Leafmining insects— characteristics and control Whitney S. Cranshaw, David A. Leatherman and James R. Feucht1 COLORADO STATE UNIVERSITY COOPERATIVE EXTENSION no. 5.548

Leafminers rarely are abundant enough to be injurious to the plants they infest. However, leaf- miner injury often is conspicuous and can make Quick Facts plants unattractive. Leafy vegetables, such as Leafminers develop and live within leaves spinach, swiss chard or beets, often are consi- of plants and injure the leaves by feed- dered more heavily injured by leafminers since ing on the soft interior tissues. edible parts are directly damaged. On woody Broadly categorized by the shape of "mines" plants, certain leafminers, such as the blackmine produced by their feeding, leafminers blotch leafminer of cottonwood, are a chronic and include many different insects. severe problem in parts of Colorado. Leafminers rarely are abundant enough to be injurious to the plants they infest. The more common leafminers in Colorado Colorado Leafminers are: spinach leafminer, cottonwood Dozens of leafminers occur in Colorado. Among blackmine blotch leafminer, lilac leaf- the more common are: miner, birch leafminer and ponderosa Spinach leafminer: This is a small fry larva pine needle miner. commonly found infesting spinach, swiss chard, beets and lambsquarters. Several generations occur during the year, but are particularly com- mon during the growing season's cooler months. Cottonwood blackmine blotch leafminer: This beetle larva produces an unusually dark and dis- tinctive blotch leafmine. The adult yellow and black beetle can be found chewing and skeletoniz- ing small areas in the exterior leaf surface from late June through July. Eggs also are laid during Leafminers are insects that develop and live this period, and later the larvae initiate mines. within the leaves of plants. Typically, the leaves Lilac leafminer: A small moth larva pro- are injured by the insect feeding on the soft inte- duces the common leafminer of lilac. Its injury is rior tissues so that only the papery-thin covering rather unusual since the older larvae often move of the exterior leaf surfaces remain. out of the mines to fold and web together the leaf. The leafminers often are categorized broadly There are several generations of the lilac leaf- by the shape of "mines" produced by their feeding. miner each year. Serpentine leafminers make mines that zig-zag Birch leafminer: The birch leafminer is a through the leaf and gradually enlarge from be- small wasp, called a sawfly, with two generations ginning to end Blotch leafminers make large per season. Many kinds of birch are very suscept- indistinctly shaped mines (Figure 1). Blotch leaf- ible to the injury of this insect, but most cultivars miner injury often is mistaken for leaf splot grown in Colorado are somewhat resistant. Birch diseases. leafminer only recently has been recorded in the Many different insects are leafminers, includ- state. ing certain flies, wasps, moths and beetles. The immature (larval) stage of these insects produces the distinctive mines. For some leafminers, the pupal stage also takes place within the leaf. More commonly, the insect larva, after it completes feeding, cuts a hole Whitney S. Cranshaw, CSU extension assistant in the mined leaf and drops to the ground to professor, entomology; David A. Leatherman, pupate. Adults move about freely to mate, lay entomologist, Colorado State Forest Service; and eggs and feed. Leafminer eggs are laid on the James R. Feucht, CSU extension professor, horti- leaves or are inserted just under the leaf surface. culture (7/85)

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with To simplify technical terminology, trade names of the U. S. Department of Agriculture, Kenneth R. Bolen, Director of Cooperative Extension, Colorado State products and equipment occasionally will be used. University. Cooperative Extension programs are available to all without discrimination. No endorsement of products named is intended nor is criticism implied of products not mentioned. Ponderosa pine needle miner: Larvae of this and mines get very big. Insecticides with sys- tiny silver moth tunnel down the needles They temic activity include acephate (Orthene), dimeth- start near the needle tips in September and by the oate (Cygon) or oxydemetonmethyl (Metasystox following July, make it two-thirds of the way to R). Carefully read labelled instructions since cer- the needle bases. The result is brown needles with tain insecticides can cause injury to plants. None green bases. Heavily-infested trees can look quite of the systemic insecticides available to home- sickly in early summer but studies have shown owners can be used on food crops. most needle miner impacts to be only temporary. Control also can he achieved with an insecti- (See Service in Action sheet no. 5.544. cide that is applied when eggs, are being laid. Adults and newly emerged larvae can be con- Control of Leafminers trolled with these treatments but they are ineffec- On ornamentals, leaf miner control is rarely tive after tunneling begins. On trees and shrubs, justified. Although unattractive, little or no dam- diazinon and carbaryl (Sevin, Sevimol) is used age to the plants occurs from the mining. Also, most often for these treatments. there is little relationship between injury from On leafy vegetables damaged by the spinach one season to the next since many natural con- leafminer, regular sprays of diazinon applied at trols effectively regulate leafminer populations. two-week intervals can limit injury. Systemic When necessary, leafminers on ornamentals insecticides can not be used on these edible crops. are controlled best with applications of insecti- Closely follow labelled instructions for applica- cides that are carried systemically throughout tion rates and observe required preharvest inter- the plant. This activity allows the insecticide to be vals (10 days, spinach; 12 days, swiss chard; 14 carried to the insect larvae, which otherwise are days, beet greens). Malathion also may be used for protected effectively within leaves. These appli- these treatments if applied at more frequent cations should be made during the egg hatch intervals. Preharvest intervals on these crops are period or shortly afterwards, before the larvae seven days.

Figure 1: Blotch leafminer. Figure 2: Serpentine leafminer.