Shield bearers—Coptodisca spp.
Larvae of this group of moths (family Heliozelidae) feed within the leaves of hosts that include apple, cottonwood, crape myrtle, oak, madrone, manzanita, mountain mahogany, poplar, Prunus species, strawberry tree, walnut, and willow.
The elongate larvae chew circular mines in foliage. Larvae are 1/5 inch or less in length and distinctly wider immediately behind the head. When larvae are present in foliage, they can be observed by examining infested leaves such that foliage is backlit. Adults (moths) are about 1/5 inch long and silvery and tan with black, brown, red, white, or combinations of these.
At least nine species of Coptodisca occur in California and are distinguished by their host plants. Photos of the adults and maps of where they have been found are presented by the Moth Photographer's Group of Mississippi State University:
The poplar (cottonwood) shield bearer (Coptodisca sp.) can be abundant on Fremont cottonwood and other Populus species. See Unnamed and Little-Known Insects Attacking Cottonwood in Southern California for more information on this pest.
Overwintering is as mature larvae (prepupae) or pupae inside of mined tissue that the mature larvae cut from host plants. Adults emerge in spring, mate, and females lay eggs into leaf tissue. After hatching, the larva chews a circular- to oval-shaped leaf mine bordered by the larger veins. Each larva consumes most of the leaf tissue in a spot about 1/5 inch in diameter or less, leaving only the thin, translucent upper and lower leaf epidermis.
The mature larva then cuts the remaining epidermis layers within which it fed. It ties the edges of the circular pieces together with silk to form a case (the "shield"). The larva drops on a silken thread and carries away its cut portion of the mine and pupates inside the case in bark crevices or litter on the ground.
Varying by species, shield bearers have one to several generations per year. For example, the madrone shield bearer has one generation per year. The cottonwood or poplar shield bearer apparently has up to three generations per year.
Shield bearer larvae chew and feed in a circular manner within foliage. While feeding, they cause mined foliage to become translucent. At maturity, each larva cuts away the epidermal layers between which it fed, leaving a circular hole in leaves up to 1/5 inch in diameter. There can be up to several dozen holes in a leaf and sometimes most of the leaves on a tree or shrub are affected. However, even high populations of shield bearers appear not to threaten the survival of otherwise healthy plants.
Plants tolerate even abundant leafminers and no control is necessary to protect plant survival. If it does not entail removing too much of a plant's foliage, pick and dispose of infested leaves on small plants where damage cannot be tolerated. No other control is known or recommended.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Leaf holes from feeding by poplar shield bearer larvae.
Leaf holes from feeding of madrone shield bearer larvae.
Larvae of poplar shield bearer feeding in their leaf mines.