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Garden Help Desk: How to deal with leafcurl ash aphids

By Usu Extension - | May 22, 2021
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Scout for aphids on fruit trees at least twice a week. The first place to check is on the undersides of leaves at the ends of twigs and branches.

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Aphid feeding can cause leaves to curl or twist. It can be hard to control aphids once this happens because they are protected inside the curled leaves.

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Leafcurl ash aphids spend the winter underground on ash roots. In the spring they leave the roots to migrate up tree trunks to reach the leaves, where they will feed until late summer.

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Large numbers of leafcurl ash aphids can sometimes be seen on ash trees as they migrate to foliage in the spring.

Question: There are white bugs on my ash tree. What are they and how can I get rid of them?

Answer: These look like leafcurl ash aphids. They spend their winters on the roots of ash trees. The aphids migrate up the trunks in the spring and feed on the foliage, causing the leaf twisting and leaflet distortion that you’re seeing. Near the end of the summer the aphids will move back down the trunk to spend the winter on the roots.

Their fuzzy appearance comes from a waxy coating that the aphids exude to protect themselves from predators. That waxy coating, combined with the curling leaves that shelter the aphids, means insecticidal sprays won’t be very effective, so spraying isn’t recommended.

The feeding damage can look terrible, but it’s usually just a cosmetic issue and seldom affects the health of the tree. If you do want to protect the appearance of your tree you can use a soil drench with a systemic insecticide next spring when the buds begin to swell. Look for a tree and shrub insect control product with the active ingredient Imidacloprid. Read the product label carefully. It will tell you how many ounces of insecticide you need to mix into a gallon of water that you’ll pour slowly at the base of the trunk.

Question: We didn’t get our peach tree sprayed with dormant oil this spring and now it looks too late. What should we do now?

In the spring each year, when the buds begin to swell, fruit trees should be sprayed with a 2% dormant oil spray to suffocate aphid eggs, mites and some other pests on fruit trees. For a peach tree, aphids are a common problem. A delayed-dormant oil spray doesn’t get rid of all the aphids, but it can reduce the population to a more manageable level. You didn’t get that done this spring, so you’ll want to keep an eye on your tree. Make a habit of checking the tree about three times a week, looking for aphids. This is a good idea every year, not just when you’ve missed the dormant spray. Focus on the young leaves near the ends of twigs and branches. The leaves will curl or roll if aphids are feeding. Also look at the undersides of the leaves; that’s a common hangout for aphids.

At the first sign of aphids, you can begin doing some aphid control. Your choice of control options — from absolutely no pesticides, soft or biological control products, or conventional insecticidal spray products — will depend on your gardening style.

The easiest, most accessible method you can try is water. You can hose off the tree with a strong spray of water (no need to power wash the tree, though). This will remove a lot of the aphids. Aphids are fragile; if you knock them off, they won’t be getting back onto the tree. Hose down the tree two or three mornings a week for a few weeks to reduce the aphid population and then keep checking the tree for new aphid problems.

You can also use softer control products — insecticidal soap or 1% horticultural oil spray. These products aren’t poisons; horticultural oil suffocates the aphids and insecticidal soap breaks down the protective surface on their bodies and damages cells. Either of these sprays can damage leaves if they’re applied in hot weather. You’ll want to spray in the evening to give the spray plenty of time to dry while temperatures are cooler. The slower drying during cooler evening temperatures also makes the spray more effective. Thorough coverage of the leaves is important because the spray only works on direct contract. There is no residual effect after it dries. Aphids that are inside the curled leaves will be protected from sprays. Repeat the application as needed at the interval recommended on the label for a few weeks.

You should get a little break from managing aphids because they usually leave the trees by mid-summer and then return in the early fall to lay eggs for the following year’s population.


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