Cabbage Looper & Imported Cabbageworm

🐛 Cabbage Looper & Cabbageworm

Trichoplusia ni / Pieris rapae · Lepidoptera: Noctuidae/Pieridae

Two caterpillars attack cabbage, broccoli, kale, and other brassicas simultaneously — and they look quite different. Both are eliminated with the same treatment at the same time.

CaterpillarVegetable PestBrassicaLepidopteraBt TreatmentLooper

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PestControlBasics Editorial Team

Reviewed by Derek Giordano · Updated 2026

Illustrated identification guide — PestControlBasics.com

🔍 Identification

Cabbage Looper (Trichoplusia ni): 35mm; pale green; moves with distinctive looping motion (arches middle of body upward with each step). Makes irregular holes throughout the leaf. Adult is a mottled grey-brown moth.

Imported Cabbageworm (Pieris rapae): 35mm; velvety pale green with faint yellow stripe; moves smoothly. Chews through the center of leaves. Adult is the common white cabbage butterfly with black wingtips.

🧬 Biology & Behavior

Both species attack the same brassica crops: cabbage, broccoli, Brussels sprouts, kale, kohlrabi, and cauliflower. Both produce green frass that's visible on leaves. Both are present simultaneously on the same plants in most areas.

⚠️ Damage & Health Risk

Irregular holes throughout leaves (looper) and deep holes in leaf heads (cabbageworm); contamination of heads with frass; severe infestations hollow out cabbage heads and broccoli florets making them inedible.

🔧 DIY Treatment

Apply Bt kurstaki (Dipel, Thuricide) when caterpillars are small — highly effective against both species with zero risk to other wildlife. Reapply after rain or every 5-7 days. Row covers prevent adult butterflies/moths from laying eggs. Hand-pick when populations are low.

👷 When to Call a Pro

Rarely warranted — Bt provides excellent control in home and commercial gardens.

❓ FAQ

What is the white butterfly on my brassicas?

That's the adult imported cabbageworm — the adult stage is the common white cabbage butterfly (Pieris rapae). Seeing these butterflies near your brassicas means they're laying eggs. Check leaf undersides for tiny bullet-shaped yellow eggs. Start Bt treatment as soon as caterpillars hatch.

Does Bt harm monarch butterflies?

Bt kurstaki can harm any caterpillar it contacts — including monarch larvae. Apply precisely to infested crops and avoid spraying milkweed or nearby wildflowers. Bt breaks down in UV within days, so precise timing and application minimizes off-target impact.

Factor	Details
U.S. Range	All or most U.S. states
Regional Detail	Distribution varies — consult your local extension service for regional prevalence data.

Factor

Details

U.S. Range

All or most U.S. states

Regional Detail

Distribution varies — consult your local extension service for regional prevalence data.

📅 Treatment Timing Guide

Treating at the right time dramatically improves results. Pest control timed to the life cycle uses less product and achieves better long-term control.

Period	Action
Spring	Inspection and perimeter treatment before pest season starts.
Summer	Active monitoring and targeted treatments as needed.
Fall	Preventive treatment before overwintering pests seek entry.

Period

Action

Spring

Inspection and perimeter treatment before pest season starts.

Summer

Active monitoring and targeted treatments as needed.

Fall

Preventive treatment before overwintering pests seek entry.

💰 Professional Treatment Costs

Service Type	DIY Cost	Professional Cost
Initial inspection	Free (self-inspect)	$75–$150 (often credited to treatment)
One-time treatment	$30–$100 in materials	$150–$500
Annual service contract	N/A	$400–$900/year
Severe infestation	Often ineffective alone	$500–$2,500+

Service Type

DIY Cost

Professional Cost

Initial inspection

Free (self-inspect)

$75–$150 (often credited to treatment)

One-time treatment

$30–$100 in materials

$150–$500

Annual service contract

N/A

$400–$900/year

Severe infestation

Often ineffective alone

$500–$2,500+

Prices vary by region, property size, and infestation severity.

❓ Common Questions About 🐛 Cabbage Looper & Cabbageworm

How do I confirm I actually have this pest (not something similar)?

The most reliable confirmation is a physical specimen — capture one and compare to reference images on this page. For cryptic pests (bed bugs, termites), look for secondary signs: frass, shed skins, mud tubes, or bites with a specific pattern. When uncertain, a professional inspection is faster than months of misidentification.

Can I treat this myself or do I need a professional?

DIY is effective for small, accessible infestations caught early. Professionals are worth the cost when: the infestation is inside wall voids or structural elements, multiple rooms are affected, you have health-risk pests (hantavirus, venomous species), or DIY has already failed twice.

How long until the infestation is completely gone?

Expect 3–8 weeks for most infestations with proper treatment. Insects with dormant life stages (pupae, eggs) extend the timeline because those stages are impervious to most insecticides. Follow-up treatments at 2 and 4 weeks catch each new cohort as they emerge.

What's the most common mistake people make treating this pest?

Treating only the visible pest population while ignoring the harborage site, entry point, or breeding location. Killing adults provides temporary relief but the population rebuilds from hidden egg cases, pupae, or new arrivals through unaddressed entry points.