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Q: Does damage from caterpillar feeding have the potential to impact tree health and cause long term damage to trees? What options are available for managing caterpillars?

Q&A: Expert insight regarding your tree health care questions

Q: Does damage from caterpillar feeding have the potential to impact tree health and cause long term damage to trees? What options are available for managing caterpillars?

— David Taylor, Sunrise Tree Care, Hawthorn Woods, Illinois


 A: Numerous caterpillar species feed on the leaves of shade trees and ornamental landscape plants. Caterpillars are in the insect order Lepidoptera and are “wormlike” larvae that hatch from eggs that are laid by moths or butterflies. Caterpillar larvae feed on the leaves or needles of trees and can cause premature defoliation. Throughout the United States, arborists are faced with managing many different caterpillar species, some of the key species of caterpillars and their common hosts are listed in Table One.


Table One: Common caterpillars and key tree hosts that they attack.

Caterpillar Species


Gypsy moth

Numerous hosts; common on oaks, birch, crabapple, sweetgum, hawthorn and can be found on spruce, pine, hemlock


Numerous hosts; common on arborvitae, honeylocust, juniper, bald cypress

Eastern tent caterpillar

Wild cherry, crabapple, plum, peach and cherry

Forest tent caterpillar

Aspen, poplar, linden, oaks, ashes, birches, alder and fruit trees

Fall webworm

Pecan, walnut, American elm, hickory, fruit trees, some maples, persimmon and sweetgum, alder, willow and cottonwood

Mimosa webworm


Fall and Spring cankerworm

Elm, hackberry, honeylocust, apple, ash, beech, birch, boxelder, hickory, linden, maple and oak

Pine tip moth

Numerous pine species




Although a low level of caterpillar feeding poses minimal threat to the long-term health of a tree, feeding by some caterpillars, such as gypsy moth, tent caterpillars and oakworms can completely defoliate entire trees under heavy pressure. Healthy and vigorously growing deciduous trees that are fed upon are rarely killed from a single generation of feeding. Conversely, a single defoliation event on conifers can cause significant decline and tree death. Conifers are not capable of quickly replacing their canopies with new growth, whereas deciduous trees may grow a new flush of leaves within the same growing season if the damage occurred early in spring/early summer. Nevertheless, premature defoliation does have its costs to the tree as the amount of energy that a tree produces throughout the growing season is reduced and new flushes of growth require the tree to use up significant energy reserves. Repeated damage in consecutive years from caterpillar feeding can impact tree health and cause dieback and decline, especially if combined with drought or other abiotic stress events. Furthermore, trees being fed on by caterpillars are more susceptible to attack by other insect and disease problems that further reduce tree health.

In addition to their impact on tree health, many caterpillars can be a nuisance for homeowners. The larvae can be unsightly as they crawl and adhere to sidewalks, buildings, driveways and hardscapes. Many caterpillars will cluster in large masses, some in their protective cocoons on the sides of trees, buildings and other hard surfaces. As caterpillars feed and enlarge, they can drop noticeable amounts of excrement which can stain surfaces. Moreover, some people and animals are allergic to caterpillar hairs and body parts.

Successful management depends on your knowledge of the pest’s biology and your ability to properly diagnosis and select an effective product(s). The majority of the feeding damage done by caterpillars occurs during the later instar stages of larval development (just prior to becoming pupae) when larvae are at their largest size. It is more difficult to control larger larvae, because as they grow and develop they must consume a larger amount of insecticide to receive a lethal dose. Therefore, in general, caterpillar treatments should target small, young, newly hatched larvae (early instar larvae) before the majority of feeding damage has occurred. Application timing of treatments is critical, especially when using products with short residual. Given the warmer spring temperatures that have been occurring in parts of the country this year, you may have noted caterpillar activity much earlier than you are accustomed. This uncharacteristic weather stresses the importance of using Growing Degree Days (GDD) or phenological plant indicators rather than calendar dates to determine application timing. For example, spray applications to control young newly hatched bagworm larvae should be timed to coincide with full bloom of Catalpa or Japanese tree lilac. Monitoring for the initial hatch of young larvae on susceptible host species is another excellent way to determine treatment timing. Many university extension offices track GDD throughout the growing season and publish weekly phenological reports with common plant species in your area.

Numerous treatment options can be effective on caterpillars when applied at the correct time. [bold>Table Two<bold] lists common active ingredients used by arborists to manage caterpillars, their respective application technique and pros and cons. Traditional broad-spectrum foliar spray insecticides containing permethrin, bifenthrin, acephate or carbaryl are extremely effective when applied to young larvae. More recently, foliar spray products such as Conserve (spinosad), Acelepryn (chlorantraniliprole), and products with the active ingredient Bacillus thuringiensis (commonly known as “Bt”) have been developed for use on caterpillars. These products have greater selectivity and have less impact on non-target organisms, including beneficial insects. Horticultural oil and insecticidal soap can also be applied to manage caterpillars. Most caterpillar sprays require at least one application to young larvae and some may require multiple applications.

Systemic products can also be used for arborists who do not want to spray or for situations where spraying is not feasible. Products containing the active ingredient acephate can be directly injected into the root flares as a tree injection treatment or applied to the soil as a soil injection treatment at the base of the tree. Systemic neonicotinoid insecticides such as imidacloprid or dinotefuran have not proven to be consistently effective on most caterpillar species and are not currently recommended.

Promoting tree health and vigor with the use of cultural practices such as proper irrigation, mulching, pruning and prescription-based fertilization practices may also aid in reducing the impact of foliage-feeding caterpillars on long-term tree health.


Table Two: Products used by professional arborists to manage caterpillars

Active Ingredient

Trade Names

Application Technique




Orthene, Acephate Pro

Foliar Spray

Trans-laminar movement on leaf (moves from one side of leaf to other)

Odor can be objectionable


Dendrex, Lepitect Infusible,

Tree Injection

Systemic; can be used on trees where spraying is not feasible or access to soil is not available

Application speed is dependent on uptake by tree



Soil Injection

Systemic; soil injection treatments are operational fast and predictable. Can be done on windy days

Odor can be objectionable

Bacillus thuringiensis


Aerial Spray, Foliar Spray

Works well in large-scale aerial spray programs to suppress a population of caterpillars

Not as effective as other options for managing caterpillars on individual high value trees


Talstar, Up-Star Gold, Onyx, Up Star TT & O

Foliar Spray

Effective on a variety of leaf-feeding pests (not only caterpillars)

More impact on beneficial insects


Sevin, Carbaryl 4L

Foliar Spray

Effective on a variety of leaf-feeding pests (not only caterpillars)

More impact on beneficial insects



Foliar Spray

Low impact on non-target organisms. Also effective as bark spray on clear winged borers

Full spectrum of pest control unknown

Horticultural oil

numerous trade names

Foliar Spray (Dormant & Growing Season)

Can be used as a dormant spray for life stages that overwinter on the tree

May cause phytotoxicity on some plant species if label directions are not followed

Insecticidal soap


Foliar Spray

100% biodegradable, as well as environmentally safe.

No residual, requires 2-3 applications, eye irritant


Talstar, Tengard, Astro

Foliar Spray

Effective on a variety of leaf-feeding pests (not only caterpillars)

More impact on beneficial insects



Foliar Spray

Derived from a fermentation process, less impactful on beneficial insects

Short residual



Foliar Spray

Insect growth regulator that is targeted to only control caterpillar larvae, less impactful on beneficial insects

Only effective on a few insects


Shawn Bernick is director of research and technical support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn. He will be answering one tree health care question in each issue of Arbor Age throughout 2010. To submit your question for consideration, please e-mail Arbor Age editor John Kmitta at jkmitta@m2media360.com.  Be sure to indicate that the question is for the tree health care Q&A, and include your name and contact information.


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