Cabbage worms: imported cabbageworm, looper and diamondback moth

Identification

Their damage looks similar (Utah State), so check the caterpillar itself. When disturbed, the cabbageworm usually stays still, the diamondback larva wiggles, and the looper inches away or whips its head around (Wisconsin).

When they arrive

Development is quick. Wisconsin says cabbageworm eggs hatch in about 5 days, larvae feed for 11 to 20 days and pupae take 6 to 11 days. UNH gives looper larvae 2 to 4 weeks, with adults emerging about 2 weeks after pupation.

How much damage matters

• Young plants. Seedlings and transplants are the most vulnerable; severe defoliation can distort or kill them (UMN).
• Older plants. They tolerate some feeding, but UMN says defoliation should not exceed 30% of leaves. Heavy feeding can prevent or stunt heads in cabbage, cauliflower and broccoli (UMN, Wisconsin).
• Heads. Larvae that tunnel into heads are hard to reach, and their bodies and frass lower quality (USU).
• Crop choice. UNH says cabbage, collards, Brussels sprouts and broccoli are more susceptible to cabbageworm than kale, turnip and Chinese cabbage.

Scouting and thresholds

Check both sides of the leaves at least weekly after planting, and more often as the season goes on (UMN). Look under leaves, especially inner leaves (USU), and check lower leaves for eggs even when no feeding shows (Wisconsin). Wisconsin advises judging infestations by larval counts rather than by damage. Published thresholds are for farm fields, mostly given as the share of plants infested:

UMN's home-garden advice is the weekly check; its thresholds are listed for farms. A few plants in a bed or pot can be checked leaf by leaf.

Row covers and netting

• Cover at planting. Lightweight floating row cover laid over plants at seeding or transplanting keeps the adults from laying eggs; remove it after harvest (UMN). Utah State puts it on hoops if needed and buries the edges along the row.
• Mind the limits. UNH says to place covers right after planting, bury the edges and watch humidity and temperature underneath, but advises against covers on summer brassicas and on unrotated ground.
• Containers. Maryland says containers are easier to shield with insect netting, so container plants usually have fewer pest problems than in-ground food gardens. Pot sizes for cabbage and broccoli are on the container sizes page.
• Clean transplants. Inspect transplants and destroy any larvae (UNH fact sheet). UNH prefers locally grown transplants, to avoid bringing in resistant diamondback moths or loopers from southern states.

Hand-picking and cleanup

1. Look along leaf midribs, where cabbageworms hide, and under leaves, where caterpillars often sit (UNH).
2. Pick caterpillars into a pail of water with a few drops of detergent (MSU) or soapy water (UMN). MSU says hand-picking works at any caterpillar size.
3. Destroy crop residue right away and remove mustard-family weeds such as wild mustard, peppergrass and shepherd's purse (UMN).

Bt and other sprays

Bacillus thuringiensis (Bt) is a soil bacterium whose proteins are toxic to some insects when eaten. NPIC stresses "it has to be eaten to work" and says people, like all mammals, cannot activate the proteins.

• Small caterpillars. Treat while larvae are young; products work less well on large ones (UMN, UNH, USU, Wisconsin).
• Coverage. Caterpillars must eat it, so leaf coverage matters (UMN); they often sit on leaf undersides (UNH).
• Strains. Utah State lists Bt var. kurstaki; UNH describes aizawai as newer and possibly more effective on diamondback moth. NPIC notes the aizawai strain can be toxic to honeybees.
• Selectivity. Bt has a short residual and is non-toxic to mammals and to beneficial parasitoids and predators (USU). UMN recommends it when diamondback moths dominate or other caterpillars are found young.
• Other options. UMN names spinosad, neem and pyrethrins (which must hit the caterpillar) as lower-risk; UC IPM calls Bt and spinosad very effective against cabbageworm. Broad-spectrum pyrethroids last longer but can kill natural enemies (UMN).
• Resistance. Diamondback moth is highly resistant to insecticides (UNH), and resistant larvae can arrive on transplants (Wisconsin). Rotate modes of action (UNH, USU).

This page gives no rates. Follow the product label, confirm the crop is listed and observe the pre-harvest interval (UMN), and ask your county extension office for current home-garden products.

Natural enemies

Parasitic wasps and tachinid flies lay eggs in the caterpillars, and lacewings, ground beetles and lady beetles eat eggs or larvae (MSU). UNH recommends flowering borders such as buckwheat, phacelia, cilantro, fennel or sweet alyssum, and avoiding broad-spectrum insecticides early in the season.

Trap crops

The WSU column says to plant trap crops before the main crop flowers, re-sow every 2–3 weeks, and check them often and remove the insects, or "the insect pests will continue to reproduce and multiply". Strong fliers can bypass a border. UVM calls perimeter trap cropping promising but says it may not give perfect control under extreme pest pressure. None of these sources tested a small bed or container. Crop choice also matters: loopers lay fewer eggs on Chinese cabbage, mustard, rutabaga and turnip than on cabbage, Brussels sprouts or collards (USU). The companion planting chart rates other pairings.

Where sources differ

For kale timing and its other pests, see the kale page.

Cabbage worm checklist

• Transplants inspected; larvae destroyed.
• Row cover or insect netting on at planting, edges buried or secured.
• Both leaf sides checked weekly; lower leaves checked for eggs.
• Caterpillars picked into soapy water.
• Mustard-family weeds and crop residue removed.
• If spraying: Bt or spinosad on small caterpillars, full leaf coverage, crop on the label.