Irene Shonle, Director CSU Extension in Gilpin County.     Have you been seeing some odd-looking leaves on trees and shrubs lately? Are you wondering what’s going on with them?

Let’s start with aspen. Eriophyid mites are common culprits of deformed aspen leaves (other plant species can also be affected by different species of the mite, but aspens are the most common hosts in the mountains). They cause anything from a few raised whitish or reddish bumps on a leaf, or they can transform an entire leaf into something gnarled and finger-like (see pictures above).

It’s quite remarkable that these tiny insects (you almost need a microscope to see them) can cause such dramatic effects. As they feed or lay eggs on young leaves, the mites transfer a growth hormone to the leaf – this is what causes the abnormal growth (gall). The galls are only formed when the leaves are small; fully expanded or mature leaves do not respond to the growth hormone. Mites then hatch and develop in the gall that they create.

Despite the drama, the problems the mites cause are usually just aesthetic. In most cases, the mites will not hit the same tree two years in a row, and they rarely cause much damage. The odd-looking leaves can be removed if desired, but this is rarely necessary. Predatory mites will sometimes take care of the problem, or the mites may simply move on.

On other species, aphids are the cause of gnarly, cupped leaves. Chokecherry aphids (see picture) and green peach aphids can build up in great numbers, and their extensive feeding causes all new growth to twist, curl and dry. They also produce a sticky, shiny honeydew which can attract yellow jackets and ants.

If the plant is heavily infested, it may not flower, since most flowers and fruit arise from new growth.  Other than that, the overall health of the plant is not usually greatly impacted. Natural controls such as ladybugs, bald-faced hornets and other predators will often take care of the aphid population, and the leaves will return to normal the following year. However, if several years go by and the damage levels do not improve, then it might be time to take action.

Both the mites and the aphids can be controlled with horticultural oil applied during early spring (before leaf buds emerge). This will smother the overwintering eggs and adults.  These oils can be purchased at a garden center; follow the directions on the label.

The CSU Gilpin County Extension Office is located at the Exhibit Barn, 230 Norton Drive, Black Hawk, CO 80422, 303-582-9106, www.extension .colostate.edu/gilpin.

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