Garlic damage in Colorado usually traces to one of five feeders: cutworms and seedcorn maggots working underground at planting, then flea beetles, grasshoppers, slugs, and spider mites working the leaves as the season runs on. Colorado State University Extension documents these among the state’s vegetable pests, but not every pest on that list has anything to do with garlic at all.
Which pests attack garlic in Colorado
Colorado State University Extension documents ten insect and mite pests active in the state’s vegetable gardens: corn earworm, cutworms and seedcorn maggots, flea beetles (Phyllotreta and Epitrix spp.), grasshoppers (Melanoplus spp.), harlequin bug, slugs, spider mites (Tetranychus urticae), spotted cucumber beetle, squash bug (Anasa tristis), and tomato/potato psyllid.
Half that list has nothing to do with a garlic bed. Corn earworm goes after corn ears and tomato fruit, structures garlic never produces. Harlequin bug is a brassica specialist, drawn to cabbage, kale, and their relatives, not alliums. Spotted cucumber beetle and squash bug both work the cucurbit family, squash, pumpkin, cucumber vines, and largely ignore a bed of strap-leaved garlic nearby. Tomato/potato psyllid feeds on nightshades. None of these five belong on a garlic problem list, whatever a general pest photo guide might suggest.
What’s left, cutworms and seedcorn maggots, flea beetles, grasshoppers, slugs, and spider mites, are the ones that actually show up on garlic in this state. They’re generalists, which is exactly why they cross over from other crops into an allium bed where a specialist wouldn’t bother. If a pest’s host list runs mainly to squash vines, tomato fruit, or cabbage leaves, it isn’t a garlic problem here, no matter how common it is in the same yard. That distinction matters more than it sounds, because a gardener who misreads a squash bug on a nearby cucumber as the cause of ragged garlic leaves ends up treating the wrong bed entirely.
It’s also worth saying plainly that this is a short list. Colorado State University Extension does not document a long roster of garlic-specific insects the way it does for, say, cucurbits or brassicas. Garlic’s tough, low-moisture leaves and underground bulb structure simply give fewer generalist feeders a reason to stay.
When each one shows up, and what it does first
Garlic in Colorado goes into the ground in autumn and comes out the following summer, which means the pest calendar has to be read against planting and lifting, not against a fixed set of months. Cloves typically go in several weeks ahead of the first fall frost window of September 30 to October 12, and because that date is a midpoint drawn from 1991-2020 records at a Denver-area station, the actual frost lands later than that window in about half of all years. That planting stretch is when cutworms and seedcorn maggots do their work below ground, either clipping a young sprout at the soil line or tunneling into a clove before it ever pushes a shoot. The first sign isn’t a chewed leaf; it’s a gap in the row where a clove should have come up.
Once the ground firms up over winter and leaves start pushing in spring, flea beetles are usually the first aboveground feeder to leave a mark, small round holes scattered across the leaf blade, sometimes described as looking shot through. As the weather warms further into late spring and early summer, grasshoppers move in, and their feeding shows up as ragged notches bitten into the leaf margin rather than clean holes, worse in dry years when other forage has thinned out.
Slugs follow the moisture, not the calendar. They show up during damp stretches, after a heavy watering or a run of rain, and they feed at night, leaving a rasped, silvery sheen on the leaf surface along with a slime trail that’s usually still visible the following morning. Spider mites do the opposite: they build up during the hottest, driest stretch of summer, right as the bulbs are sizing up before harvest, and their feeding shows as fine yellow stippling across the leaf, sometimes with faint webbing at the base where the leaves meet.
Telling the damage apart from Everything else
A garlic bed can look stressed for reasons that have nothing to do with an insect, and mixing those up leads to spraying a plant that just needs water or a different fertilizer. A soil test, not a bug, explains an evenly yellowed leaf more often than any pest on this list. The table below lays out the symptom, the usual insect suspect, and the detail that actually separates it from a look-alike cause.
| Symptom | Possible insect cause | What distinguishes it |
|---|---|---|
| Missing or clipped seedling | Cutworm or seedcorn maggot | The stem is cut cleanly at soil level, or the clove itself shows tunneling; a clove that simply rotted shows no insect and no clean cut |
| Small round holes across the leaf | Flea beetle | Holes are clustered and shot-hole shaped, often with tiny beetles that jump when the leaf is disturbed; storm-related tears look random and coincide with a specific weather event |
| Ragged notches along the leaf edge | Grasshopper | Bite marks are irregular and concentrated on outer leaves; uniform tan, dried tips point to wind or drought stress instead |
| Yellow speckling with fine webbing | Spider mite | The stippling feels slightly rough under a hand lens and webbing sits at the leaf base; even yellowing from tip to base with no texture change usually points to a nutrient issue |
| Slime trail and rasped surface | Slug | The trail is visible in early morning and the surface looks scraped, not spotted; fungal disease produces defined lesions with a clear border instead |
None of this replaces an on-the-ground look. The table narrows the possibilities; it doesn’t confirm a diagnosis, and a county extension office can do that with a sample in hand better than any list can.
What actually works here, and what is a waste of time
On Colorado’s short growing season, timing beats any treatment applied after the fact. Getting a floating row cover over the bed right after planting, and keeping it on through the fall before the ground locks up, matters more than anything sprayed on later, because it blocks the adults from laying eggs near the cloves in the first place. Clearing old plant debris out of the bed after harvest removes the damp cover that slugs and overwintering cutworm larvae rely on, and it costs nothing but time.
Handpicking works for grasshoppers and slugs on a small bed, especially early in an infestation before numbers build. Shallow dishes set at soil level catch slugs overnight and are worth checking each morning during a damp spell. Rotating garlic and other alliums away from a bed that had heavy seedcorn maggot pressure the previous year also cuts risk, since the maggots are drawn to decaying organic matter left behind in soil that’s been recently disturbed.
What doesn’t help: watering more in response to leaf stippling, on the assumption it’s drought stress, when the actual cause is spider mites, since extra moisture does nothing for mites and can invite fungal problems instead. Broad spraying without confirming which pest is present is also a common misstep; it knocks back the predators that would otherwise keep flea beetles and mites in check, often making the following season worse. This site doesn’t name products or rates for a reason: what’s labeled and legal to use changes, and only the state’s extension service tracks that current list accurately.
Where to check this for your own county
Colorado State University Extension is the source for everything listed on this page, and its county offices are the next stop for anything specific to a single garden. A pest documented at the state level doesn’t mean it’s present in a given yard; presence varies block by block, let alone county by county, and only a local office or a submitted sample can confirm what’s actually feeding on a particular bed.
The main extension site, extension.colostate.edu, links out to individual county offices, which handle sample identification and answer questions tied to local growing conditions rather than statewide averages. That local layer matters especially in a state where mountain towns can frost in July while the plains hold on into October, and where the western slope runs on its own calendar entirely.
Before spraying anything, or before assuming the worst about a chewed leaf, a call or a dropped-off sample at the county office settles the question faster than guesswork from a list like this one.
Common questions
Does garlic attract the same pests as onions in Colorado?
Garlic and onions share several feeders on this list, since both are alliums and draw the same generalist insects, cutworms, seedcorn maggots, and grasshoppers among them. Colorado State University Extension doesn’t document a garlic-only pest separate from other alliums.
Can squash bugs or cucumber beetles move onto garlic if squash is planted nearby?
Not based on what Colorado State University Extension documents. Both are cucurbit specialists tied to squash, pumpkin, and cucumber vines, and they don’t shift onto allium foliage even when planted in the same bed.
Is fall the riskiest time for garlic pests in Colorado?
Fall planting is the window when cutworms and seedcorn maggots do underground damage, since that’s when cloves and young sprouts are most exposed. Summer, closer to harvest, is when spider mites tend to build up instead, so the risk shifts rather than disappears.
Do slugs really bother garlic, given how tough the leaves are?
Slugs are on Colorado State University Extension’s documented list and do feed on garlic during damp stretches, though the tougher leaf tissue means the damage is often lighter rasping rather than the heavier feeding seen on softer greens nearby.