Insects from several orders including Coleoptera, Lepidoptera, Diptera and Hymenoptera
Pest description and damage Leafminer larvae feed between the upper and lower surfaces of leaves, or under the epidermis of stems or leaf petioles. Mines may appear as surface blotches, or serpentine (winding) trails. Some leafminers are solitary, others feed gregariously, and the mines may coalesce to form one large mine. For some species, the insect first mines a needle or leaf, but then shifts to tunneling down the petiole, under bark and then enter bud or stem tissues. Larval identification is based on host, type of mine, and the pattern of the frass within the mine. To get a positive identification, it may be necessary to rear some to the adult stage and send them to a taxonomist specializing in that order or group. The damage that the leafminers do is variable and ranges from minor cosmetic damage to total defoliation. There are several orders of insects that have developed the leaf mining lifestyle; control products may vary with the order of insects causing the mines.
- Beetle and weevil leafminers include some flat-headed borers, weevils and leaf beetles.
- Fly leafminers are generally maggot-shaped without a head capsule. Examples include columbine leafminer, holly leafminer and boxwood leafminer.
- Moth leafminers have a distinct head capsule are commonly found on madrona, including the serpentine and madrona blotch miner, as well as laburnum leafminer, and tentiform leafminers.
- Sawfly leafminers include the elm leaf miner, birch leafminer and alder leafminer.
Biology and life history Mines are often scattered and the leafminers controlled by parasitoids, so leafminers are usually inconsequential to plant health. Occasionally plants become heavily infested, but the plants generally recover. Other leaf miners have two to five generations in a year. Most adults emerge from soil and lay eggs in spring, although some may winter over as larvae in the mine.
Pest monitoring Watch for the adults or the occurrence of the first mines early in the spring as the leaves unfold. Scouting for damage is most critical for leafminers with multiple generations. If early leaf mining activity is minimal, no action is needed, but the plants should be inspected when the next generation is due again. Pheromone traps developed for some leafminers will help determine if a pesticide will be needed. In the case of the elm leafminer, there is a degree day model for predicting egg-laying by adult sawflies.
Low numbers of leafminers provide food for the parasitoids that, in turn, keep number of mines low. Handpick and remove mined leaves or squish larvae within mines. Select cultivars that are less susceptible to leafminers. There is evidence that leafminers thrive when plants are stressed, so ensure that landscape plants are planted in the right place and receive water adequate for their needs.
Many leafminers, even those that occasionally build to large populations, have natural enemies, such as parasitoids, that attack them. Often, about the time damage is intolerable, parasitoids readily find prey in which to lay eggs and by the next generation the leafminer population collapses. Healthy plants can recover from heavy defoliation. A plant under heavy attack will "dump" its leaves by forming a premature abscission layer dropping the leaf from the plant. The leafminer will perish unless it is already nearing pupation. Parasitoids may be seen in or on the leaf mining larvae.
See Table 3 in:
Chemical Control of Landscape Pests
For more information
Buss, E. A. 2006. Leafminers on Ornamental Plants. University of Florida, IFAS Extension, Publication ENY-326 (http://ufdcimages.uflib.ufl.edu/IR/00/00/28/84/00001/MG00600.pdf)
Byers, J.A. 2006. Leaf-mining insects. Chemical Ecology.