Managing Cottonwood Leaf Beetle
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[Chrysomela scripta Fabricius]

R. Gary Smith, Urban IPM Project

Charles R. Ward, Extension Entomologist Emeritus

L.M. English, Extension Entomologist

Circular 552
This Publication is scheduled to be updated and reissued 9/03.

Description

Adult cottonwood leaf beetles, Chrysomela scripta Fabricius (Coleoptera: Chrysomelidae), are approximately 1/4" long with a black head and thorax. The thorax has reddish margins. The elytra (wing covers) are yellow with broken black stripes. Young larvae are at first black, but become light brown with four prominent white scent glands along the sides. These scent glands secrete a milky, foul-smelling fluid that repels predators when the beetle is disturbed, and can be retracted back into the larva-s body when danger has passed. Mature larvae are approximately 1/2" long.

Fig. 1. Cottonwood leaf beetle life stages showing newly hatched larvae, mature larvae, and adult.

Distribution

The cottonwood leaf beetle is found throughout the United States.

Life Cycle

Cottonwood leaf beetles overwinter under litter or in bark crevices. As the host trees begin leaf expansion in the spring, the beetles fly in to mate and feed on new growth. The female lays her yellow eggs on the undersides of leaves in clusters of 25 or more. As the small, black larvae hatch, they feed on the underside of the leaf and generally pupate in two weeks. The larvae attach themselves to twigs or leaves, hanging upside down to pupate. In 5­10 days the adults emerge and the cycle repeats. In New Mexico, four or more generations may be completed in a season; however, after the first generation, leaf beetle populations are usually small and control is seldom warranted.

Hosts

Cottonwood leaf beetles are serious pests of poplars, aspens, elders, and willows.

Damage

The young larvae live in groups and skeletonize the undersides of leaves before eventually separating and consuming all but the large leaf veins. Adults chew holes in the leaves and attack new shoots, causing stunting and multiple-branched trees. The beetles also feed on the young bark of seedlings, causing severely stunted growth and death. Cottonwoods and other suitable hosts are most severely damaged within the first three years of growth.

Action Threshold

Control is generally not necessary in New Mexico, but damage should not exceed 20% defoliation in the spring or 40% in the summer. Fall applications generally are not warranted.

Inspection and Control

Ants, ladybird beetles, lacewings, spiders, wasps, and various parasites feed on cottonwood leaf beetle larvae and eggs, offering some natural controls.

When populations reach action threshold levels, insecticide applications may be necessary. Young larvae are susceptible to insecticidal soaps and horticultural oils. Mature larvae and adults may need to be managed with applications of acephate, carbaryl, chlorpyrifos, or diazinon (table 1).

Table 1. Control suggestions for cottonwood leaf beetle in New Mexico.


Material
Orthene 9.4% E.C.
Sevin 50% W.P.
Liquid Sevin 27%
Dursban 12.6% E.C.
Rate
3 Tbsp/gal water
2 Tbsp/gal water
1 Tbsp/gal water
per label directions
Comments

Least toxic controls for leaf feeding beetles include the "San Diego" and "tenebrionis" strains of Bacillus thuringeiensis and products that contain azadirachtin, the active ingredient in neem oil.

Additional Reading

Cranshaw, W., D. Leatherman, and B. Kondratieff. 1993. Insects that Feed on Colorado Trees and Shrubs. Colorado State University Cooperative Extension. Bulletin 506A. pp. 22­23.

Furniss, R.L. and V.M. Carolin. 1980. Western Forest Insects. USDA FS. Publication No. 1339. U.S. Government Printing Office. Washington, D.C. 20402. p.317.

Johnson, W.T. and H.H. Lyon. 1988. Insects that Feed on Trees and Shrubs. Cornell University Press, P.O. Box 6525, Cascadilla St., Ithaca, NY 14851-6525. p.226.



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Printed July 1998
Electronic Distribution September 1998