The Q&A Archives: Rose Foliage Has Translucent Holes

Question: I've had this problem with my climbing roses for many, many years. Each summer, beginning around late June, the leaves on my climbing roses begin to develop many small translucent patches (visible on both sides of the leaf) and then evntually the leaves develop holes. I'm not certain if the damaged patches turn into the holes or not because often there will be both holes AND patches simultaneously. Over time, the leaves disappear (are consumed?). The damage begins at the tips of the canes and works back towards the roots. The canes generally remain healthy looking but will blacken at the tips. If I do not cut off the diseased looking parts, the damage will travel towards the roots.

I occasionally find a small green caterpillar on the leaves. There also seems to be tiny white insects on the underside of the leaves. The roses are draped upon a fence and do not come into contact with the ground. I feed them beginning in the spring and ending in August. No matter how much damage they sustain over the summer, they always come back vigorously the following year.

I've never been able to fit this problem neatly into any rose pest/disease category. They are so beautiful right now, but in the past week or so the above mentioned symptoms have begun to appear again. Any solutions?

Answer: I found the answer! Your culprits are roseslugs. They aren't slugs at all -- they are sawfly larvae. The adults, which are wasplike, lay eggs between the leaf layers, and the larvae emerge to feed, leaving "windows" on the leaves. As they grow, they consume entire leaves. The larvae drop to the soil to pupate, and in some places there may be multiple generations during a summer.

Fortunately, your good care has kept your rosebush hearty. You can help it further by spraying a neem-based insecticide /repellent may repel the adults. If the tiny larvae do emerge and start eating, a dose of neem should alter their appetite. Insecticidal soap is another option.

If, despite your control of the roseslugs, the cane tips still blacken, then you may have rose midges feasting there. Tiny orange larvae emerge from eggs laid by tiny flies, and feast on the tender tips. Pruning is the best method of control, though perhaps the neem may repel them as well. Best of luck!

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