Paper birch (Betula papyrifera), is the native birch of northern forests and river valleys. A mature paper birch will reach a height of 10 to 12 meters and has an upright growth habit. Paper birch is often grown as a clump with three or more main trunks. The bark on mature trunks is white, papery and peels away naturally to create an attractive contribution to both the summer and winter landscape. Because it is native to our area, paper birch has greater resistance to the local population of insect and disease pests than does the European birch.
River birch (Betula nigra) is less frequently seen in landscapes but is probably the more resistant to insect and disease pests than any of the other birches. Its attractive bark has a strong tendency to peel in long horizontal strips and is darker in colour than the white bark of other birches.
Description & Life History:
Multiple species of exotic birch leaf miners have been introduced accidentally to North America since the early 1900’s. Since their introduction, forestry workers along with gardeners have had to live with unsightly and sometimes sick birch trees. Our predominant species out here seems to be Fenusa pusilla.
The birch leaf miner (BLM) is related to wasps, bees and ants however it doesn’t have a constricted abdomen like the rest of its relatives. Adult BLM are small black wasps, about 3/16 of an inch long (Figure 1). The first generation of adults emerges in April and May. Adults begin to oviposit eggs into newly developing leaves causing wounds as seen in Figure 2. The eggs hatch and the sawfly larvae begin to feed on the leaf tissue. Larvae look like creamy-white, slightly flattened caterpillars (Figure 3). The head and thorax are slightly wider than the abdomen. There can be dark markings on the thorax; the different markings are used to distinguish between different species. After feeding, the larvae either spin down to the ground on a silken web or fall with the leaf. Larvae pupate in the soil and emerge as adults to start the cycle over again two weeks later. F. pusilla have two to three generations per year.
Damage:
Larvae feed on the mesophyll of the leaves, leaving the outer epidermis intact. Having the epidermis intact provides a nice protective house for the leaf miner to feed safely. The upper surface of the leaf can appear transparent to allow viewing for your own leaf miner at work. Areas where leaf miners are active appear as blotchy brown leaves. Due to the fact that these leaf miners can produce several generations through the season, damage to your birch tree can appear severe as fall approaches. During a normal year, trees tolerate this severe damage because they will soon drop their leaves and go dormant for winter.
In August 2002, birch trees were beginning to defoliate because of extensive leaf miner damage. Some trees may have tried to produce new leaves to replace the damaged ones. This is can be risky for the tree’s health because fall and winter is just around the corner and the tree needs to go dormant, not produce new leaves. High, late season populations last year and recent mild winters most likely contribute to higher than normal populations this summer. My birch tree is already experiencing very high populations. Successive years of defoliation may seriously compromise the tree’s health and lead to mortality. This could be a bad year for birch leaf miner infestations.
Monitoring & Management:
As leaves begin to emerge in April, inspect leaves for ovipositional wounds. Once mining damage becomes evident, you can monitor leaf miner development simply by viewing them through the leaf. If it is difficult to see, hold the leaf against a light and you will find the larvae. In Late May and early June, follow larval development by sampling trees weekly. Once you’ve noticed that the mature larvae have vacated the leaves, make your management decision. Ask yourself, “is the population severe or not.” A severe problem would have almost every leaf being mined and the tree has experienced problems for a few years. A minor infestation would be over 25% of the leaves being mined. No problem would be less than 25% of the leaves being mined.
Cultural Management
Pinch the leaves of small trees to kill the leaf miner. If you are planning on planting birch trees, consider installing resistant species and varieties. Betula costata, B. davurica, B. maximowicziana, B. utilis var jacquemontii, B. nigra, B. schmidtii, and the varieties ‘Crimson Frost’ and ‘Purple Rain’ all show resistance to BLM infestations. Birch trees enjoy having their roots in a moist and shady location. Be sure to select the best site to plant your birch and grow a healthy tree.
Chemical Management
Foliar insecticides used to manage BLM target the adult stage and hatching larvae. When the leaves are fully formed in late April or early May, this is your first opportunity to use insecticides. Managing leaf miners at this time can significantly reduce the chance of a problem later in the season. In June, if populations are severe, time your insecticide application to coincide with the second period of adult flight. Once you’ve noticed that the larvae have left the leaf, start to look for adults emerging two to three weeks later. Apply insecticides when most of the adults have emerged. Using insecticides to manage late season generations is generally not worth it. If late season problems are severe, consider an insecticide application next spring. Consult recommendations given on Hortsense (
http://pep.wsu.edu/hortsense/) for using insecticides.
Biological Control
A few parasitic wasp species have been released in North America to biologically control BLM larvae. These parasitoids can be quite effective. Some parts of North America, where BLM has a problematic history, have been relieved from severe BLM problems. Additionally, small insectivorous birds, like chickadees, can be found feeding on the developing larvae. Nothing more entertaining than to watch these birds work over a tree. Generalist soil-dwelling predators, like ground beetles will devour BLM pupae. Conserve your good guys by reducing pesticide usage and providing favorable habitat.
To reach Todd Murray please call (360) 676-6736 or e-mail him at tamurray@coopext.cahe.wsu.edu.
Local to Red Deer 403-505-3478 
