Leafminers

Adults are small, golden brown moths with white bands or spots that give them a silvery appearance when they fly in sunlight. In spring eggs are laid in young leaves during the period from the tight cluster bloom stage through petal fall. Larvae develop within the leaf tissue. The first three instars feed on sap within the leaves and are called sap-feeders. Sap-feeders have a white, flat, legless body with a brown, wedge-shaped head. They form snakelike mines in the leaf that are visible only on the lower surface of leaves. Fourth and fifth stage larvae are known as tissue-feeders because feeding is concentrated more on leaf tissue than on sap. Tissue feeders have both legs and prolegs and a round head. These older larvae tie the sides of the leaf together with silk to form tents.

Leafminers overwinter within mines in leaves on the orchard floor.

Leafminer damage is restricted to foliage. In a heavy infestation, over 60% of the leaf tissue can be destroyed. Larvae feed on cells between upper and lower epidermal layers of the leaf, leaving only the thin epidermal layers. The upper side of the leaf takes on a light, spotted appearance. Infestations greater than an average of 5 to 10 mines per leaf may cause premature defoliation. Even if trees are not defoliated, leaf function is impaired, and fruit may fail to size or color.

Low levels of leafminer populations are present in most orchards every year; populations are usually kept at low levels by several species of parasites. Leafminers only become pests when their natural enemies are disrupted by the use of broad-spectrum insecticides for codling moth control.

Parasitic wasps such as Pnigalio flavipes and Sympiesis stigmata are very important in controlling tentiform leafminers. One of the common leafminer parasites lays its eggs in the leaf mines when the leafminers have reached the tissue-feeding stage or fourth instar. After hatching, parasitic larvae attach themselves to the outside of leafminer larvae. The parasite grows rapidly and consumes the leafminer by its fifth instar. It then pupates within the leaf mine about the same time the leafminer would have. The parasite resembles the leafminer pupa in size and color, but it has a larger head and eyes, and it is flat and naked in the leafmine, whereas leafminer pupae are inside a silk cocoon. Although adult parasites are very small, they are easy to spot if populations are abundant because they fly in groups that hover near infested trees.

Biological control and sprays of the Entrust formulation of spinosad are the primary means of controlling leafminers in an organically certified crop.

To determine treatment needs, monitor both the number of sap-feeding mines and parasitization levels of the tissue-feeding stage. First generation larvae normally do not require treatment in California. It is only necessary to monitor parasitism levels of this generation in April. Collect 50 mines with tissue-feeding larvae and examine the larvae for parasites. Sample again in May as soon as sap-feeding mines appear. Randomly collect four leaves from 25 trees. Use the parasitization rate from the first generation sample in conjunction with the number of sap-feeding mines in the current generation to make a treatment decision. Treat if leaves average more than five mines per leaf, or if two or more mines per leaf were present and less than 10% of the first generation was parasitized.

If more than 10% of the first generation was parasitized, delay treatment and reevaluate the level of parasites as soon as the current generation reaches the tissue-feeding stage. At this time, examine leafminer larvae in 50 mines for the presence of parasites.

To determine if treatment is necessary for the third generation, sample sap-feeding larvae as soon as they appear in July. Collect four leaves from 25 trees. Treat if the leaves are averaging more than five mines per leaf or if parasitization was less than 30% when parasite levels were reevaluated in the previous sample. Generally, treatment is not recommended for fourth generation larvae unless parasite levels are low or sap-feeding mines exceed an average of five per leaf.

Leafminer pheromone traps are available and can be used to monitor flights of this pest. Although no correlation has been developed between number of moths trapped and the occurrence of leaf mines, the traps can help pinpoint the start of the different generations.