Oak Ribbed Casemaker

Larvae of this moth (family Lyonetiidae) are probably the most common species among the many leafminer species that can feed on oaks.

Identification

First instars (larvae) form circular white cocoons on leaves. The last (third) instar forms an elongate, cigar-shaped cocoon with distinct longitudinal ribs. These pupal (last instar) cocoons can occur on oak bark and leaves and on nearby objects and plants.

Larvae are green, olive-green, or yellowish with rows of pale spots. They have distinct segments and a small, pale brown head. The mature larva and its pupal cocoon are about 1/4 inch long.

Adults (moths) are mottled brown, tan, and silvery to whitish. They have a wingspan of 1/3 inch. When wings are spread, numerous hairlike setae line the rear margin of wings, especially the hind wings.

Life cycle

Oak ribbed casemaker overwinters as a prepupa or pupa in an elongate cocoon. Adults emerge in the spring and after mating females lay eggs on leaves of various oak species. First instars mine inside the leaf. The second and third instars feed externally on the lower leaf surface then pupate. The summer generation adults emerge and mate and the females lay eggs. The hatching larvae feed until they form the overwintering pupae. There are two generations per year.

Damage

Larval chewing inside oak leaves or on the leaf underside causes damaged foliage to appear brown, translucent, or tan. First instars form pale, narrow, linear mines inside leaves. Late instars chew on and skeletonize the underside of foliage causing circular discolorations.

The oak ribbed casemaker in California occurs on at least California black oak, coast live oak, interior live oak, and valley oak. The chewing of its larvae discolors oak foliage. However, even extensive leaf chewing and mining appear not to threaten the survival of otherwise healthy oaks.

Solutions

No control is generally warranted to protect the health of oaks. If leaf mining damage is intolerable, Bacillus thuringiensis (Bt) can be applied to thoroughly cover the underside of leaves using a high pressure sprayer. Time applications for when the roundish cocoon of first instars appears on foliage. Because Bt is of short persistence and not all larvae are in a susceptible stage at the same time, a second application about 7 to 10 days after the first can be made. Other types of insecticides are not recommended because they may adversely affect the natural enemies that keep many of the numerous oak-feeding insects under good biological control.

For effective control on large oaks, a professional applicator must be hired. For tips, consult Pest Notes: Hiring a Pest Control Company. For example, tell the company the particular insecticide that you want applied, Bacillus thuringiensis.

Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM) and A Field Guide to Insects and Diseases of California Oaks from the USDA Forest Service.