Controlling Caterpillars on Trees and Shrubs

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Yellow neck caterpillars often feed in large groups.

In addition to peak hurricane season, September is also peak caterpillar season. While things have been very quiet on the hurricane front this year, the opposite is true for caterpillars. Many different types of caterpillars can currently be found munching on tree and shrub leaves in our area. In most cases, caterpillars feeding causes no lasting damage to plants and these insects can be left alone, but there are some situations where control is needed. If you have a caterpillar outbreak that requires treatment, several organic insecticides are available that can do the job.

Understanding Caterpillars

Caterpillars, also referred to as worms, are the immature life stage of butterflies and moths. Many gardeners are reluctant to leave caterpillars untreated, perhaps haunted by memories of Eric Carle’s ‘The Very Hungry Caterpillar’, and fearful that if left alone caterpillars will soon devour everything in sight. This is highly unlikely since most caterpillars are extremely picky eaters and are only interested in feeding on certain species of plants. In addition, most caterpillars will only feed for two to three weeks before pupating (turning into a chrysalis or cocoon).

Caterpillars are easy to detect by the ragged holes their feeding activity leaves in plant foliage. For healthy, established perennials, trees, and shrubs losing a portion of their leaves is not a problem, especially this late in the growing season. In addition, caterpillars are an excellent food source for migrating birds, which can help provide control naturally. Spiders, parasitic wasps, and other natural predators also help keep caterpillar levels from getting out of control.

Control Options

Sometimes caterpillars come in masses, and can strip the foliage from a plant in a few days. This type of feeding damage is more serious and may warrant control, especially on small or recently planted plants. If large numbers of caterpillars are rapidly stripping foliage from a tree or shrub that is small enough for you to spray, it is usually best to treat. This is particularly true for young plants that have been transplanted in the last year or two and may not yet be fully established.

Controlling caterpillars can be as simple as picking them off the host plant and dropping them into a bucket of soapy water or squishing them, but most gardeners would rather not take such a hands-on approach. Many pesticides will kill caterpillars, including several organic products. Advantages of using organic pesticides include that they are less harmful to beneficial insects and break down quickly so do not leave behind residues.

Eastern Tiger Swallowtail Butterfly

Caterpillars are the immature stage of butterflies and moths.

Organic insecticides effective for caterpillar control include those containing B.t. and spinosad, both derived from naturally occurring bacteria. B.t., which is short for Bacillus thuringiensis, only harms caterpillars and is most effective when applied in late afternoon, since it degrades in sunlight. Younger, smaller caterpillars are more susceptible to B.t., so be sure to apply it when you first notice damage. Examples of products containing B.t. include Dipel and Wormwhipper. Spinosad is a relatively new organic insecticide made from a rare type of filamentous bacteria. It persists for three to five days and can be used to control caterpillars and Colorado potato beetles. Brand names include Captain Jack’s Dead Bug Brew and Monterey Garden Insect Killer.

Other organic pesticides are derived from plants. Pyrethrin is extracted from the flowers of certain species of chrysanthemum, though not the type we commonly grow for their fall flowers. Pyrethrin will help control a wide range of insect pests, including caterpillars. Neem oil is derived from the seeds of the neem tree, a native of southern Asia. It is effective for treating several common pest insects including caterpillars, aphids, true bugs, and some beetles.

Many of the synthetic pesticides available today are manmade versions of pyrethrin. Known as synthetic pyrethroids, this group of insecticides includes those with the active ingredients bifenthrin, permethrin, cyfluthrin, and cyhalothrin. Older insecticides that control caterpillars include Sevin and malathion. To determine the active ingredient in any pesticide product check the label, which will also tell you how it should be mixed and applied, as well as any hazards the product poses to other non target organisms (pets, wildlife, aquatic life forms).

When using any pesticide be sure to read and follow all label directions. If bees are actively working flowering plants in the area wait until late evening to spray, after bees have returned to the hive, and never spray open blossoms.

Please Note: Any recommendations of brand names or listing of commercial products in this article are included solely as a convenience to the reader and do not imply endorsement by N.C. Cooperative Extension nor discrimination against similar products. Before applying any chemical, always obtain current information about its use and read the product label carefully.

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