Colorado potato beetle and seedcorn maggot are the two insects documented for potato in this border region, according to University of Maine Cooperative Extension. One chews leaves once plants have broken the surface; the other tunnels into seed pieces before they ever sprout. Everything else on the region’s general pest list feeds on trees, lilies, or squash vines, not potato. Timing here tracks the average last spring frost near Moncton, not a calendar month, because planting itself starts around that date.
Which pests attack potato in New Brunswick
University of Maine Cooperative Extension documents ten insects for this border region: Browntail moth (Euproctis chrysorrhoea), Cabbage maggot, Colorado potato beetle, Emerald ash borer, Lily leaf beetle (Lilioceris lilii), Seedcorn maggot (Delia platura), Spotted wing drosophila, Squash vine borer, Striped cucumber beetle, and Winter moth. That’s the full list the service tracks in the area. Most of it has nothing to do with a potato patch.
Browntail moth caterpillars feed on hardwood leaves, oak and apple and cherry among them, not garden vegetables. Cabbage maggot larvae tunnel into the roots of cabbage-family plants like broccoli, radish, and turnip, a different root system entirely. Emerald ash borer only bores into ash trees. Lily leaf beetle sticks to lilies and fritillaria foliage. Spotted wing drosophila lays eggs in soft-skinned fruit such as raspberries and blueberries, not tubers. Squash vine borer and striped cucumber beetle both work squash, pumpkin, and cucumber vines. Winter moth caterpillars defoliate fruit trees and hardwoods in early spring, before most vegetable seedlings are even in the ground.
Only two of these ten insects have any documented connection to potato itself. Colorado potato beetle feeds on foliage after the plants emerge, stripping leaves from the edges inward when populations build. Seedcorn maggot works underground, tunneling into the cut seed piece before a shoot ever reaches daylight. The service’s list for this region is short on potato specifically, and there’s no reason to stretch it further.
That narrowness matters for how a gardener reads the rest of this page. A leaf with a shot-hole pattern, a stem chewed at the base, a vine collapsing overnight, none of those match either of the two species named here. They point toward something else in the garden, possibly a pest this service documents on a different plant altogether, or possibly no insect at all.
When each one shows up, and what it does first
Both insects key off soil temperature and moisture more than the calendar. The reference point that matters here is the average last spring frost near Moncton, which lands around May 23. That figure comes from Environment and Climate Change Canada’s climate normals, and it’s a mean, not a guarantee. In one year out of two, the actual last frost falls later than that date, and a province this size runs weeks behind that average in northern or upland districts. Treat it as the midpoint of a window, not a safe planting signal.
Seedcorn maggot risk is highest in the cool, wet weeks bracketing that average last frost, before shoots break the surface. The adult fly is drawn to decaying organic matter and to cut seed pieces sitting in slow-to-warm soil. The first sign a gardener actually sees isn’t the insect at all: it’s a seed piece that simply never sprouts, or a weak shoot that pushes up chewed and stunted because the piece underneath has been tunneled through before it could feed the plant.
Colorado potato beetle works on a different clock. The adults overwinter in the soil at field edges and hedgerows, then walk or fly into the rows once the ground has warmed past that same frost window and the first potato foliage is up. The earliest sign is a handful of orange-and-black striped adults clustered on the first true leaves. That’s followed by clumps of bright orange eggs on the underside of leaves, and then by reddish, humped larvae that skeletonize foliage from the leaf margins inward, working through a planting row by row rather than all at once.
Neither insect announces itself with damage to the tuber before that point. The earliest visible sign in both cases is above or just below the soil surface, which is exactly where most gardeners aren’t looking in the first weeks after planting.
Telling the damage apart from everything else
A lot of things in a potato patch look like insect damage and aren’t. Sorting them out starts with what you can actually see, not with a guess at the name.
| Symptom | Likely cause | What to check |
|---|---|---|
| Seed piece never sprouts | Seedcorn maggot tunneling, or simple rot in cold wet soil | Cut the piece open. Tunnels with small white maggots point to the insect; a soft, uniformly black, mushy piece with no larvae present points to rot |
| Leaves skeletonized early in the season | Colorado potato beetle larvae feeding | Look for the reddish, humped larvae themselves on the underside of affected leaves, not just the pattern of damage |
| Leaf margins yellowing, whole plant looks pale | Nutrient stress or drought, not insect feeding | No chewing marks anywhere on the plant, and the discoloration is even across the row rather than concentrated on a few leaves |
| A single stem wilting while neighbors stay green | Mechanical injury from cultivation, or a vascular disease | Check the stem base for a cut, a crush mark, or discoloration inside the stem; no beetles or eggs present nearby |
None of this replaces an actual look at the plant in front of you. A photo or a sample taken to the county extension office settles a doubtful case far faster than working through a list at home, and it avoids the common mistake of treating for an insect that was never there to begin with.
What actually works here, and what is a waste of time
Starting with certified seed pieces matters more than almost anything else on this list. Uncertified seed can carry virus into the planting before an insect ever touches it, and no amount of pest control afterward fixes that. It’s a decision made before the ground is even opened.
For Colorado potato beetle, hand-picking adults and crushing the orange egg clusters in the first weeks after emergence works because the population is still small, visible, and concentrated on a few plants. That window closes once larvae hatch in numbers across the planting. Row covers over the rows during those same early weeks keep flying adults from reaching the plants at all, an option that costs nothing but timing and a bit of fabric. Rotating potato out of ground that grew potato or tomato the year before reduces the number of overwintering beetles nearby, since they don’t have to travel far to find the next crop.
For seedcorn maggot, the decisive move happens before planting, not after: waiting until the coldest, wettest stretch around the average last frost has passed reduces the time a seed piece sits vulnerable underground. Planting into cold, waterlogged soil does the opposite of protecting the crop, because it slows emergence and gives the maggot more time to work before a shoot can outgrow the damage.
What doesn’t help: treating an entire planting on a fixed calendar without checking for beetles or eggs first, which wastes effort on rows that never had a problem. Neither does assuming a cold spring alone will keep seedcorn maggot away. Cold, slow-to-warm soil is exactly the condition the fly favors, not a deterrent.
Where to check this for your own county
This list comes from University of Maine Cooperative Extension, a service on the neighboring side of the border, and it’s offered here because it documents insects for this general region in detail that isn’t otherwise published for the province garden by garden. What’s documented at that scale is never a guarantee of what’s in a specific yard. A garden near Moncton, close to the climate station behind the frost date on this page, sits in a different microclimate than one further north or at higher elevation, where the same average last frost lands weeks later.
The next step for a gardener seeing damage that doesn’t match anything on this page is a call or visit to the local county extension office, not another search online. Bring a sample if possible, a few affected leaves, an intact seed piece cut open, or a photo taken close enough to show the insect or the exact damage pattern. Extension staff working county by county see what’s actually present that season, which shifts year to year with weather in a way a general list never captures.
That’s also the right place to ask about anything not covered here, generation counts for Colorado potato beetle in a given season, or which insecticide options are labeled for potato in the province, since this page names neither. Those answers change by year and by product registration, both things a local office tracks and a general reference page doesn’t.
Common questions
Does Colorado potato beetle produce more than one generation per season here?
That figure isn’t published in the sources used for this page. A county extension office can speak to what’s typical locally in a given year.
Is seedcorn maggot only a threat to potato?
No. It’s a generalist pest of seeds and seedlings broadly, not a potato specialist, which is part of why cold, wet soil conditions at planting time raise its risk across more than one crop.
Should potato go in the ground before or after the average last frost date?
Seed pieces tolerate cooler soil than frost-tender transplants do, but an exact planting date isn’t published in the sources behind this page. Since the May 23 figure is a mean, not a safe cutoff, a county extension office is the better source for a planting window suited to a specific garden.
Does hand-picking Colorado potato beetle actually work on a small home planting?
On the scale of a home garden, checking plants regularly in the first weeks after emergence and removing adults and egg clusters by hand is a practical, workable approach, precisely because the population starts small and visible before it spreads.