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If your plant leaves appear like a kid was doodling squiggly lines, you have leafminers. Leafminers are larval (maggot) stage of insects that live in and eat the leaf tissue of plants. They appear in spring, like young robins and daffodils, but are nowhere near as welcome. The vast majority of leaf-mining insects are moths (Lepidoptera), sawflies (Hymenoptera), flies (Diptera), though beetles (Coleoptera) also exhibit this behavior.
Host plants include apple, beans, beets, blackberries, cabbage, citrus plants, Swiss chard, lettuce, garlic, peppers, tomatoes, onions, spinach and a variety of ornamental flowers especially chrysanthemum and nasturtium, trees and shrubs.
On vegetable crops, the most common leafminers belong to the genus Liriomyza.
Adults are small (1/10th of an inch or 2.5 mm long) black and yellow flies with clear wings and are similar in appearance to small, hunch-back house flies. Sometimes the flies are more silvery-gray in color than black.
The only leafminers that lay masses of eggs on the underside of leaves in gardens are Pegomya fly species (Anthomyiidae), on Amaranthaceae (spinach, chard, etc.) and Polygonaceae (docks, garden sorrel; occasionally rhubarb, but the larvae always seem to die pretty quickly in rhubarb leaves). All agromyzid flies (such as Liriomyza) insert their eggs in plant tissue, and these are impossible to see, although sometimes you can see discolored spots where the female inserted her ovipositor. They are tiny, cream-colored and oval.
The larvae of leafminers are bright yellow or pale green, stubby, translucent maggots that are about 2 mm long when they emerge from the leaves to pupate. Pupae look like tiny brown grains of rice. Leaf-mining flies though, don’t make cocoons.
Adults emerge from overwintering cocoons in early spring and lay their eggs in clusters just under the surface of the leaf epidermis. The larvae mine leaves for 1 to 3 weeks till mature, then drops down to the soil and pupates for 2 to 4 weeks. There are normally 2 to 3 generations per year; more in greenhouses.
Larvae mine between upper and lower leaf surfaces, creating winding, whitish tunnels that are initially narrow, but then widen as the larvae grow. This may cause leaves to dry out, resulting in sun-burning of fruit and reduction and yield and quality. Excessive damage from larvae on young plants and seedlings can cause them to die due to lack of chlorophyll and reduced photosynthesis.
Mines and feeding punctures also open up ways for pathogenic organisms to enter. Excessive leaf mining may cause death of the plant.
Organic Control and Prevention of Leafminers
1. Check your transplants for leaf mining damage before planting them out in your vegetable garden.
2. Cover plants with floating row covers to keep adults from laying eggs on leaves. Protect small seedlings by covering them with a protective cloth or a cold cap.
3. Pick off and destroy infested leaves.
4. Do not plant vegetables in areas where you’ve had previous leafminer infestations.
5. Cut forage crops, remove remaining old plants and till the ground after harvesting your crops. This can significantly reduce leafminer populations by destroying pupae in the soil and eliminating several sources.
6. Scout for egg clusters on susceptible plants regularly and destroy them as soon as they are visible.
7. If available, plant vegetables varieties that are less susceptible to leafminer infestation, such as tomato with curled leaves.
8. Leafminers on vegetable crops have been kept under control by their natural enemies, in particular parasitic wasps such as Diglyphus begini and Chrysocharis parksi. These wasps feed on leafminer larvae and can significantly reduce leafminer populations. Diglyphus isaea is a commercially available beneficial insect that will kill leafminer larvae. Encourage these parasitic wasps by planting nectar-and pollen-rich flowers such as dill and yarrow.
9. Leafminer infestation can be reduced or prevented by planting trap crops near the plants to be protected. Lamb’s-quarter, columbine and velvetleaf will distract leafminers, drawing them to these plants and thereby reducing the incidence of attacks on nearby crops.
10. Spray infected plants with Spinosad, an organic insecticide, which will control the leafminer. Spinosad does not kill on contact but will have to be ingested by the leafminer to take effect.
11. Spray Neem oil to control or prevent leafminer infestations. Neem oil will break the leafminer’s life-cycle by preventing larvae from reaching maturity. It also acts as an anti-feedant, a repellant and it interferes with egg laying activities.
12. Use sticky yellow traps to catch egg laying adults.
13. Cover the soil under infested plants with plastic mulch to prevent the larvae from dropping down into the soil and pupating.
14. If using trap crops to distract leafminers is not on your agenda, control weeds such as lamb’s-quarter and dock which are known leafminer hosts.