Colorado State University Extension documents seven pests that actually feed on peppers here, and they show up on a schedule tied to the state’s short, hot growing season rather than to a calendar month. Flea beetles and cutworms hit transplants first. Spider mites and grasshoppers build through the hottest stretch of summer. Corn earworm and psyllids arrive once fruit is forming. Slugs work the edges whenever the soil stays damp.
Which pests attack pepper in Colorado
Colorado State University Extension documents ten insect and mollusk pests on this territory’s vegetable list, and only seven of them actually feed on pepper. The full list is: 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. It’s a short list compared to what some states carry, and there’s no reason to pad it.
Three of those ten don’t belong on a pepper page at all. Harlequin bug is a cole-crop specialist, it targets cabbage, kale, broccoli and their relatives, and pepper foliage isn’t on its menu. Spotted cucumber beetle and squash bug are both cucurbit pests, built around squash, pumpkin, and cucumber vines, and neither is documented here as a pepper feeder. A gardener who spots a squash bug on a pepper plant is looking at a bug that wandered over from the neighboring cucurbit bed, not one that’s actually working the pepper.
That leaves seven species worth watching on pepper in this state: corn earworm, cutworms and seedcorn maggots, flea beetles, grasshoppers, slugs, spider mites, and tomato/potato psyllid. Cutworms and seedcorn maggots matter almost entirely in the first two weeks after transplant, when stems are still soft. Flea beetles chew tiny pits in young leaves and can slow a transplant’s recovery. Grasshoppers and spider mites are the two that scale up fastest in a dry, hot Colorado summer. Corn earworm and psyllid both wait for fruit and foliage to mature before they become the bigger concern.
The decision that matters most here isn’t which pest shows up, it’s when the transplant goes into the ground. Peppers need soil at 60°F and a frost-free start, and that timing shapes when every one of these seven pests gets its first shot at the plant.
When each one shows up, and what it does first
This territory’s last spring frost falls between May 1 and May 11, based on the 30-70% freeze-probability band for the Denver reference station. Pepper transplants that have spent 8 to 10 weeks indoors go into the garden after that date, once soil has reached 60°F. Everything below is timed off that transplant window, not off a fixed month, because mountain towns and the western slope run their own, later schedules.
Cutworms and seedcorn maggots are the first threat a transplant faces. Within the first one to two weeks in the ground, a cutworm can clip a young stem clean off at the soil line overnight, leaving a seedling toppled with no visible insect around it the next morning. Flea beetles arrive in that same early window, riddling young leaves with small round shot-holes that make the foliage look like it’s been hit with fine buckshot.
Slugs show up whenever the soil stays damp, which in Colorado usually means cool spring mornings before the heat sets in, or after an irrigation cycle. Ragged holes in leaves and a silvery slime trail on the soil surface are the tell.
Grasshoppers and spider mites both build as the season heats up, and their timing tracks the state’s heat load directly. Denver’s reference station averages 39.6 days a year at or above 90°F, and that stretch of hot, dry days is exactly when spider mite populations explode, showing up first as fine stippling and a bronzed cast on leaves, with webbing visible on the undersides in heavier infestations. Grasshoppers ramp up over the same weeks, moving in from field edges and dry ground to notch and shred leaf margins, worse in dry years when their natural food sources dry out first.
Corn earworm and tomato/potato psyllid both wait for the plant to mature. Earworm damage shows up as a bored entry hole in developing fruit, once peppers have started to size up. Psyllid feeding shows up earlier and more subtly, as a yellowing or purpling discoloration on upper leaves paired with stunted new growth, a pattern known as psyllid yellows.
Telling the damage apart from everything else
Not every ugly leaf or stalled plant is a pest. Colorado’s combination of alkaline soils, hard water, and swings between cold nights and hot days produces symptoms that mimic insect feeding closely enough to send a gardener down the wrong path.
The clearest example is fruit set failure. A pepper plant loaded with flowers that never turn into fruit looks like it’s under attack, but fruit set fails at both ends of the temperature range: nights below 55°F or days above 90°F, with no insect involved at all. This site doesn’t carry summer night minimums for Colorado, so if plants are dropping blossoms on a cool stretch, that’s a call for watching your own overnight temperatures, not for guessing at them from the season or the almanac.
| Symptom | Possible cause | What tells them apart |
|---|---|---|
| Blossoms drop, no fruit forms | Heat or cold stress, not a pest | Check for chewing, boring, or stippling on the flowers themselves; none of the seven documented pests target blossom drop directly |
| Yellowing, purpling upper leaves | Psyllid feeding, or nutrient deficiency | Psyllid damage comes with stunted new growth; deficiency symptoms are usually more uniform across the whole plant |
| Wilting despite watering | Root or vascular disease, or underwatering | None of the documented pests here cause plant-wide wilt; check soil moisture at root depth first |
| Stem severed at soil line | Cutworm | Look for the caterpillar curled just under the soil surface within an inch of the stem |
| Fine speckling, bronze cast on leaves | Spider mites, or drought stress | Webbing on the underside confirms mites; without webbing, check soil moisture first |
When a symptom doesn’t match cleanly to one of the seven pests documented here, the safer move is a diagnosis from someone looking at the actual plant, not a guess extended from a list.
What actually works here, and what is a waste of time
Colorado’s growing season is short enough that timing beats treatment for most of these pests. A physical barrier over transplants for the first two weeks blocks cutworms and flea beetles during exactly the window when they do the most damage, and it costs nothing more than the material and the time to set it up.
That barrier has to come off once flowers open, because pepper fruit set depends on pollinators reaching the blossoms, and a cover left on through flowering does more harm than the pests it was blocking. Handpicking works reasonably well against grasshoppers and larger caterpillars on a small planting, especially in the cooler part of the morning when grasshoppers move slowly.
Spider mites are the pest most often made worse by the wrong response. They thrive in hot, dry, dusty conditions, so a stressed, underwatered plant during one of those 39.6 average days above 90°F is far more vulnerable than a well-watered one. Keeping plants adequately watered through the hottest stretch of summer does more for mite prevention than most treatments applied after the fact.
What’s a waste of time: treating for a pest before confirming it’s actually there, and reaching for anything without checking what’s legal to use on food crops in this state. This site doesn’t name products, active ingredients, or rates, because that call belongs to the state’s extension service, which tracks what’s currently registered for use on peppers in Colorado. Blanket treatments applied on a schedule rather than in response to an actual sighting tend to hit pollinators and beneficial insects harder than the target pest, which is its own kind of setback on a plant that needs those pollinators to set fruit at all.
Where to check this for your own county
Colorado State University Extension is the source for everything on this page, and its county offices are where a gardener gets an answer specific to their own garden rather than to the state as a whole. Pest pressure documented at the state level doesn’t mean every pest shows up in every county, or every yard within a county. A garden in a dry, wind-exposed spot on the eastern plains sees different grasshopper pressure than one tucked into an irrigated suburban lot along the Front Range.
The next step, if a pepper plant is showing damage that doesn’t clearly match one of the seven pests above, is a call or a sample drop-off at the local CSU Extension county office, not a guess based on a state-level list. County agents can confirm an identification from an actual leaf or insect sample, and they know what’s currently registered for use in Colorado if treatment turns out to be warranted.
Extension offices also track season-specific alerts, which matter more than a fixed calendar in a state where the frost-free window itself runs from May 1 to May 11 at the reference station and shifts considerably in the mountains and on the western slope. A pest calendar built for Denver’s elevation and frost timing isn’t the same one that applies two thousand feet higher up a canyon road.
Common questions
Do squash bugs or spotted cucumber beetles ever damage pepper plants in Colorado?
Neither is documented as a pepper pest here. Both are cucurbit specialists built around squash, pumpkin, and cucumber vines, so a sighting on a pepper plant most likely means the insect wandered over from a nearby cucurbit planting rather than that it’s feeding on the pepper itself.
Why do pepper flowers drop without setting fruit even when no pest is visible?
Fruit set fails at both temperature extremes, nights below 55°F and days above 90°F, with no insect involved. Since this site doesn’t track Colorado’s summer night minimums, blossom drop during a cool stretch calls for watching your own overnight temperatures rather than guessing from the season.
When is the earliest a pepper transplant is at risk from pests in Colorado?
The first one to two weeks after transplanting, once soil has reached 60°F following the last frost (May 1 to May 11 at the reference station), is when cutworms, seedcorn maggots, and flea beetles do the most damage to a young plant.
Is it worth covering pepper plants all season to keep pests off?
No. A cover left on past the start of flowering blocks the pollinators peppers need to set fruit. Covers work best for the first couple of weeks after transplant and should come off once blossoms appear.