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Birch Leaf Miner ((NEW))

Some of the most prevalent, and widespread causes of browning of birch leaves in Maine has been by one or the other of these European insects. Damage is a result of feeding between the upper and lower leaf surface by the larval stages. Early symptoms of attack are tiny grayish or discolored blotches in the leaf tissue surrounding the eggs. Eggs of M. nana are laid along the serrated leaf margin and larvae mine toward the center of the leaf. Eggs of F. pusilla are laid away from the leaf margin and larvae mine toward the margin. Mines appear roughly ten days after the eggs are laid and are translucent green at first. As the irregularly shaped mines become progressively larger they turn brown and cause the leaves to become wrinkled and distorted.

birch leaf miner


While both species can be an aesthetic problem especially on ornamentals, only M. nana appears to impact forest stands where it is often associated with the birch casebearer, Coleophora serratella. The combination of these two species can result in tree stress and may hasten infestation by the more destructive bronze birch borer, Agrilus anxius.

The predominant miner on white birch in recent years has been Messa nana. This species also attacks gray and yellow birch as well. Fenusa pusilla, on the other hand prefers gray birch or gray birch hybrids. The ornamental European white birch and its cut-leaf varieties are attacked by both of these sawflies. Trees growing in open, sunny areas such as around homes and along roadsides seem to be preferred and frequently suffer the heaviest browning.

Whereas M. nana appears to have only a single generation in any one year there may be 3-4 generations of F. pusilla. Both sawflies appear to commence activity at roughly the same time each spring however. The small black sawflies, which somewhat resemble black flies, start crawling over leaves in late May, soon after the foliage develops, and lay eggs in the leaf tissue. Emerging larvae mine as described above. The small flat pale colored larvae have various dark spots as they mature and leave their blackish specks of frass (waste material) within the mines. Larvae eventually leave the mines, drop to the ground and pupate in the soil. Pupae of M. nana remain there until the next spring whereas those of F. pusilla produce a new generation of adults to start egg laying again within 10-14 days.

Good control depends on early, timely spraying for the first generation of sawflies and larvae to prevent appreciable numbers in any later generations. The best way to time the first application is to watch the new leaves as they first develop in May. Areas in leaves where eggs have been laid will develop a grayish cast rather than the normal green of the leaf. Developing mines will show up as tiny translucent spots of a lighter green color than the rest of the leaf when leaves are held up to the light. As soon as either the transparent spots or offcolor areas appear, foliar treatments should be applied thoroughly over the whole tree. A repeat spray may be necessary 10 to 14 days later if new symptoms appear. A number of insecticides including carbaryl, malathion, and acephate will all provide effective control against birch leaf miners, however, first check the insecticide label on the container for specific use and apply only in accordance to label instructions and precautions.

A systemic insecticide, disulfoton granular also provides excellent control against birch leaf miners. However, it is a restricted use pesticide to be sold only by licensed dealerships to certified pesticide applicators only. Disulfoton is applied as granules spread over the ground beneath the affected tree and thoroughly soaked with water in accordance with specific label use instructions and precautions. Disulfoton should be applied by mid-May to be most effective.

The birch leaf miner (Fenusa pusilla) feeds primarily upon three host plants: gray birch, paper birch, and European white birch. This European native was introduced into the U.S. pre-1923 and has grown to economic levels as one of the most important nursery insect pests of New Hampshire.

Birch leaf miners prefer sunny areas but can attack susceptible trees almost anywhere. Birches tolerate leaf miner attacks best when they are planted in areas that are shady, cool, and moist. Formerly, attacks could seriously weaken trees, so good fertility and horticultural care were necessary.

The tiny larvae of this leaf mining sawfly can incur complete defoliation on the host plant. Repeated infestations, in consecutive years, can weaken the host to the point where it is invaded by secondary pests such as the bronze birch borer.

In Massachusetts the adults appear in May, and their appearance is closely timed to the emergence of new foliage on the host plant. Adult females require tender, newly expanding foliage within which to lay their eggs. The eggs quickly hatch and mining begins, which initially appears as small yellow spots on the foliage. Larval feeding occurs for about 4 weeks, whereupon, they chew a hole through the bottom of the leaf and drop to the soil. One leaf may support a dozen or more larvae and be completely browned from their feeding. One moderate-sized landscape tree may support tens of thousands of larvae in the first generation. Most of the larvae will stay in the soil until the following spring but a limited number (less than 30%) will pupate and emerge about a month later as the second generation. There is even a third generation in southern New England but it is smaller still.

Birch leafminers (Amharic: የበርች ቅጠል ቆጣሪዎች, Persian: برگ بران توس; Profenusa thomsoni and Fenusa pumila) are sawflies, which are closely related to bees and wasps. They are among the most common insect pests affecting Birch trees (Betula spp.) in North America. Areas inside the leaves are consumed by the larvae, affecting the leaves' ability to produce food. Yearly browning of birch leaves are noticed in mid July and August, but the leafminers have been feeding inside the leaf tissue since early spring.

Leafminers overwinter in the soil as prepupae. Adults emerge in May to late June to early July, depending on temperature and humidity. Oviposition (egg-laying) peaks during the last week of June. Adult birch leafminers are small (about 1/8 to 1/4 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 larvae. These immature larvae feed individually between the leaf surfaces, creating blotchy kidney shaped mines. The immature leafminers feed for several weeks, then drop to the ground where they enter the soil layer to develop into pupae. They pupate and remain there until the following spring. After overwintering as prepupae in the soil below the tree, the adults emerge just as the birch trees are leafed out. Adults are almost all females.

The areas of leaves that are consumed by the amber marked birch leafminer larva turn brown. Because people often do not see the early signs of birch leafminer feeding, it often appears the tree has suddenly dried up or become diseased. This browning is caused by the outer layers of the leaf drying out after the leaf miner larva has consumed the green tissue between the outer layers of the leaf. 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. Feeding over several weeks causes the blemish to take on a blister-like appearance. A single leaf can contain as many as 40 larvae whose mines may merge to destroy the total photosynthetic area of the leaf. Heavy infestations of leafminer larvae can seriously affect a tree's photosynthetic capacity. Repeated attacks will generally cause stress which may induce susceptibility of the tree to other injurious agents.

There are two species mainly responsible for defoliation and browning of birch trees in the United States and Canada. In Northern forests, it is the amber-marked birch leaf miner, Profenusa thomsoni, which were accidentally introduced from Europe to North America early in the 1900s. The other is the birch leafminer, Fenusa pumila, which is more common in Eastern forests.

Presently there is no commercially available biological control agent to control Amber marked birch leafminers, however Canadian trees in the Edmonton area have been successfully controlled with releases of a parasitic wasp, Lathrolestes luteolator. Populations of the tiny parasitoid wasp selectively attack the most damaging birch leafmining pest (Profenusa thomsoni) have developed and drastically reduced the problem in the Edmonton area of Canada.

Following trials in 1995 that supported a dramatic reduction in birch leafminer damage by the first parasitoid, the City of Edmonton, Alberta, Canada discontinued pesticide treatments to almost 3,500 city birch trees in 1996 and 1997. These trees continue to show very little leafminer damage without any treatment.

Spinosad can be used to control birch leafminers prior to extensive damage. Spinosad is a new chemical class of insecticides derived from a soil dwelling bacterium discovered in 1982. It is considered practically non-toxic to humans, pets, and beneficial insects. Unlike other insecticides, Spinosad will not harm beneficial insects including the Amber Marked Leafminer parasite.

Horticultural oil applications applied at the right time may help kill eggs or tiny larvae within the leaf tissue. Oil applications should be made as soon as adults have emerged in the spring and egg laying has occurred and should continue weekly until mid June. Pesticides made with botanical plant oils may be especially useful to prevent egg laying. Neem oil acts as a repellent and may interfere with the egg laying activity of female leaf miners. 041b061a72


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