Blotch leafminers make large indistinctly shaped mines. Blotch leafminer injury often is mistaken for leaf splot diseases. Many different insects are leafminers, including certain flies, wasps, moths and beetles. For some leafminers, the pupal stage also takes place within the leaf. More commonly, the insect larva, after it completes feeding, cuts a hole in the mined leaf and drops to the ground to pupate.
Adults move about freely to mate, lay eggs and feed.
Leafminer eggs are laid on the leaves or are inserted just under the leaf surface. Leafminers rarely are abundant enough to be injurious to the plants they infest. However, leafminer injury often is conspicuous and can make plants unattractive. Leafy vegetables, such as spinach, swiss chard or beets, often are considered more heavily injured by leafminers since edible parts are directly damaged. On woody plants, certain leafminers, such as the blackmine blotch leafminer of cottonwood, are a chronic and severe problem in parts of Colorado.