Tent Caterpillars & Web Worms

A large web covering tree branches, covered in caterpillars, photo by Matthew Fells

Here are simple, least-toxic methods to manage tent caterpillars and web worms.

Manual and Mechanical Controls

One of the best methods to control an infestation is to collect and destroy egg masses and larvae in branches and trunk.1 The larvae can be killed by probing the hole with a long, flexible wire.2 You can also scrape the nest onto the ground or into soapy water (make sure to wear gloves!).3 The best time to remove and destroy larvae and their nests is evening and early morning because the caterpillars will gather into their nests at night.4

Biological Controls

Another non-chemical method is the use of nematodes, which are insect-eating worms. “Create a mixture of nematodes and water and place directly in the hole. The most effective method is soaking cotton strips in the solution and using tweezers to place as deeply in the hole as possible.”2

Bt Insecticide

Bacillus thuringiensis (Bt) is often recommended for controlling caterpillar infestations and other insects. Bt is a biological pesticide that is naturally occurring at low levels in soil. A variety of Bt, Bacillus thuringiensis var. kurstaki (Btk), has been found to survive in the upper respiratory tracts of humans; however, there is no evidence of Btk-related illnesses. Therefore, NCAP maintains the precautionary principle that there is not sufficient evidence of Bt and Btk’s safety to humans to merit exposing human populations to it.5 

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  1. U.S. Forest Service, Rocky Mountain Region. “Forest Tent Caterpillar.” 2011. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5343832.pdf
  2. Beyond Pesticides. “Least-toxic Control of Tree-Boring Caterpillars.” https://www.beyondpesticides.org/assets/media/documents/alternatives/factsheets/TREE%20BORING%20CATERPILLAR%20CONTROL.pdf
  3. Shetlar, David J.  “Eastern and Forest Tent Caterpillars and Their Control.”  Ohio State University Extension Fact Sheet.
  4. WSU Extension. “Biology and Control of Tent Caterpillar.” Gardening in Western Washington.
  5. Northwest Center for Alternatives to Pesticides.  “Bacillus Thuringiensis (B.t.)” Journal of Pesticide Reform Vol. 14, No. 3: 1994. https://assets.nationbuilder.com/ncap/pages/26/attachments/original/1428423315/bacillus.pdf?1428423315

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  • Michael Curtice
    commented 2016-05-01 20:32:43 -0700
    If Bt doesn’t have sufficient data indicating it’s ability to be used safely around humans, then why are they spraying the Chemical Industries unproven products containing Bt whenever they deem. Of course they DO know that Bt kills caterpillars (I.e. Monarch, Swallowtail…future butterflies) and earthworms and continue their assault to control insect infestation and the general publics wallets. A sad thing to admit to , but a lot of the insect issues today are man-made due to unsustainable practices and inflated lifestyles. Nonetheless, how can you expose people to something that you can omly assume is safe and do it in a ‘matter of fact’ fashion. When will this government say NO to the lobbyists running amok around Capitol Hill and looting the American public of their health, rights and tax dollars. It’s absolutely criminal how they carry on in politics and business in this country with little or no regard to very people who voted for them or bought their slick misleading advertised products. You can’t eat money and you need the ecosystem that you are destroying with chemicals/organisms to grow food. Harmony is achieved through balance, not greed.