ISSUE 9   July 8, 2010


Over the last few weeks, we’ve begun to observe scale insects feeding on ash trees. The specific scale we’ve seen is the European fruit lecanium scale (Parthenolecanium corni). What people most often notice is the reddish brown protective cover of the female scale (Figure 1).

Figure 1. Female scales on ash twigs. Insecticides
are not effective against the hard-shelled adult scales.

However, female scale insects are not the life stage that causes the damage. Crawlers hatch in June and July. They disperse from underneath the female scale to feed on the underside of the leaves next to the main-order veins (Figure 2), which may damage or even kill twigs and branches. Crawlers are also the life stage that is most susceptible to insecticide treatments. In late summer, crawler move back to the twigs to overwinter. There is one generation a year.

Figure 2. Crawlers of the European fruit lecanium
scale on the underside of a green ash leaflet.  High
populations of crawlers may warrant control with insecticides.

Small populations of the scales rarely deserve control. However, where populations are high, control may be warranted for either individual trees or perhaps entire shelterbelts. Trees that were hard-hit by ash anthracnose earlier this year are particularly susceptible to additional damage from the scales.

Egg hatch occurs for only a short period of time, so monitoring for crawlers and proper insecticide timing are crucial for effective control. To monitor for crawlers, place a white piece of paper underneath the branch and vigorously tap the branch to dislodge crawlers onto the paper or wrap the branches with a wide, double-sided sticky tape near active European fruit lecanium scale insects. Inspect tape often for crawlers. Once crawlers settle and insert their coiled mouthparts into the plant usually 1-3 days, it will be difficult to dislodge them. Crawlers are small and white to yellow (more mature). Also, look for any honeydew which is excreted by scale insects, and any twig or branch dieback. If beneficial insects (lady beetles, minute pirate bugs, lacewings, predaceous midges, parasitoids, etc.) are common, use short duration, low residual insecticides, such as horticultural oil, insecticidal soap, or insect growth regulators (IGR – pyriproxifen) against crawlers to conserve natural enemies. If scale populations are high, a broad-spectrum foliar insecticide application may be needed to rescue the tree. Some common insecticides include: acephate, bifenthrin, lambda-cyhalothrin, deltamethrin, permethrin, imidacloprid, carbaryl, malathion and others. These insecticides are effective in controlling only the crawlers. The window for applying an insecticide is relatively short (1-2 weeks depending on temperature). Insecticide application should be targeted at the active crawler stage (crawlers moving out onto the leaves) for the optimal control. Soil-applied systemic insecticides or trunk injections with imidacloprid (Trade names - Merit, Bayer Advanced Tree & Shrub) should be applied in the fall for control of spring crawlers. Systemic insecticides are generally less harmful to beneficial insects than foliar-applied broad spectrum insecticides. Summer oils that smother exposed eggs, crawlers and immature females and dormant season oils can also be used for control of scale insects. Oils are less harmful to beneficial insects.

Disclaimer: Always read and follow directions and safety precautions on labels. Mention of a pesticide product does not constitute an endorsement, nor is criticism meant for any products not mentioned.

Joe Zeleznik
Extension Forester

Jan Knodel
Extension Entomologist

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