Leafrollers are larvae (caterpillars) of certain moths (Tortricidae). They primarily chew foliage and often feed and pupate within leaves they curl and web with silk.
Correctly identify the cause of damage before taking action. The biology and management of leafrollers differs from that of the other caterpillarlike larvae. Grasshoppers, katydids, leaf beetles, sawflies, walking stick insects, weevils, and certain other insects can cause damage resembling caterpillar chewing, but these other insects do not produce silk strands or webbing.
About 100 species of leafrollers occur in California. Pests of fruit and nut trees and various ornamental include fruittree leafroller, light brown apple moth, obliquebanded leafroller, and omnivorous leafroller. Adults and larvae of many species are similar looking and for positive identification must be submitted to an expert. Distinguishing leafroller species may be possible based on differences in body color and markings.
Adults (moths) of most leafroller species are a mottled mix of colors and at rest appear bell shaped when viewed from above. Larvae have pairs of leglike appendages (prolegs) on abdominal segments 3 through 6 and the last (rear) segment. When disturbed, larvae wriggle vigorously and often drop suspended on on a silk thread.
Larvae develop through five or six increasingly larger instars (immature growth stages). Older caterpillars construct a new leafroll nest frequently, often daily, eventually pupating inside a nest or on bark in a thin, silken cocoon. Adult females lay eggs overlapping in flat masses resembling fish scales on leaves and limbs.
The fruittree leafroller has only one generation per year, and the larvae are present only during spring; it overwinters as eggs on bark. Most other leafroller species have two or more generations per year, and can be present as larvae throughout the growing season; they overwinter in protected places, primarily as larvae or prepupae (inactive, mature larvae).
Leafroller feeding gives foliage a ragged or curled appearance. Larvae prefer leaves but sometimes chew blossoms, fruits, and nuts, causing them to drop or become distorted or scarred. Unusually high populations can completely defoliate trees, and the leafroller's silken threads can entirely cover the plant.
For the major pests, species-specific pheromone-baited traps can be used to determine whether adults of a particular species are present and to time insecticide applications targeting young larvae. Adult captures coincide with egg laying and the approximate time of egg hatching for most species except fruittree leafroller.
Conserve natural enemies, which often keep leafroller populations low. Parasitic wasps and tachinid flies are especially common in leafroller larvae and pupae. Trichogramma parasites kill some eggs. Assassin bugs, green lacewing larvae, ground beetles, and certain other beetles and spiders are common predators of leafrollers.
Otherwise healthy trees tolerate the loss of some leaves, and low numbers of leafrollers do not warrant insecticide application. If fruittree leafroller has been a problem, apply horticultural oil in January or February to thoroughly cover limbs and small twigs infested with overwintering eggs. Because oil spray can damage dormant plum and walnut trees, spray infested foliage on these hosts during the growing season if leafrollers are a problem.
A properly timed foliar spray of Bacillus thuringiensis (Bt) or spinosad controls leafroller larvae. Bacillus thuringiensis kills only caterpillars (butterfly and moth larvae). A second application of Bt about 7 to 10 days after the first is recommended because of its short persistence.
Spinosad can adversely affect bees and certain natural enemies. Because it is toxic to bees for several hours after the spray has dried, do not apply spinosad to plants that are flowering.
If larger larvae in rolled leaves are abundant, use a high-pressure sprayer so the insecticide penetrates into rolled foliage. Because fruittree leafroller has only one generation per year, by the time chewed leaves are obvious or trees are defoliated, the caterpillar stage might be almost completed and spraying leaves may be of little benefit. For more information, consult Pest Notes: Leafrollers on Ornamental and Fruit Trees.