P&PDL Picture of the Week for
August 18, 2005

Cottony Maple Scale

Cliff Sadof, Extension Ornamental Entomology Specialist, Purdue University

The maples in the photos beloware infested with cottony maple scale. The sap is the excrement of these sucking insects. The white cotton tufts on the twigs are egg masses. There has been an increase in these this year. My guess, based on some recent research papers, is it has something to do with increased applications of mosquito adulticides killing the natural enemies of scales.

Right now the scales should be in the nymphal stage feeding on the undersides of leaves. You should find them (without cotton) attached to leaf veins. I suggest you do the following:

  1. Assess the natural enemy population. Inspect the trees for twice stabbed lady beetles. These black beetles with orange (red) spots feed on scales. The larvae may also be found in the egg masses themselves. The larvae resemble mealybugs, as a way to hide from the ants feeding on the honeydew. Also check the scale nymphs on leaf undersides. Some may be parasitized and appear black.

  2. If you can find many natural enemies, then just leave the problem alone. It should lessen over the next 1-2 years and be gone by yr 3.

  3. If you find few or no natural enemies, then apply imidacloprid to the soil in early spring. This will kill the scales next year. Applications at this time of year may be marginally effective due to poor uptake by scales who really do not feed much in the summer. The soil applications should stop the honeydew dropping at this time.

Please see E-29 for more details.

Click on image to enlarge

Larval stages of predatory beetle that feed on cotton maple scale and crawlers on leaf

Cottony maple scale egg mass on twig

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service