How to Manage Pests
Pests in Gardens and Landscapes
The redhumped caterpillar, Schizura concinna, is found throughout much of California. Although the climate of the coastal regions usually doesn’t favor development of destructive populations, it can be a serious problem in the warm Central Valley. This pest most commonly attacks liquidambar (sweet gum), walnut, and plum trees, but you also can find them on almond, apple, apricot, birch, cherry, cottonwood, pear, prune, redbud, willow, and others, especially where insecticides applied to control other pests have killed their natural enemies.
The redhumped caterpillar has four stages of development—egg, larva (caterpillar), pupa, and adult (moth). Adults lay eggs, which are nearly spherical and pearly white to cream colored, in groups of 25 to 100 on the undersides of younger leaves.
Caterpillars are 1 to 1 1/2 inches long when fully grown and have a base color of yellow. Longitudinal white, reddish brown, or sometimes black stripes mark the body. The head is usually orange or brick red, as is the fourth body segment, which is distinctly humped and has two prominent, black tubercles (spines). Each body segment also has less distinctive black tubercles. Caterpillars rest with their hind end elevated.
The pupa is reddish brown, a little more than 1/2 inch long, and enclosed in a silken cocoon in the soil or in the layer of organic debris covering the soil.
Adult moths have a wingspan of 1 to 1 3/8 inches. The forewings are reddish to grayish brown and often are darkest along the hind margin. The hind wings are off white to light gray or brown.
In autumn, caterpillars drop to the ground and spin silken cocoons. They remain inside the cocoons during winter and transform into pupae in spring.
Moths begin emerging from pupae in April and May. They mate, and each female can lay more than 200 eggs. The eggs hatch into tiny caterpillars that feed, grow, and then drop to the ground to pupate. There are often as many as four or five generations per year. Redhumped caterpillars seem to be more abundant after a warm winter.
Upon hatching, caterpillars feed in groups on lower leaf surfaces and skeletonize the leaves. As the larvae become larger, they tend to disperse and consume the entire leaf, leaving only the tough, woody veins. When infestation is light, larvae eat leaves on only a few branches, but occasionally a heavy infestation develops that defoliates entire trees.
A number of parasitic wasps, including Cotesia (Apanteles) species and Hyposoter fugitivus, use the redhumped caterpillar as a host and often provide effective natural control. General predators including spiders, lacewings, bigeyed bugs, and damsel bugs also feed on eggs and caterpillars. However, in some instances additional control measures are necessary. The simplest of these is to cut off the foliage that contains caterpillars while the insects are still young and active. At this stage you’ll need to prune off only small branches in order to destroy a large group of caterpillars. Then either burn the foliage or crush the caterpillars.
If you choose to use insecticides, choose ones that are least toxic to the caterpillar’s natural enemies. Bacillus thuringiensis—which is sold as a variety of products including Safer Brand Caterpillar Killer or Greenlight BT Worm Killer—is an effective spray for redhumped caterpillar control. Bt, as it is commonly known, is a bacterial preparation that causes a diseaselike condition in many types of caterpillars. They stop eating several hours after feeding on a sprayed leaf and die a couple of days later.
Spray with Bt after the first skeletonized leaves appear. Bt is most effective on the smallest caterpillars. Be sure the caterpillars are present before spraying. A complete coverage spray throughout the entire tree is necessary for effective control. Bt won’t harm the natural enemies of redhumped caterpillars and other insects as the more toxic, broad-spectrum insecticides do.
Another environmentally friendly insecticide that controls redhumped caterpillar is spinosad (Monterey Garden Insect Spray). Like Bt, complete coverage of leaves is required for effective control.
Experience has shown that sprays for redhumped caterpillars are often applied too late to have any effect. Unless many non-parasitized caterpillars are present on trees, it’s best to delay spraying until the next generation appears and is feeding on the leaves. If you find significant numbers of parasitized redhumped caterpillar pupae, biological control alone will likely control the pest and sprays should not be needed.
Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2004. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3359.
Flint, M. L. 1998. Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide, 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3332.
Moore, W. S., and C. S. Koehler. 1981. Redhumped Caterpillar—A Pest of Many Trees. Oakland: Univ. Calif. Agric. Nat. Res. Leaflet 21064.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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