Parasitic Wasps for Cabbage Worms: Cotesia & Trichogramma
By Paul Smith, pemdasgarden.com
If you find a green caterpillar covered in tiny yellow cocoons on broccoli, kale, or cauliflower, do not pick it off. That caterpillar may already be losing a fight you could not see: tiny parasitic wasps lay eggs inside cabbageworm eggs or caterpillars, and their young develop inside the host until it stops feeding and dies. The wasps do not sting people, and they are a strong reason to hold off on a broad spray when caterpillars first appear. This guide covers those wasps only — which species attack which stage, what parasitized caterpillars and cocoons look like, a five-minute check, how to keep wild wasps alive, and what bought releases can and cannot do. For the pest itself, see our cabbage worm identification and control guide; worm identification is kept brief here on purpose.
One caution first: a yellow cocoon cluster is not always a success story, and a green caterpillar is not always an imported cabbageworm. If you find ragged holes and a smooth green caterpillar on a brassica leaf, this looks most like imported cabbageworm feeding, but cabbage looper or diamondback moth larva damage is also possible, so treat any single sighting as a working guess, not a settled diagnosis.
What parasitic wasps actually do
A parasitoid wasp is not a predator that eats its prey outright. The adult female lays her eggs in or on a host — an egg, a caterpillar, or a pupa. Her larvae feed inside the host, which usually keeps feeding for a while, then weakens, stops, and dies. That delayed kill is the most misunderstood part of biological control: a parasitized caterpillar may still chew leaves for days, so a garden that looks unchanged in the first week may already have wasps at work.
That delay is why wasps fit the biological step of IPM rather than the emergency step. In IPM order — prevention, cultural, mechanical, biological, then least-toxic last — wasps come after covers, rotation, and handpicking. They pay off over the season, especially from midseason onward when parasitism rates rise (Cornell University, University of Minnesota Extension). A broad spray at the first holes may kill wasps along with worms and leave the next worm generation with no enemies.
Which wasp attacks which cabbageworm stage
Three wasp groups matter most on brassicas, attacking different stages. Confusing them causes two classic mistakes: expecting egg wasps to fix hatched caterpillars, or destroying larval cocoons that look like pest eggs. The table below is an original compilation for this article.
| Wasp group | Cabbageworm stage attacked | Visible sign in the garden | What to do when you see it |
|---|---|---|---|
| Trichogramma spp. (egg wasps) | Eggs, before they hatch | Host eggs turn dark instead of hatching; adults are under 1 mm and rarely seen | Leave darkened eggs alone; they hold wasps, not worms about to hatch |
| Cotesia glomerata (gregarious larval wasp) | Young caterpillars, especially first instars | A cluster of 20 to 30 small yellow cocoons on or beside a dead caterpillar | Leave caterpillar and cocoons in place; they are the next generation |
| Cotesia rubecula (solitary larval wasp) | Caterpillars, killing the host around the fourth instar | One small cocoon near a dead caterpillar, not a cluster | Leave it alone; one cocoon means one wasp, not failed control |
| Pteromalus puparum (pupal wasp) | Pupae (chrysalids) | Pupae turn brown and produce no butterfly | Leave pupae in place; they cut the next generation, not current leaf damage |
The timing difference matters for your harvest. Cotesia glomerata hosts keep feeding until about the fifth instar, after much feeding has happened, while Cotesia rubecula hosts die around the fourth instar, before most feeding damage (Colorado State University, University of Minnesota Extension). Cotesia glomerata is still a major mortality factor over the season (Cornell University), but its benefit lands mainly on the next generation, not on leaves already chewed.
Do not expect a particular species on your plants. Colorado State University places Cotesia rubecula as far west as North Dakota, while Cotesia glomerata is confined mainly east of Illinois in that account, and University of Massachusetts Extension reports Cotesia rubecula has largely displaced Cotesia glomerata in Massachusetts. Judge by the cocoons you actually see, not by a species name in a book.
Do not mistake cocoons for pest eggs
This is the costliest identification error in this topic, and it is easy to make. Cabbageworm eggs are tiny, pale yellow, and laid singly on leaf undersides; a Cotesia glomerata cluster is 20 to 30 small yellow cocoons on or beside a dead caterpillar (Kansas State University Extension, Cornell University). Crush every yellow thing on a leaf and you destroy the wasps and spare the worms.
Use three checks before you crush anything yellow. Count: a single egg alone on a leaf underside is a cabbageworm egg; a cluster of many small cocoons on a caterpillar is wasp work. Look for the host: cocoons sit on or next to a shrivelled caterpillar, eggs on bare leaf. Look at the caterpillar: a parasitized one often looks sluggish, swollen, or off-color. If you find a caterpillar with a yellow cluster, this looks most like Cotesia parasitism, but a diseased caterpillar is also possible — either way, leave it in place and check back in a few days.
The same logic applies to darkened eggs. Trichogramma wasps develop inside the host egg, and the egg darkens as the wasp matures (University of Hawaii, McGill University). A dark egg is a wasp about to emerge, not a dead egg to clean up. Crush only pale yellow, singly laid eggs you are confident are fresh cabbageworm eggs.
The 5-minute check for wasp activity walkthrough
Do this check weekly once white butterflies are flying, and again a few days after any caterpillar surge. Bring a hand lens, not a spray bottle: the point is to decide whether you need to act at all.
- Scan for butterflies first. White butterflies over the bed mean fresh eggs are being laid now. Note the plants they visit most; inspect those first.
- Flip five to ten leaves. Look underneath for singly laid pale yellow eggs and darkened eggs. Crush pale eggs; leave darkened ones — they may hold Trichogramma wasps.
- Look for caterpillars with company. Check midribs, leaf undersides, and plant centers for green caterpillars, looking for yellow cocoon clusters, a single cocoon nearby, or a swollen, sluggish body.
- Count cocoons, not just caterpillars. Any caterpillar carrying a yellow cluster means wasps are already active. Record roughly how many of the total were parasitized — for example, three of ten — so next week's check shows whether the share is rising.
- Decide, then wait before spraying. If most caterpillars are parasitized and damage is light, hold off and recheck in a few days. If most are healthy, feeding is heavy, and heads are at risk, use handpicking and the other IPM steps in our cabbage worm identification and control guide, not a broad spray that would kill the wasps you just found.
The photograph above shows the sign to learn: a caterpillar no longer a threat, plus the yellow cocoons that are the next generation of help. Note date, plant, and parasitized count to turn an impression into a trend.
How to tell wasp activity from a failed control
Gardeners often conclude biological control has failed because leaves still show holes a week after wasps first appeared. That mistakes the timeline: parasitized caterpillars keep feeding for days, Cotesia glomerata hosts feed until about the fifth instar, and pupal wasps do not reduce current feeding at all — they reduce the next butterfly generation (University of Minnesota Extension). Holes already made will not heal.
Judge wasps by the next generation, not today's holes. Working: the parasitized share rises weekly, fresh clusters appear, butterflies fall without spraying. Not enough alone: healthy caterpillars far outnumber parasitized ones, feeding reaches heads, or young plants are stripped. Then wasps are simply outnumbered — use handpicking, covers, and least-toxic options in IPM order, not a broader spray that resets the wasp population to zero.
For hole patterns when the caterpillar is hard to find, see our holes in cabbage leaves guide and green caterpillars on vegetables guide; for crop-specific behavior, see cabbage worms on kale and cabbage worms on broccoli. If the holed plant is not a brassica at all, the suspect list changes entirely — see our sunflower leaves with holes guide for a non-brassica example.
IPM step 1: prevention that protects wasps
Prevention comes first in IPM order; here it means preventing conditions that force you to spray. Put floating row covers over transplants at planting, before butterflies lay eggs, and seal the edges (Utah State University Extension). Rotate brassicas yearly and clear debris at season's end, since the pest overwinters as pupae near debris. Planting and harvest times vary by climate region, so time covers and checks by butterfly activity in your own bed and your county extension office, not by a single national date.
When you plan the next brassica round, start from seed so you control timing and put covers on from day one. For broccoli, browse our broccoli seeds collection and choose a sowing date that puts head formation ahead of the heaviest butterfly pressure in your area; for rotation beds, use our kale seeds and cauliflower seeds collections instead of reusing the same bed.
IPM step 2: cultural control and habitat
Cultural control for wasps is habitat control. Adults need nectar and pollen to fuel flight and egg-laying, and flowering landscapes free from broad-spectrum insecticides encourage their presence (University of Missouri Extension, Cornell University). Plant small flowers that bloom from early spring through fall near your brassicas — sweet alyssum, buckwheat, and marigolds are commonly recommended nectar sources — and keep an undisturbed margin at the bed edge for shelter. A patch a few feet across is better than none.
Two habits undo that habitat. Clearing every flowering margin at once removes nectar and shelter in one pass; leave part standing at any one time. Treating the whole garden, including flowering borders, with a broad-spectrum spray is worse: UC IPM warns natural enemies can be more adversely affected by pesticides than the pests, because they move widely and pick up a higher total dose. If you must treat, treat affected plants only, in the evening, and keep spray off flowering plants.
For the kit that makes weekly checks practical — hand lens, containers, row cover fabric — see our gardening tools page. For a fast succession brassica beside the main crop, our arugula seeds collection is a useful short-season slot.
IPM step 3: mechanical control first, wasps second
Mechanical control — handpicking, crushing fresh pale eggs, and a strong jet of water — buys wasps time. Pick healthy, unparasitized caterpillars off by hand and drop them into soapy water, but leave parasitized caterpillars and their cocoons in place: removing them removes the wasps. The ones you most want to keep are the ones carrying cocoons.
Do not tidy the bed by removing every pupa either. Pupae parasitized by pupal wasps look brown and lifeless, but they are why fewer butterflies emerge next generation. Leave pupae in place where they do no harm, and clear debris only at season's end.
IPM step 4: biological control — conservation vs. purchased releases
Conservation — protecting the wasps already in your garden — is the default recommendation here, and purchased releases are the exception. University of Missouri Extension states that parasitoid wasps usually are not purchased for home release; the best approach is helping the existing population thrive. Cornell University adds that Cotesia glomerata is not commercially available at all, so more of it can only come from supporting wild populations.
Trichogramma is the exception. Commercially purchased Trichogramma can be released at peak butterfly flight, and Utah State University Extension describes that timing as effective, while University of Minnesota Extension notes timely mass releases during peak flight could be effective. Cards are hung in the crop so emerging wasps can search for fresh eggs — but the limits are real, and supplier optimism should not be taken at face value.
| Approach | How it works | Best when | Limits and risks |
|---|---|---|---|
| Conservation of wild wasps | Nectar flowers, undisturbed margins, no broad sprays; wild wasps build up over the season | Wasps present, damage light to moderate, and you can wait weeks | Slow early; parasitism is low in the first generation, rising later (Cornell University); nothing for heads already infested |
| Purchased Trichogramma releases | Cards hung in the crop release egg wasps that parasitize fresh eggs | Peak butterfly flight, fresh eggs laid daily, releases repeatable through the flight period | Useless on hatched caterpillars; timing must match egg-laying; results vary with weather and release quality; no result is assured |
Verdict: choose conservation if you already see cocoon clusters or darkened eggs, if damage is still light, or if you want season-long help with no repeat cost — plant the nectar border, stop broad sprays, and let the wasps build. Choose purchased Trichogramma releases if you are at peak butterfly flight, see fresh pale eggs daily, and can hang cards on time and repeat them — while still handpicking and covering, because released wasps prevent future caterpillars, they do not rescue heads already full of larvae. Judge a release by whether fewer new caterpillars appear in two weeks, not by whether today's holes disappear.
Timing decides whether a release helps at all. Trichogramma wasps attack eggs, so cards hung after most eggs have hatched arrive too late for that generation. Hang cards while egg-laying is active, and repeat through the flight period, since each card covers a limited window. Do not spray around a release: the spray that kills caterpillars kills the wasps you just paid for. No release, wild or purchased, is assured to control cabbageworms on its own, and any claim that it will should be treated with skepticism.
IPM step 5: least-toxic options last
Only after prevention, cultural, mechanical, and biological steps should other treatments enter the picture, least-toxic first. Bacillus thuringiensis var. kurstaki (Bt) must be eaten by actively feeding larvae to work, and spinosad, the active ingredient, is listed by Utah State University Extension and University of Minnesota Extension as effective on young caterpillars. Apply only where young larvae are present, cover leaf undersides, apply in the evening, and follow label directions exactly. No treatment works every time.
The wasp-specific rule here is simple: if a plant holds cocoon clusters or darkened eggs, skip it when you treat, or delay and recheck in a few days. Spraying it trades a free, self-renewing control for a one-time kill. Broad-spectrum insecticides are the worst trade: UC IPM and Cornell University warn they harm natural enemies and pollinators, sometimes more than the pests. If treatment is necessary, choose the most selective option, spot-treat only the plants that need it, and keep it off flowering borders.
What wasps cannot do alone
Wasps cannot do four things. First, they cannot save a head already infested: parasitized caterpillars keep feeding, and caterpillars inside florets are sheltered from wasps and sprays, so the floret check in our cabbage worms on broccoli guide still matters. Second, they cannot act fast in early season, when parasitism is low; covers and handpicking carry the early season. Third, they cannot overcome a bed with no nectar, no shelter, and regular broad sprays — habitat is a precondition, not a bonus. Fourth, they cannot stop eggs arriving from a neighbor's unmanaged brassica patch across the fence.
None of that makes wasps minor. Over a full season, a bed with nectar borders, undisturbed margins, and no broad sprays can cut each successive worm generation — not a worm-free garden, but fewer worms later, less pressure on heads, and fewer sprays, with handpicking and covers doing the urgent work wasps do slowly.
Frequently asked questions
Will parasitic wasps sting me or my children?
No. These wasps are tiny — Cotesia adults are about 3 to 7 mm long — and Cornell University states parasitoid wasps are not a sting risk to humans. They seek caterpillars and nectar, not people.
Should I remove a caterpillar covered in yellow cocoons?
No, leave it in place. Those yellow cocoons hold the next generation of Cotesia wasps, and that caterpillar has already stopped feeding or is about to. Removing it destroys the wasps and reduces nothing.
How do I tell wasp cocoons from cabbageworm eggs?
Count and context. Cabbageworm eggs are laid singly on leaf undersides; wasp cocoons appear as 20 to 30 small yellow cocoons on or beside a dead caterpillar. A cluster on a caterpillar is wasp work; a single pale egg on bare leaf is a pest egg. When in doubt, leave it and recheck in a few days.
Why are there still holes if wasps are working?
Because parasitized caterpillars keep feeding for days, and holes do not heal. Judge wasps by the next generation: a rising share of parasitized caterpillars, fresh cocoon clusters, and fewer butterflies in following weeks mean they are working, even if today's leaves still show damage.
Are purchased Trichogramma cards worth buying?
They can help, but only in the right window: they attack eggs, so they must be hung while butterflies lay fresh eggs, with repeats through the flight period. They do nothing against hatched caterpillars, and no result is assured. If you already see cocoon clusters, conservation alone may serve you better.
Do I need to buy Cotesia wasps?
Usually you cannot, and you do not need to. Cornell University reports Cotesia glomerata is not commercially available, so more of it comes only from supporting wild populations with nectar flowers, undisturbed margins, and no broad sprays. Yellow cocoon clusters mean Cotesia wasps are already there.
What flowers should I plant for parasitic wasps?
Small-flowered plants that bloom across the season — sweet alyssum, buckwheat, marigolds — are commonly recommended nectar sources. Plant them near the bed, keep part of the margin standing at all times, and do not spray the flowers.
Sources
- Cornell University — Imported cabbageworm parasitoid (Cotesia glomerata): cals.cornell.edu/.../imported-cabbageworm-parasitoid-cotesia-glomerata
- Utah State University Extension — Imported cabbageworm: extension.usu.edu/.../imported-cabbageworm
- University of Minnesota Extension (VegEdge) — Imported cabbageworm: vegedge.umn.edu/.../imported-cabbageworm
- Colorado State University — Imported cabbageworm: agsci.colostate.edu/.../imported-cabbageworm
- University of Massachusetts Extension — Imported cabbageworm: umass.edu/.../imported-cabbageworm
- Kansas State University Extension — Imported cabbageworm: bookstore.ksre.ksu.edu/.../mf3727-imported-cabbageworm
- University of Missouri Extension — Parasitic wasps offer precision pest control: extension.missouri.edu/.../parasitic-wasps-offer-precision-pest-control
- UC IPM — Protecting natural enemies and pollinators: ipm.ucanr.edu/.../protecting-natural-enemies-and-pollinators
- University of Hawaii — Trichogramma wasps (egg parasitoids): ctahr.hawaii.edu — Trichogramma Wasps (PDF)
- McGill University — Practical approaches to the biological control of crucifer pests: eap.mcgill.ca/CPCP_1.htm
- Koppert — Maximizing the effectiveness of parasitic wasps: koppert.com — maximizing the effectiveness of parasitic wasps
Ready for a wasp-friendly season? Cover the next planting from day one, plant a nectar border, and check weekly for yellow cocoons before any spray: start from our broccoli seeds collection, with kale seeds, cauliflower seeds, and arugula seeds for rotation beds.
