Leafminers

Leafminers, Birch leafminers are related to wasps. The adults are small, black, four-winged sawflies about 3 mm (1/8 inch) long. The name “sawfly” is derived from the saw-like egg-laying organ of the female.

Leafminers are insects that have a habit of feeding within leaves or needles, producing tunneling injuries. Several kinds of insects have developed this habit, including larvae of moths (Lepidoptera), beetles (Coleoptera), sawflies (Hymenoptera) and flies (Diptera). Most of these insects feed for their entire larval period within the leaf. Some will also pupate within the leaf mine, while others have larvae that cut their way out when full-grown to pupate in the soil.

The alfalfa blotch leafminer (Agromyza frontella (Rondani))is a relatively new pest in Wisconsin alfalfa. It was first found in the state during the summer of 1996 with positive confirmations in five counties. A 1997 survey of the state showed that the alfalfa blotch leafminer had spread to an additional 34 counties. A 1998 survey shows that most, if not all, alfalfa producing counties in the state of Wisconsin have been infested at sometime during the three-year survey.

Life Cycle: Birch leafminers normally produce two generations a year. They overwinter in the soil as pupae. Adults emerge about mid-May or approximately when the leaves first start expanding. Adult birch leafminers are small (about 1/8 inch long), black and fly-like. Females deposit their eggs singly in slits cut in the central areas of young leaves, usually near the tips of branches. More than one female may lay eggs in a leaf.

The eggs hatch into legless, worm-like insects. These immature larvae feed individually between the leaf surfaces, creating kidney-shaped mines. Early mines appear as light green or whitish discolorations on the leaves. Larvae sometimes can be seen easily when leaves are held up to sunlight, especially as the mines and larvae grow larger. The immature leafminers feed for about two weeks, then drop to the ground to develop into pupae. The areas of leaves that are consumed turn brown. Because people often do not see the early signs of birch leafminer feeding, it often appears the birch has suddenly dried up or become diseased.

The leafminers remain as pupae about two to three weeks. The second generation adults emerge around mid-June in central Minnesota (including the Twin Cities) and lay eggs in newly developing leaves. The larvae again feed for about two weeks and then drop into the soil. They pupate and remain there until the following spring.

Damage: Larvae of the citrus leafminer form serpentine mines in the tender new flush leaves of all varieties of citrus. The mining causes the new flush to twist and curl and prevents it from expanding fully. The citrus leafminer has a short developmental time (2-3 weeks), and as many as 7-8 generations occur in California. In Florida, where bacterial canker is present, leaf mining activity creates opportunities for canker to enter, increasing the incidence of this disease. Canker is not present in California. In California, thus far, the damage has been primarily to nursery trees and new plantings because the larvae must have young citrus foliage to mine and feed on.

The pattern of CLM infestation of mature citrus has been predominantly in the fall months. Mature citrus trees, except for coastal lemons, can tolerate heavy fall flush damage with little impact to fruit production. Coastal lemons have continuous flushing of leaves and produce multiple crops of fruit. The continuous flushing allows prolonged infestations of CLM that reduce vigor of trees and fruit production.

Biological controls:General Predators and Parasitoids. Naturally occurring biological control of citrus leafminer has been important in keeping this pest from reaching economically damaging levels in Arizona. In one study in December 2002, over 60% of citrus leafminers were killed by predators and parasitoids in Yuma, AZ. It appears that many of the predators and parasitoids that prey on citrus peelminer prey on citrus leafminer as well.

In The parasitoid Cirrospilus coachellae has been reported preying on leafminers. This parasitoid is common, particularly during late summer and early fall. Predators also appear to be important biological control agents of leafminers. The predacious mites, the Yuma spider mite and a Tydeus sp. have been observed feeding on citrus leafminer larvae though the leave’s upper epidermal layer.

Commonly used Chemicals: Although rarely required in Arizona, chemical control of citrus leafminer has proven effective. The addition of narrow range spray oil with the insecticide will significantly increase efficacy.

Abamectin: 7 day PHI. Abamectin (AGRI-MEK) is applied at a rate of 0.006 to 0.024 lbs-ai per acre. It cannot be applied to nurseries or by aircraft. It should always applied in combination with a horticultural spray oil at a rate of not less that 1.0gal per acre. Abamectin is relatively nontoxic to beneficial insects and mites. Do not exceed three applications or 0.048 lbs-ai per acre per year, and allow at least 30 days between applications. The restricted entry interval for abamectin is 12 hours.

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