Tentiform leafminers—Phyllonorycter spp.
At least 15 Phyllonorycter species (family Gracillariidae) occur in California. At least two of these, P. crataegella and P. mespilella, feed on and mine inside of apple and pear foliage. Phyllonorycter crataegella is officially named apple blotch leafminer.
Adults, mature larvae, and pupae are about 1/4 inch long. Adult tentiform leafminers have bands of brown and white separated by black bands. The hind wings have a fringe of gray hairs and when flying in sunlight, the moths appear silvery. At rest the wings are held rooflike over the body.
Eggs are flattened, ovate, and about 1/75 inch (0.3 mm) in diameter. They are translucent when laid then become creamy to pale yellow. Eggs occur on the underside of leaves of apple and less frequently on pear.
The first three instars (larval stages) are referred to as the sap-feeding stage. These larvae are legless, flattened, and have a brown, wedge-shaped head. The larvae are pale gray to whitish, but the dark contents of their food can be visible through their covering. As they feed the outer layer of the leaf undersurface separates from the rest of the leaf tissue. Their mines initially appear as a thin, winding line, then enlarge to a blotch. These mines are visible only from the underside of the leaf.
Fourth and fifth instars are called the tissue-feeding stage. These older larvae have a round head, thoracic legs, and fleshy protuberances (prolegs) on their abdomen. While tissue feeding, they cause an elongate mine. Inside leaves they tie the sides of the mine together with silk, giving mines a tentlike appearance for which the insect is named. If the mines are opened, numerous small pits chewed by the larvae can be observed in leaf tissue.
Before pupating, the larvae change from whitish to yellow. The elongate pupae are initially dark yellowish then become dark brown. Pupae occur in a thin silken cocoon in the leaf mines. Just before emerging as an adult the pupa cuts a hole in the leaf's lower surface where the pupa partly protrudes from the leaf mine. After the adult emerges, the pupal skin remains attached to the leaf.
Tentiform leafminers develop through four life stages: egg, larva, pupa, and adult. They overwinter as prepupae (mature larvae) or pupae in mines in fallen leaves. The adults begin emerging in late February and can be observed resting on trunks and nearby low-growing vegetation. In apples, adult females begin laying eggs when buds are still in a tight cluster. In pears, egg laying begins when the first leaves begin to unfold. In both hosts egg laying can continue through petal fall.
The hatching larvae chew into leaf tissue where they feed and develop through five increasingly larger instars. Apple- and pear-feeding tentiform leafminers in California have four generations per year.
The larvae chew and feed inside apple and pear foliage. Infested leaves develop mines that are initially winding then become elongated blotches. Mines of young larvae are visible only on the underside of leaves. Mines of older larvae are visible from both sides of the leaf. Blotched leaves may become wrinkled and drop prematurely.
A total of about five or less mines per leaf over the entire growing season has no discernable effect on fruit quality or yield. Especially if trees are stressed by other pests such as mites, more than five mines per leaf can retard fruit color development and reduce fruit size and sugar accumulation. If mines are numerous, leaves may drop prematurely, and this can result in sunburned fruit and reduced yield the following growing season.
Tentiform leafminers are generally not pests in organic apple or pear production and where broad-spectrum, persistent insecticides are avoided for all pests on the trees. The reason is that tentiform leafminers have very effective parasitoid (parasitic) wasps if this biological control is not disrupted by insecticides.
Important parasitoids include Pnigalio flavipes and Sympiesis stigmata (family Eulophidae). These small wasps during their larval stage feed within mines and externally on the leafminer larvae. Mature wasp larvae pupate near the shriveled skin of the leafminer larva that they fed upon and killed. The parasitoid pupae resemble leafminer pupae in color and size, but the immature wasp has a larger head and more prominent eyes. They are flattened and naked in the leaf mine, unlike leafminer pupae that occur within thin silk. Some leafminer larvae are also killed by an insect pathogen.
No control action aside from conserving parasitoids and avoiding the application of broad-spectrum, persistent insecticides is recommended for home apple and pear trees. See Protecting Natural Enemies and Pollinators for more tips on how to improve biological pest control.
If the leafminers are intolerably abundant, foliage can be sprayed with spinosad, which moves short distances into leaf tissue. Adding horticultural oil to the spray mix can increase the efficacy and persistence of spinosad. Spinosad is toxic to bees and certain beneficial predatory insects for at least several hours after its application. Wait until after petal fall to make any application.
Adapted from Integrated Pest Management for Apples and Pears, Pest Management Guidelines: Apples, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).