Maine’s home gardens run into a fairly consistent short list of insects, and University of Maine Cooperative Extension tracks ten of them closely enough to keep a dedicated fact sheet on each: browntail moth, winter moth, lily leaf beetle, Colorado potato beetle, seedcorn maggot issues, Cabbage maggot Maine problems, striped cucumber beetle damage, squash vine borer pressure, spotted wing drosophila pressure and emerald ash borer populations in Maine. Two of those move on a schedule UMaine Extension has actually documented: browntail moth caterpillars are active after overwintering in their nests, with adult moths flying in July and August, and lily leaf beetle adults emerge anywhere from very early spring through June. Walk your lily bed and check any browntail webbing on branch tips as soon as the ground is workable, since both pests move fast once temperatures climb.
What Maine gardens actually deal with
Maine’s cool, damp spring and its mix of native hardwoods, old orchard trees and long-established perennial beds give several of these pests exactly the conditions they need to build up numbers before a gardener notices. Browntail moth and Winter moth populations both overwinter as young larvae or as eggs on woody hosts, so a mild winter followed by a wet spring lets more of them survive to the leaf-chewing stage. Lily leaf beetle populations in Maine, an import that has spread through the Northeast, thrive wherever true lilies and fritillaria are grown as garden staples, which they are throughout the state, while vegetable beds face steady pressure from the Colorado potato beetle Maine gardeners have come to expect on potato and tomato plants. Gardeners just over the state line deal with much of the same pressure. Massachusetts, for instance, sees the same browntail and winter moth cycle play out on a similar calendar, covered in Garden Pests in Massachusetts: What Shows Up, When, and How to Stop It. The pest list looks completely different in a hot, dry climate. In Garden Pests in New Mexico: What Shows Up, When, and How to Stop It, moisture-loving maggots barely register and the pressure comes from a different set of insects entirely.
Browntail moth
The browntail moth caterpillar is dark brown to blackish, up to about 1.5 inches long, with two orange-red spots near the tail end and a coat of fine hairs that cause a poison-ivy-like rash on skin contact. Colonies overwinter inside a tightly woven white silk web at the tip of a branch, most often on oak, apple, crabapple and other fruit and shade trees. UMaine Cooperative Extension notes the caterpillars become active in spring straight out of that nest, feeding through late spring before pupating, with the adult moths flying in July and August. Damage shows up as entire branches stripped to bare wood. Cut and destroy the winter webs before caterpillars emerge in early spring, and never handle them bare-handed.
Winter moth
Caterpillars are pale green inchworms with a faint white stripe down each side, about an inch long at full size, feeding on expanding buds and new leaves of maple, oak, and fruit trees in spring. The adult male is a small tan-brown moth that flies at night in late fall; the female is wingless and climbs the trunk to lay eggs, which is exactly what makes sticky trunk bands effective. Damage looks like ragged holes and skeletonized new leaves appearing all at once just as buds open. A dormant oil spray applied before bud break can reduce the following spring’s hatch.
Lily leaf beetle
The adult is small, glossy and bright scarlet red, about a quarter to three-eighths of an inch long, with a black head and legs. Larvae are dull orange-brown grubs that often carry a coating of their own dark excrement on the undersides of leaves. UMaine Extension’s Bulletin 2450 notes adults emerge anywhere from very early spring through June, and the beetle completes one generation per year in Maine. Both stages chew notches and skeletonize lily and fritillaria foliage, and a bad infestation can strip a stand down to stems. Hand-picking adults in early morning, when they are slower, and checking leaf undersides for eggs and larvae are the most reliable controls home gardeners have.
Seedcorn maggot
The larva is a small yellowish-white, legless maggot about a quarter inch long that feeds inside seeds and germinating seedlings in the soil, most damaging in cool, wet spring conditions. The adult is a nondescript grayish-brown fly. Damage shows up as poor germination and weak or missing seedlings, often blamed on bad seed when the real cause is underground. Delaying planting until soil has warmed and dried a little, and avoiding fresh manure worked in right before sowing, cuts the risk substantially.
Cabbage maggot
The larva is a small white, legless maggot about a third of an inch long that tunnels into the roots of cabbage, broccoli, and radish. The adult fly is grayish and slightly humpbacked, similar in size to a housefly. Wilting or stunted transplants with tunneled roots are the giveaway. A floating row cover or a simple collar around the base of each transplant, put in place at planting time, blocks the egg-laying fly before it ever finds the stem.
Colorado potato beetle
The adult is a rounded, domed beetle about three-eighths of an inch long, yellow-orange with ten black stripes running the length of its wing covers. Larvae are humpbacked, reddish-orange grubs marked with rows of black spots. Both stages chew potato, eggplant, and tomato foliage and can defoliate a planting within days once larvae hatch. Hand-picking adults, crushing the orange egg clusters on leaf undersides, and rotating nightshade beds each year keep numbers down without a spray.
Striped cucumber beetle
The adult is slender and oblong, about a fifth of an inch long, yellow with three bold black stripes and a black head. It chews seedling leaves and stems hard enough to kill a young cucumber or squash transplant outright, and it spreads bacterial wilt as it feeds, which is often the more serious damage. Row cover kept on until flowers open, then removed for pollinators, is the standard defense.
Squash vine borer
The adult is an odd-looking day-flying moth, wasp-like, with a stout orange-red and black body and metallic greenish-black forewings. Larvae are white to cream grubs with a brown head, up to an inch long, tunneling inside the stem. A vine that suddenly wilts despite moist soil, with sawdust-like frass piled near the stem base, means a borer is already inside. Checking stem bases weekly through early summer and slitting the stem to remove a larva while damage is minor can save the plant.
Spotted wing drosophila
The adult is a tiny tan fruit fly, about an eighth of an inch long, and the male carries a single dark spot near the tip of each wing. Unlike most fruit flies, it lays eggs in fruit that is still ripening, so raspberries and blueberries that look fine on the outside turn soft and collapse from the inside within days. Picking ripe fruit promptly and refrigerating it right away slows larval development enough to matter.
Emerald ash borer
The adult is a narrow, flattened, metallic emerald-green beetle about half an inch long. Larvae are cream-colored and segmented, feeding under the bark in winding S-shaped tunnels that eventually girdle the tree. Canopy dieback, bark splitting, and D-shaped exit holes are the signs, and an infested ash can die within a few years of the first symptoms. This is an invasive species, so report a suspected find to UMaine Extension rather than treating on a guess.
Telling damage apart
Reading the pattern
Chewed leaf edges with visible bite marks mean a chewing insect, something with mandibles working from the outside of the leaf in, like Colorado potato beetle or lily leaf beetle larvae. Stippling, a fine speckled bleaching of the leaf surface, points to something that sucks or scrapes rather than bites, and shows up long before any hole does. Clean round or irregular holes punched through the leaf blade, without ragged edges, usually mean flea beetles or young caterpillars rasping small sections clean through. Skeletonized foliage, where only the veins remain, is the signature of a heavy caterpillar infestation like browntail or winter moth working through new growth fast.
Insect, disease or water
Wilting on its own is the trickiest symptom to read, because it can mean a borer has severed the stem’s plumbing from the inside, a soil-borne fungus has done the same from the roots, or the plant is simply dry. The distinction matters because the response is completely different. Check the stem base and root crown before doing anything else: sawdust-like frass at a squash stem points to squash vine borer, a soft brown mushy stem points to a fungal rot, and firm tissue with no boring signs points back to water. Sticky residue on leaves, often followed by a black sooty coating, is honeydew left by sap-feeding insects rather than any disease, and it washes off once the insect causing it is controlled. A yellowing that starts at the leaf margins and moves inward, spread evenly across the whole plant, is more often a nutrient or watering problem than a pest, since most Maine insect damage shows up as a localized, uneven pattern rather than an even discoloration.
When each one shows up
Maine’s pest calendar runs on soil and air temperature far more than on the calendar date, so the same pest can show up a week or two apart from one part of the state to another. UMaine Cooperative Extension’s monitoring, drawn mainly from around Portland, has browntail moth caterpillars active through spring as they leave their overwintering nests, with the adult moths not flying until July and August. A garden farther north or well inland, where spring arrives later, should expect that same activity to lag behind the Portland timing rather than match it exactly.
Early spring, as soon as soil can be worked, is when seedcorn maggot risk is highest, because cool wet soil slows seed germination and gives the maggot more time to find it. That same stretch is when lily leaf beetle adults start turning up, a window UMaine Extension’s Bulletin 2450 puts anywhere from very early spring through June for a single generation. Cabbage maggot flies follow close behind, laying eggs at the base of newly set brassica transplants.
Once soil has warmed and cucurbits and potatoes are up and growing, striped cucumber beetle and Colorado potato beetle adults appear on schedule with the crop rather than the date, moving in as soon as the right foliage is available. Squash vine borer adults begin flying as squash and pumpkin vines start to run, usually a few weeks after the cucumber beetles show up. Spotted wing drosophila pressure builds through summer as raspberries, blueberries, and other soft fruit start to ripen, and it keeps building into fall as long as fruit keeps coming. Emerald ash borer adults are active for a shorter window in early summer, but the damage they cause is invisible until the tree has been infested for a year or more, which is why watching bark and canopy condition matters more than watching for the beetle itself.
What actually works
Prevention comes first
Rotating nightshades, brassicas, and cucurbits to a different bed each year breaks the cycle for soil-dwelling stages like cabbage maggot and Colorado potato beetle, since neither travels far to find a host on its own. Floating row cover put on at planting and sealed at the edges keeps cucumber beetles and cabbage maggot flies off young plants entirely, and it only has to come off once flowers need pollinating. Clearing garden debris and old lily foliage in fall removes some of the overwintering sites lily leaf beetle and squash vine borer use to get through winter. Encouraging ground beetles, parasitic wasps, and birds by leaving some undisturbed edges and skipping broad-spectrum sprays keeps a natural check on caterpillar and beetle numbers that a spray program would otherwise wipe out along with the pest.
- Rotate nightshade, brassica, and cucurbit families to a new bed each season
- Install row cover at transplanting, remove it once flowering starts
- Clear fallen leaves and old stems where pests overwinter
- Leave unmown edges and skip broad sprays to protect predatory insects and birds
Targeted intervention
Hand-picking works well for slow-moving pests like Colorado potato beetle and lily leaf beetle adults, especially early in the morning when they are sluggish. Cutting and destroying browntail moth winter webs before caterpillars emerge removes a whole colony in one pass. Ultrasonic pest repellers and bug zappers do nothing measurable for garden insect damage, since none of the pests on this list navigate by sound and most zappers kill far more beneficial insects than pests. Garlic and hot pepper sprays can make foliage briefly unpalatable but wash off with the next rain and need reapplying constantly to matter. Where a spray is genuinely warranted, matching the product to the pest’s life stage and timing it to when that stage is actually present matters more than which product is chosen.
What control costs
Hand-picking costs nothing but time, and for pests like lily leaf beetle and Colorado potato beetle, a few minutes every few days through the active season is usually enough to keep numbers below the point of real damage. Row cover and simple stem collars are a one-time material cost per bed that can be reused for several seasons if stored dry, making them the cheapest option per season once the first purchase is made. Biological controls, where available, such as encouraging or releasing predatory and parasitic insects, take patience rather than repeated spending, but they establish slowly and won’t stop an infestation already underway. Sprays, whether organic or synthetic, carry an ongoing cost because most need to be reapplied every one to two weeks through the pest’s active period, and timing a spray to the vulnerable life stage takes more attention than any other method on this list.
| Method | Money cost | Time cost | Repeat frequency |
|---|---|---|---|
| Hand-picking | None | Low, a few minutes per check | Every few days in season |
| Row cover / barriers | Low, one-time per bed | Low, install and remove once | Reusable for several seasons |
| Biological controls | Low to moderate | Moderate, slow to establish | Ongoing, minimal upkeep once set |
| Sprays | Moderate, recurring | Moderate, timing-sensitive | Every 1-2 weeks through active period |
Local help
UMaine Cooperative Extension identifies specimens and tracks outbreak activity across the state, and its home and garden IPM program is the right first stop for anything on this list that doesn’t match the descriptions above. Reach it through extension.umaine.edu, and for other seasonal tasks around the state, our Maine guides cover the rest of the growing calendar. Confirm identification before treating, especially with emerald ash borer, since misdiagnosis wastes a season and, with an invasive pest, delays reporting that matters statewide.
Look Closer at Specific Pests
Everything above gives you the broad shape of the season, but some pests deserve a closer look on their own terms. If your brassicas have been wilting for no obvious reason, that’s often the work of cabbage maggot, which starts turning up in Maine gardens in early to mid-May, right as the soil warms past 50°F for a few days running. That timing matters, because it’s the first of two or three generations the pest works through over the season, and knowing when the clock starts is half the battle in getting ahead of it. Rather than trying to cram all the details about egg-laying windows, soil temperatures, and generation counts into a general overview, it’s worth reading the dedicated page so you can match your defenses to the actual biology at play instead of guessing from a calendar date alone.
Common questions
Do I need to spray every year even if I didn’t see pests last year?
No. Most of the pests on this list build up gradually or arrive from nearby overwintering sites, so scouting first and treating only when you actually find damage or the insect itself avoids wasted sprays and protects the beneficial insects that keep numbers down on their own.
Is it the browntail moth caterpillar hairs that cause the rash, or something else?
The hairs themselves cause it, on contact or even airborne in late summer when old nests break apart. Long sleeves and gloves when pruning or removing nests, and washing exposed skin promptly, reduce the reaction, which can last for days on sensitive skin.
My lily leaves have holes but I never see any beetles. What’s going on?
Check the undersides of the leaves rather than the tops. Lily leaf beetle larvae feed there and coat themselves in dark excrement that camouflages them against the leaf, so a quick glance from above misses them almost every time.
Can I treat for emerald ash borer myself once I spot the D-shaped holes?
Get the identification confirmed through UMaine Extension first rather than treating on your own, since the exit holes appear only after the tree has already been infested for a while, and a misidentified beetle wastes the treatment window on the wrong problem. This is also an invasive species the state extension tracks, so a confirmed report matters beyond your own yard.