How to Manage Pests
Pests in Gardens and Landscapes
The redhumped caterpillar, Schizura concinna (Notodontidae), is found throughout much of the United States. In California, high populations are usually limited to the Central Valley.
This pest most commonly chews leaves of liquidambar (sweet gum), plum, and walnut. It also feeds on almond, apple, apricot, birch, cherry, cottonwood, pear, prune, redbud, willow, and other deciduous trees and shrubs.
Adult females lay eggs in groups of 25 to 100 on the underside of leaves. The eggs are spherical, cream-colored to yellow, and about 1⁄16 inch in diameter. After hatching, the empty egg shells are translucent to white with an irregular, ragged hole. First-generation eggs are laid on lower leaves, nearest the ground. Later in the season, egg clusters can occur throughout the tree.
Newly hatched (first-instar) caterpillars are approximately 1⁄8 inch long and often feed in groups. Young larvae have dark brown to black heads, yellowish-orange bodies with faint dark, lengthwise stripes, and rows of dark bumps (tubercles) with bristles along the body.
Older caterpillars reach 1 to 1–1/2 inches long and have yellowish-orange bodies with lengthwise black and white stripes. Second through four instars have blackish or red heads, and the fifth (last) instar has a red head. In late instars, the fourth segment behind the head (the first abdominal segment) has a reddish hump with two prominent black spines. The dark tubercles and bristles along the length of the body are more striking in larger (later-instar) caterpillars. Larger caterpillars commonly hold their rear end raised above the plant surface.
The pupa is brown to red and about 1⁄2 inch long. Pupae occur in a thin, silk cocoon in topsoil or organic debris on the ground at the base of the tree, or sometimes on leaves.
Adult moths are about 7⁄10 inch long and have a wingspan of 1 to 1–3/8 inches. The forewings are a mix of reddish to gray and brown. The hind wings are whitish to gray or pale brown.
The redhumped caterpillar has four stages of development—egg, larva (caterpillar), pupa, and adult (moth). Moths emerge from pupae in April to May and mate. Each female lays several groups of eggs on host foliage.
The eggs hatch into caterpillars that feed on and chew leaves. Caterpillars grow through five increasingly larger instars (immature stages) by shedding and regrowing their outer cuticle (skin), then pupate.
In the autumn, full-grown caterpillars move to the ground, spin a silk cocoon, and remain inside over the winter as inactive, mature larvae (prepupae). During spring, they undergo metamorphosis into pupae, then become adult moths that emerge and seek mates. The redhumped caterpillar has two or more generations per year in California’s Central Valley.
Young caterpillars commonly feed side-by-side in groups, chewing on the lower leaf surface. As the larvae grow, they tend to disperse and feed in smaller groups or individually. Skeletonized leaves are a common result, as the older caterpillars chew all the way through and consume leaves, leaving only the larger, tough veins. Unlike certain other caterpillars that may feed on the same hosts, redhumped caterpillars do not tie leaves with webbing or leave silk strands on foliage; the exception is when silk-covered pupae occur on leaves.
When their abundance is low, larvae eat leaves on only a few branch terminals. Occasionally, heavy infestations develop and defoliate entire trees during the summer. Usually only scattered individual and young trees are severely defoliated. If severely defoliated, trees that are otherwise healthy usually recover.
Redhumped caterpillars do not feed on fruit or nuts, but these may become sunburned and drop prematurely if defoliation is severe. Yield quantity may be reduced the season after fruit or nut trees are defoliated.
Biological control and spot-pruning of infested shoots are sufficient to manage this pest in most situations. Insecticide application can be warranted if caterpillars are abundant and their feeding damage is intolerable, especially when trees are small, already stressed, or unhealthy from other causes.
On hosts where this insect has been a problem, look for redhumped caterpillars in April and May. This is when the first-generation caterpillars may be present. Inspect the underside of terminal leaves for egg masses and groups of small caterpillars at least once a week.
From a distance, look for green shoots with ragged (chewed) or few leaves, or leaves that turn brown, which may indicate the presence of redhumped caterpillars. Leaves can turn brown from many other causes, so inspect the underside of leaves for the insects themselves and their chewing damage. If you find eggs or larvae of the first generation, prune out and destroy the infested shoots.
Monitor again in late June and July for second-generation eggs and caterpillars. If you find these, look for evidence of parasitism as described below. If caterpillar parasitism is abundant, no further control action is needed. If parasitization is low, prune out and destroy infestations if they are limited, or consider applying an insecticide if caterpillars are abundant.
Parasitic wasps (parasitoids) are important natural enemies of the redhumped caterpillar and often appear to prevent this pest from becoming abundant. Female wasps of several species lay eggs in caterpillars, which hatch into larvae that feed inside the host.
Cotesia (Apanteles) schizurae (Braconidae) feeds as multiple larvae inside each caterpillar; they emerge and pupate in white, silken cocoons in a group near the caterpillar they killed. Hyposoter fugitivus larvae feed singly and pupate inside the skin of the caterpillar, which becomes swollen and mottled gray with black.
Because these parasites do not change their host appearance until shortly before the caterpillar is killed, when obviously parasitized caterpillars are observed, other nearby caterpillars may also contain parasites too immature to have affected caterpillar appearance. When a caterpillar appears lethargic or not actively feeding, especially when others nearby appear parasitized, grasping the caterpillar at each end and pulling it apart may expose one or more pale, maggotlike, parasite larvae inside.
Trichogramma spp. wasps feed as larvae inside moth eggs, causing the normally pale-colored eggs to darken. When a Trichogramma adult emerges, the egg has a smooth-edged, roundish hole instead of the irregular, ragged hole chewed when a caterpillar emerged.
Conserve parasites and predators by avoiding the use of long-residual (long-lasting), broad-spectrum insecticides such as carbamates, organophosphates, and pyrethroids. Plant flowering species as insectary plants near commonly infested trees, because parasites of redhumped caterpillar live longer, lay more eggs, and can parasitize and kill more caterpillars when the adult parasites are provided with flower nectar, as discussed by Pinnock and others (see References).
Clip off shoots that contain caterpillars while the insects are young and feeding in groups. At this stage, you'll need to prune off only the infested shoot terminals to destroy a large group of caterpillars. Crush the caterpillars, or dispose of them in a plastic bag in the trash.
Provide trees and shrubs with a good growing environment and proper cultural care to encourage their healthy regrowth and increase plants' tolerance to redhumped caterpillar chewing and damage by certain other pests. See the management recommendations at ipm.ucanr.edu for more information.
If applying insecticides, select active ingredients that are the least toxic to the caterpillar's natural enemies and other nontarget organisms. These insecticides include Bacillus thuringiensis and spinosad, which can be sprayed on foliage alone, in combination, or mixed with horticultural oil (petroleum or narrow-range oil, e.g., All Seasons Horticultural & Dormant Spray Oil). Because redhumped caterpillar infestations tend to be very localized, spot treatment (e.g., spraying only the outer shoots on infested hosts) is usually sufficient.
Bacillus thuringiensis (Bt) is a group of naturally occurring bacteria that kill certain insects. To be effective, Bt must be eaten by the caterpillars, and it is most effective on younger caterpillars. Caterpillars stop eating within one day after feeding on a leaf sprayed with Bt; then they darken and die within a couple of days.
Thoroughly spray the underside of infested leaves with Bt when caterpillars and their chewing first appear. A second application 7 to 10 days after the first is recommended, because Bt degrades quickly and not all the moth eggs hatch at the same time. Bt has little or no toxicity to humans, bees, and most other animals, including the natural enemies of redhumped caterpillars.
Bts are produced commercially by fermentation and sold for pest control. The caterpillar-killing Bt subspecies aizawai and Bt subspecies kurstaki are available in a variety of products, including Garden Safe Brand Bt Worm & Caterpillar Killer, Monterey B.t., and Thuricide Bacillus thuringiensis.
Agree WG Biological Insecticide, Jackpot WP, and certain other products containing Bt subspecies aizawai are available only to professional applicators. If caterpillar-infested trees are large or tall, hiring a pest control company with the equipment and experience to effectively spray trees can be a good idea. Discuss in advance with the company how they plan to control your pest problem. Request they apply caterpillar-selective Bt, or spinosad as discussed below if the plants to be sprayed are not blossoming.
The active ingredient spinosad is a mixture of spinosyns, which are fermentation products from the bacterium Saccharopolyspora spinosa. Spinosad quickly kills insects that contact or eat it, including most caterpillars, flies, thrips, and certain species of beetles and wasps. It can kill a larger percentage of the caterpillars than Bt, especially when spraying older caterpillars.
Spinosad (e.g., Captain Jack's Deadbug Brew, Monterey Garden Insect Spray, Ortho Insect Killer Tree and Shrub Concentrate) remains effective for about 1 week after application. Adding horticultural oil to the spray mix and using water with a pH of 6 to 8 may increase the persistence of spinosad.
Spinosad has relatively low toxicity to people but can be toxic to pollinators and natural enemies, including parasites of caterpillars. Applying Bt instead of spinosad where natural enemies are active may provide better long-term control by conserving natural enemies. It is toxic to bees for several hours after the spray has dried, so do not apply or allow it to drift to plants that are flowering. To minimize its adverse effects on honey bees, apply spinosad in the late evening if permitted by the label and regulations.
Insecticides to Avoid. Avoid foliar sprays of carbamate insecticides (carbaryl*), organophosphates (e.g., malathion), or pyrethroids (e.g., bifenthrin, cyfluthrin, fluvalinate, and permethrin). These materials are highly toxic to natural enemies and pollinators and can cause spider mite outbreaks.
Because their use in landscapes and gardens can run or wash off into storm drains and contaminate municipal wastewater, these insecticides are being found in surface water, and are adversely affecting nontarget aquatic organisms.
*As of August 1, 2020, pesticides containing the active ingredient carbaryl are restricted use materials in California. A valid pesticide applicator’s license is required for their possession and use. For more information see the California Department of Pesticide Regulation website.
Dreistadt, S. H., J. K. Clark, T. A. Martin, and M. L. Flint. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 3rd ed. Oakland: UC ANR Publication 3359.
Moore, W. S., and C. S. Koehler. 1981. Redhumped Caterpillar—A Pest of Many Trees. Oakland: UC ANR Leaflet 21064.
Packard, A. S. 1895. Monograph of the Bombycine Moths of America North of Mexico. Family 1.-Notodontidae. Memoirs of the National Academy of Sciences 7: 5–390.
Pinnock, D. E., K. S. Hagen, D. V. Cassidy, R. J. Brand, J. E. Milstead, and R. L. Tasson. 1978. Integrated pest management in highway landscapes. California Agriculture.
University of California Statewide Integrated Pest Management Program. Insectary Plants.
Wilen, C. A., D. L. Haver, M. L. Flint, P. M. Geisel, and C. L. Unruh. 2006. Pest Notes: Hiring a Pest Control Company. Oakland: UC ANR Publication 74125.
Pest Notes: Redhumped Caterpillar
Produced by University of California Statewide IPM Program
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