Growing Things: How to deal with lilac leaf miner

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Q: This summer, my lilacs had leaves that curled up, and upon examining them, there seemed to be a worm inside. The leaves then turned brown. I tried to pick off the affected leaves, but the lilacs are too tall to do this. Is this some sort of miner?  What can I do to prevent this from occurring again next year?  Is there some kind of spray to use before they appear?

A: The problem you are describing is caused by the lilac leaf miner. The adult of this insect is a brown-coloured moth about 6 mm long. They usually appear around the end of May to the beginning of June, laying their eggs on the underside of the leaves. These eggs then hatch, and the small, green larvae burrow into the leaves and begin to mine in the leaves.  Initially, you will notice the feeding as brownish, discoloured spots that progress into larger blotches.

After approximately 3 weeks of chowing down, the larvae emerge and roll up the leaves and then feed on the inside of the rolled leaf.  A few days later, they drop to the ground and pupate in the debris just below the soil surface. A second generation of moth then emerges, and the cycle is repeated. The second generation continues to feed until mid-September, at which time the mature larvae drop to the ground and spend the winter in the pupal stage. They awaken in the spring, and the cycle begins again.

I have offered this amount of detail to show it is very difficult to treat these pests with any pesticides. When mining the leaf or inside the rolled leaf, it is impossible for any spray to reach the insects. The way I treat my leaf miners is to pick off and destroy any rolled leaves. If you check your shrubs on a regular basis and remove any rolled leaves immediately, this can be a very effective way of dealing with the problem.

Q: We have a friend who wishes to give us a peony. What is the ideal time of the year to transplant the plant? Should it be placed in a sunny location? What soil conditions does a peony prefer? At the time of transplanting, should bone meal or a slow-release fertilizer be added to the soil?

A: The fall before freeze-up is a good time to transplant peonies and other perennials. It should definitely have a sunny location, as they perform best in full sun. The soil should be one that is rich in organic matter, so when planting, work in lots of compost or well-rotted manure. The area you are planting in should also be well drained. Peonies do not like to have their “feet” wet. I always put a handful of bone meal into my perennial planting holes.

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Learn more by emailing your questions to filipskigerald@gmail.com, reading past columns or my book Just Ask Jerry. You can also follow me on X (Twitter) @justaskjerry01.