Caterpillars on Trees and Shrubs

Sphinx moths (family Sphingidae)

Also called hawkmoths, the caterpillars (hornworms), often bear a single, stiff horn-like projection at the end of their body. (The horn is harmless to people.) The defensive posture of the caterpillar is a raised upper body with its head tucked under.

Wild silkmoths (family Saturniidae)

Despite their common name, this family of moths is not related to the domesticated silkworm. These are some of Maryland's largest caterpillars and adult moths.

Prominents (family Notodontidae)

Other moth and butterfly families

Each caterpillar instar eats progressively more food because the insect is growing rapidly. Early-instar (young) caterpillars may not cause noticeable plant damage, while late-instar (older) caterpillars can quickly defoliate branches or entire plants when populations are high (or when the host plant is still small).

Many caterpillars can be ignored because they do not cause substantial damage and the plant can afford to lose some leaves, especially later in the growing season (closer to when the plant goes dormant). Lots of chewed or disappearing leaves may look alarming, but trees and shrubs typically recover just fine with replacement growth.

In some cases a high-value plant might warrant intervention to control caterpillars, such as when a conifer cannot replace lost foliage. Although one defoliation event usually will not cause the death of a deciduous shrub or tree, repeated defoliations can significantly weaken or kill the plant, especially if it is already struggling from another issue, such as drought or dieback from a disease.

Which caterpillars could warrant control?

The following caterpillars may warrant intervention when they cause prominent damage to trees and shrubs. Each species links to a resource page with life cycle details and management recommendations.

Caterpillars are easiest to manage while they are still young, especially when using an organic or reduced-risk insecticide. For example, Bacillus thuringiensis (Bt, specifically strains aizawai and kurstaki), is only effective on early-instar caterpillars.

Early-season caterpillars which can have population outbreaks which defoliate trees before they've had time to store much energy for recovery should be controlled when larvae are small and before the damage becomes extensive. Examples include spongy moth and spring cankerworm.

Mid- to late-season caterpillars generally do not require control, except potentially on very young trees or if heavy defoliation is occurring. Examples include fall webworm, yellow-necked caterpillars, mimosa webworm, and orange-striped oakworm. Bagworms feeding on deciduous trees can usually be ignored; on most needled evergreens (spruce, juniper, arborvitae, etc.), their feeding can result in more permanent damage since the eaten needles will not grow back.

How do I remove or treat caterpillars?

• Pick off caterpillars by hand on shrubs and young trees within reach. (Use gloves in case they have irritating spines or hairs.) You can toss them elsewhere for wildlife to eat, or where the caterpillars may be able to find other plants to eat. Invasive species like spongy moth, euonymus leaf notcher, and box tree moth can be dropped into a container of soapy water or squished.
• Prune off the silken "nests" of gregarious caterpillars. Pruning removes one or more years of branch growth, so select the location of pruning cuts carefully.

Insecticide as a last resort

• Hire a professional (a certified arborist or certified pesticide applicator) when insecticide is used on larger trees. Good control of a caterpillar outbreak on mature trees can be difficult, especially when a contact spray is used.
• Contact-type insecticide sprays like horticultural oil or insecticidal soap need to directly contact the caterpillars in order to work, so thorough spray coverage is essential. Caterpillars hidden inside a silken web or the fold of a leaf will be shielded from such sprays.
• Systemic insecticides are absorbed by the plant, and the caterpillars are exposed as they ingest the pesticide. Residual pesticides linger on the surfaces of the plant that was sprayed, and for a time can expose caterpillars that walk over the residues or eat treated leaves before the residue degrades. Insecticides in these categories could potentially harm other organisms that are beneficial or not pests.

References

Host plant information was sourced in part from:
Wagner, D. L. (2005). Caterpillars of Eastern North America. Princeton University Press.