Recently a local plant nursery owner brought a problem to me that someone was having with a single leaf ash. This tree, a Utah native, appeared to have something eating and causing an off color to the leaves. Pictures I saw showed damage to the leaves and black marks so I asked if the marks could be rubbed off since they appeared to be from an insect. So the homeowner reexamined the plant and found two insects feeding on the leaves including a type of green fruit worm and the ash plant bug.
There are a number of different green fruit worms, which are caterpillars, that feed on ash but I don’t currently know which genus/species this specific one belongs to. When you hear that common name, “green fruit worm,” the expectation is it feeds on fruit tree leaves and that is where it’s considered the biggest pest. That doesn’t stop it though from feeding on other plant leaves, including ash trees and shrubs like forsythia and roses. This particular caterpillar looks to be about 1.5 to 2 inches long, with a white or cream-colored stripe down the side and white speckles on the top.
The other pest found was the ash plant bug that feeds only on ash trees (Fraxinum spp). In the case of this insect the term “plant bug” is actually a part of its name, not just because it’s a bug on that plant. Plant bugs are a group of insects that use their piercing sucking mouthparts to pierce the leaves of plants and feed on the sap. The insect is somewhat oval and grows to a quarter-inch long. They can come in a variety of colors from pale yellow to brown or black. The nymphs or young are similar in appearance but much smaller and could be mistaken for aphids. These insect pests are most abundant just as the leaves are starting to open, expand and grow. Their feeding can cause leaf discoloration as well as stippling and distortion with some potential browning.
Luckily, neither of these insects is considered a big problem since their feeding largely affects the appearance of the leaves. Controls start with just living with it but can lead to spraying the leaves with a moderately strong stream of water to disrupt and kill the young. If numbers reach higher levels and you want to do more you can spray with an organic product such as a pyrethrin or with a soapy water solution.
The main issue here is cosmetic but it’s important to properly care for any plant. Even with a plant native to Utah, if it was planted and didn’t grow naturally, the plant needs more care to be at its best health, which includes appropriate and timely irrigation.
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