them, and install protective measures as necessary based on your knowledge of prior damage or infestations. Here is a quick overview of how to scout for each pest:

• Allium Leafminer: Keep an eye out for the distinctive oviposition marks that female adults create when laying eggs, typically on the outermost tips of leaves. Scout the allium crops that are overwintered or well established in the field at the time 630 GDD33.8F are met in your area. Images of damage and more information about ALM are available in the May 7 newsletter of this year. Please contact Peyton Ginakes by email at peyton.ginakes@maine.edu if you see any suspicious looking marks.
• Onion Maggot: Be on the lookout for individual plants that suddenly wilt and die, followed by neighboring plants that do the same. If this occurs, you can dig up plants and examine the roots for maggots, or submit the whole plant to the UMaine Plant Disease Diagnostic Lab (if no pests are present, they can diagnose any potential root diseases that may be causing the problem). If onion maggot has damaged your allium crops in the past, you can keep the flies out of your current crop with row cover, provided the field is not where a previous infestation occurred. Use the GDD model to time row cover installation before flies emerge at 390 GDD40F.
• Onion Thrips: Unlike the two pests already mentioned, onion thrips are likely to be present near you since they feed on a wide range of plants, despite their name. Scouting is critical for good control; thrips can be found well before their damage is noticeable from afar, at which point the population is typically beyond the action threshold of 1 to 3 thrips per leaf on average. Once GDDs reach 140 GDD52.7F, closely examine leaves for small, light yellow to yellow nymphs, which are very indiscreet. They typically hide in leaf folds along the midvein of garlic, and near leaf axils in garlic as well as round-leaved alliums. Thrips thrive in hot and dry conditions, so the rainy weekend forecast may keep them at bay even where the GDD threshold is met. The next issue will discuss onion thrips biology, crop damage, and management in more detail.
• Leek Moth: This pest is not listed in the table above because it emerges at 50F instead of following a growing degree day model. Like ALM, leek moth has not recently been found in Maine, but it is best to keep an eye out since it is now consistently found in bordering states and Canada. Larvae leave behind ragged chewing marks and lots of frass, and pupae have distinct cocoons that look like cargo nets. More information on leek moth is available in one of last year’s newsletters. Please contact Peyton Ginakes by email at peyton.ginakes@maine.edu if you see any suspicious looking feeding damage, larvae, or pupae.

Peyton Ginakes, PhD
Research Associate
University of Maine Cooperative Extension
Highmoor Farm
52 US-202
Monmouth, ME 04259
(207) 933-2100

peyton.ginakes@maine.edu

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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