Will Bt hurt butterflies I want to keep, like monarchs or swallowtails?

Bt is caterpillar-specific but not species-specific, so it will affect any caterpillar that eats a treated leaf, including ones you'd rather keep. The fix is targeting: only spray the specific plants with the pest caterpillar you've identified, and avoid treating host plants like milkweed or parsley where you know desirable butterfly larvae are feeding. Identification before treatment is what protects the caterpillars you want while still controlling the ones causing real damage.

How often do I need to reapply Bt once I start spraying?

Bt breaks down quickly in sunlight and has only about a 24 to 48 hour window of effectiveness, so a single application rarely finishes the job. Plan on reapplying every few days during active feeding, and always reapply after rain washes residue off the leaves. Because it only works when eaten, thorough coverage of the leaves the caterpillars are actually feeding on matters more than spraying the whole plant evenly.

Is it too late to use row covers if I already see caterpillar damage?

Largely, yes — row covers work by physically blocking adult moths and butterflies from laying eggs on the plant in the first place, so once caterpillars are already feeding, the eggs have already hatched and a cover traps the problem inside rather than keeping it out. At that point handpicking or Bt on the current generation is the right move; save row covers for the next planting or the next generation of pests, put on before adults are active.

What if handpicking and Bt aren't keeping up with the damage?

First confirm you're targeting the right stage — Bt works best on young, actively feeding larvae and is less effective on larger, later-instar caterpillars — and make sure you're reapplying often enough given its short residual. If damage is still severe on a high-value planting after a consistent staged effort, that's the point to consider spinosad as a step up, or bring in an ISA-certified arborist for a valuable tree rather than escalating straight to a broad-spectrum insecticide that also kills your free pest control.

Daniel Brady · NH-Licensed Pesticide Applicator · License #782664

How Can I Stop Caterpillars From Eating Leaves?

Needs a plan

Caterpillars are controllable with a staged approach — identify first, then escalate from handpicking to row covers to Bt — and you rarely need a broad-spectrum spray.

At a glance

The verdict: Needs a plan
Who fixes it: You
Category: Leaf-Pest ID & Environmental Factors

The short answer

Yes, you can stop caterpillars, and the honest order of operations matters: identify first, since some caterpillars are future butterflies worth tolerating, then escalate from least-invasive to most. Handpicking, floating row covers, and the biological spray Bt (Bacillus thuringiensis) handle the large majority of leaf-chewing caterpillars without harming pollinators. Broad-spectrum insecticides are usually unnecessary and can backfire by killing the parasitic wasps and predators that keep caterpillar populations in check on their own.

The honest router

Who actually fixes this?

YouGarden centerUNH ExtensionCertified arboristPest-control pro

Start here

You

Handpicking, row covers, and Bt application are standard homeowner garden tasks that don't require professional pest control.

Garden center — You need to buy Bt, spinosad, or row-cover fabric, or want product-specific application advice.
UNH Extension — You want the caterpillar identified before deciding whether to treat it or leave it for the butterflies.
Certified arborist — The infestation is on a large, high-value tree rather than a garden bed or shrub you can reach yourself.

Straight talk from Anchor

Caterpillars chewing garden and landscape leaves are a plant-care issue, not something we treat, and we'd tell you that on the phone. Anchor works on the structural and yard pests around your home — rodents, ants, ticks, moisture pests — not caterpillars on your plants.

The full answer

The details, straight

Step 1: Identify before you act

Not every caterpillar is a pest worth killing. NC State Extension makes the point directly: caterpillars "identified as the larvae of Eastern tiger swallowtail butterflies" may be worth tolerating rather than removing, since a little leaf loss on the right host plant is the cost of having butterflies in the yard. Match the caterpillar to the plant it's on and take a photo if you're unsure — UNH Extension's Insect ID service can confirm it — before you decide handpicking, exclusion, or Bt is warranted.

Step 2: Scout and handpick

For most home gardens, checking leaf undersides twice a week during warm months and dropping any caterpillars you find into soapy water is the least-invasive control and University of Minnesota Extension lists it as a primary method. It's slow but has zero drift risk to pollinators or beneficial insects, and it doubles as your early-warning system for catching a bigger problem before it grows.

Step 3: Exclude with floating row covers

Lightweight fabric laid over vegetables and susceptible plants physically blocks adult moths and butterflies from laying eggs in the first place. UConn IPM guidance notes the edges must be buried or weighted down to keep adults from finding a way underneath, and covers need to come off once plants flower so pollinators can reach them — an exclusion method that only works before eggs are already on the plant, not after.

Step 4: Use Bt for active feeding

Bacillus thuringiensis is a naturally occurring soil bacterium that caterpillars must eat directly off a treated leaf to be affected. Per UC Master Gardener guidance, it's "safe for people, pets and beneficial insects and bees," and it works best on young, actively feeding caterpillars. It has only a 24–48 hour residual, so thorough leaf coverage and reapplication after rain are what make it effective — it's not a one-and-done spray.

Why broad-spectrum and systemic sprays are usually the wrong call

Broad-spectrum and systemic insecticides kill the parasitic wasps and predatory insects already doing free control work in your yard, and neonicotinoid systemics in particular are toxic to bees and parasitic wasps, especially when applied to flowering plants. For the ordinary caterpillar pressure most home gardens see, that trade-off isn't worth it — reserve stronger products for a confirmed, severe infestation that handpicking, covers, and Bt haven't controlled, and treat spinosad (an organic option that's also toxic to bees when wet, so apply in the evening) as a step up from Bt rather than a first resort.

The gist

Key takeaways

Identify the caterpillar first — some are future butterflies worth tolerating on the right host plant.
Handpicking into soapy water is the lowest-risk control and doubles as scouting.
Floating row covers exclude egg-laying adults but must go on before eggs are present, and come off at flowering.
Bt (Bacillus thuringiensis) is caterpillar-specific and safe for pollinators, but must be eaten off the leaf and reapplied — it has only a 24–48 hour residual.
Broad-spectrum and systemic insecticides usually aren't warranted; they kill the natural enemies keeping caterpillars in check.

Not sure which caterpillar you have?

Some caterpillars are future butterflies worth keeping. Identify before you treat, then use the staged plan that protects pollinators.

See the staged caterpillar plan

Local to New Hampshire

Caterpillar timing in New Hampshire

New Hampshire context

UNH Cooperative Extension tracks several leaf-feeding caterpillars across New Hampshire, and outbreak years for pests like forest tent caterpillar and spongy moth can strip foliage in early summer before the insects mature. Because timing drives what management works, UNH Extension's Insect & Arthropod Identification service is worth using to confirm which caterpillar you're dealing with. Anchor Pest Services handles the structural and yard pests around your home, not caterpillars on garden or landscape plants — that stays with a garden center or Extension.

NH-licensed applicator #782664 (NHDAMF, RSA 430) · NEPMA member · family-owned since 2017.

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Caterpillars Are Garden Work — But Your Home Is Our Job

Stopping caterpillars from eating your plants is garden care, and we'll always say so plainly. Where Anchor Pest Services genuinely helps is the pests around your home — rodents, ants, ticks, and moisture pests drawn to yard debris and foundation clutter. Family-owned since 2017, NH license #782664, serving Manchester, Nashua, Concord and nearby towns.

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Honest routing — caterpillars on plants are garden care, and we say so
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Keep Exploring

How Can I Stop Caterpillars From Eating Leaves?

Who actually fixes this?

The details, straight

Step 1: Identify before you act

Step 2: Scout and handpick

Step 3: Exclude with floating row covers

Step 4: Use Bt for active feeding

Why broad-spectrum and systemic sprays are usually the wrong call

Key takeaways

Not sure which caterpillar you have?

Caterpillar timing in New Hampshire

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Caterpillars Are Garden Work — But Your Home Is Our Job

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