In Maine, potato foliage is eaten mainly by the Colorado potato beetle, which strips leaves from shortly after emergence through midsummer. Seed pieces planted into cool, wet spring soil face a different problem: seedcorn maggot larvae tunneling into the piece before sprouts ever break the surface. Both are documented for potato by the University of Maine Cooperative Extension, and both track the region’s frost calendar more than the calendar month.
Which pests attack potato in Maine
The University of Maine Cooperative Extension documents ten pests across Maine’s landscapes and gardens: Browntail moth, Cabbage maggot, Colorado potato beetle, Emerald ash borer, Lily leaf beetle, Seedcorn maggot, Spotted wing drosophila, Squash vine borer, Striped cucumber beetle, and Winter moth. That is a broad list built for the whole state, not a potato menu, and most of the names on it have nothing to do with a potato patch.
Emerald ash borer feeds only on ash trees. Lily leaf beetle is a lily and fritillary specialist. Winter moth and browntail moth chew the leaves of oaks, apples, and other trees and shrubs, not potato foliage. Spotted wing drosophila targets soft, thin-skinned fruit like berries and cherries. Squash vine borer and striped cucumber beetle are cucurbit pests, tied to squash, pumpkin, and cucumber vines. Cabbage maggot goes after brassica roots, cabbage, radish, turnip, not potato tubers. None of those seven belong on a page about what’s eating a potato crop, and if you spot one of them in the garden, it’s feeding on something else nearby.
That leaves two pests the extension service documents that actually reach a potato plant. The Colorado potato beetle is the one growers meet first and see most often, a hard-shelled beetle with black-and-yellow stripes whose larvae, humped and orange-red, do the heaviest defoliation. The seedcorn maggot works underground and earlier in the season, its larvae boring into the cut seed piece itself before the plant has even sent up a sprout, favoring soil that stays cold and waterlogged after planting.
Scouting leaf undersides for egg clusters before larvae hatch is the single most useful habit a home grower can build around the first pest; for the second, the decision that matters happens at planting, not afterward, since damage to a buried seed piece is rarely visible until the row fails to come up evenly.
When each one shows up, and what it does first
Timing here follows the frost-free window, and that window is not the same everywhere in the state. In the Portland area, the last spring frost falls between April 22 and May 1, based on NOAA NCEI’s 1991-2020 Climate Normals for that station. Inland and northern Maine run substantially later in spring and earlier in fall, so a grower in Aroostook County should expect both pests to arrive later on the calendar than a grower near the coast, even though the sequence of events stays the same.
Seedcorn maggot risk sits at the very start, in the days after seed pieces go into the ground and before sprouts emerge. The larvae feed inside soil that has stayed cool and wet, tunneling into the cut surface of the seed piece. The first sign a grower sees is not an insect at all, it’s a gap in the row where a plant should have come up and did not, or a weak, yellowed sprout that never gains strength. By the time the damage is visible above ground, the maggot has often already finished feeding below it.
Colorado potato beetle shows up later, once the plants are up and growing. Overwintered adults emerge from the soil as temperatures warm following the frost-free date and move into newly planted potato rows to feed and lay eggs. The first thing most growers notice is small, ragged holes chewed into young leaves, followed within days by clusters of bright orange-yellow eggs on the underside of leaves nearby. Left alone, those eggs hatch into the humped larvae that do the real defoliation, working from the edges of leaves inward until whole plants are reduced to stems and veins.
The gap between emergence and the first chewed leaf is short, which is why a grower who waits for obvious damage before checking has usually already missed the egg stage, the point where control is easiest.
Telling the damage apart from everything else
Not every ragged leaf or thin stand of potatoes is a pest problem, and treating a nutrient issue or a dry spell as an insect infestation wastes effort without fixing anything. The clearest rule: a diagnosis needs the insect, an egg mass, or a larva actually present, not just a symptom that looks similar to something else.
| Symptom | Likely cause | What tells them apart |
|---|---|---|
| Leaves chewed to the veins, orange eggs or humped larvae on leaf undersides | Colorado potato beetle | Insects, eggs, or larvae are visible on the plant itself |
| Seed piece rotted or hollowed out below the soil, sprout never emerges | Seedcorn maggot | Soil stayed cold and wet after planting; larvae are visible inside the piece if dug up |
| Leaves yellowing evenly, starting low on the plant, no holes | Nutrient deficiency | No chewing marks, no insects present, pattern spreads gradually across the whole plant |
| Leaf edges browning and curling during dry weather | Drought stress | Soil is dry to the touch; damage follows the driest part of the row |
| Dark, water-soaked or ring-patterned lesions on leaves, worse after rain | Fungal disease | Lesions, not chewed edges; spreads fastest in wet, humid stretches |
| Broken stems or torn foliage in a straight line through the row | Mechanical damage | Pattern matches the path of a hoe, tiller, or cultivator, not random feeding |
A home grower doesn’t need to settle every case alone. When a symptom doesn’t clearly match one of these, the safest next step is a call or a sample to the county extension office, since a photo or a plant sample lets someone trained in Maine’s specific pest and disease pressures make the call instead of a guess made over a garden fence.
What actually works here, and what is a waste of time
On a short growing season, timing beats almost any product decision. Planting after the soil has warmed rather than into cold, wet ground is the single change that lowers seedcorn maggot risk the most, since the larvae need exactly that cool, saturated soil to feed successfully on a seed piece before it sprouts. Waiting a few extra days for soil to dry and warm costs little and removes the window the maggot depends on.
Certified seed potatoes matter here too, and not as an afterthought. Uncertified seed can carry virus into a planting invisibly, with no insect involved at all, so starting with certified stock each season removes a problem that no amount of pest management later on can fix.
For Colorado potato beetle, hand-picking adults and crushing egg masses on leaf undersides works well precisely because Maine gardens tend to be small enough that the task is manageable, and because catching the beetle at the egg stage prevents the larval stage that does the real damage. Row covers placed over young plants at planting, then removed once flowering begins to allow pollination, physically block adults from reaching the crop during the weeks damage tends to start. Crop rotation, moving potatoes to a different part of the garden each season, reduces the number of overwintered adults that emerge right next to a fresh planting.
What doesn’t help: spraying on a fixed calendar without checking the plants first, since beetle populations and timing shift from one season to the next and a treatment applied at the wrong stage misses the insects entirely. Tilling in a damaged row after the fact doesn’t recover a stand lost to seedcorn maggot, it only confirms the loss. Any decision about a specific product or treatment threshold belongs with the University of Maine Cooperative Extension, which knows what’s registered and appropriate for use in the state.
Where to check this for your own county
Everything above comes from the University of Maine Cooperative Extension, and that’s also where a Maine grower should go next with a specific question. The extension service documents these pests at the state level, but presence and pressure shift from one county to the next, and even from one side of a county to the other, depending on soil, elevation, and how late the frost-free window runs locally.
A statewide list doesn’t say what’s actually in a particular garden this season. That’s the gap a county extension office fills, with staff who track local conditions and can look at a sample or a photo directly rather than working from a general description. For a grower in Aroostook County, where the frost-free window runs later than coastal Maine, or for a grower near Portland working with the April 22 to May 1 window, the right move is the same: bring the question, and ideally a sample, to the local office rather than guessing from a symptom alone.
The next step is straightforward. Find the county extension office through the University of Maine Cooperative Extension, and use it before planting if the concern is seed piece rot, or as soon as a chewed leaf or egg cluster shows up if the concern is beetle damage. Waiting until a stand has already failed removes the option of catching either pest at the stage where action still makes a difference.
Common questions
Does the Colorado potato beetle attack other garden crops too?
The University of Maine Cooperative Extension’s list here covers pests across many crops, but only Colorado potato beetle and seedcorn maggot are documented against potato specifically. Other pests on the broader list, like striped cucumber beetle or squash vine borer, target unrelated crops such as squash and cucumber.
Can seedcorn maggot damage be seen before the potato sprouts?
Rarely. The larvae feed inside the seed piece underground, so the first visible sign is usually a missing plant in the row or a weak sprout, not anything seen while digging unless the piece is deliberately checked.
Does the frost-free date matter if I’m planting indoors first?
Potatoes are planted directly from seed pieces, not started indoors and transplanted, so the frost-free window applies directly to when seed pieces go into the ground outdoors.
Is it worth spraying for Colorado potato beetle before eggs appear?
Scouting for eggs and hand-removing them tends to work well in home-garden-scale plantings and avoids treating before there’s anything to treat. Any decision about a broader treatment should go through the University of Maine Cooperative Extension, since timing and product choice depend on what’s registered in the state.