(News article for October 29, 2022; edited)
I was recently asked to take a look at some mint plants that had distorted, curled leaves. They had been growing in a greenhouse but had been placed outside, since the grower could tell there was something wrong with them.
There are several common causes of distortion or curling of plant leaves. Among these are some plant viruses. For example, tomato yellow leaf curl virus (TYLCV) and tomato spotted wilt virus (TSWV) can cause distortion of tomato leaves. Spray drift or soil residues from herbicides in the synthetic auxin group (2,4-D; dicamba; picloram; etc.) can also cause leaf curling.
In this case, the culprit turned out to have been aphids, often called “plant lice.” Aphids are among a number of insects and mites that get nutrients by piercing plant tissues and sucking sap out of plant cells. This type of feeding sometimes causes leaf distortion.
By the time I looked at these plants, though, I didn’t find any living aphids. Instead, I saw a couple of things we often see when aphids have been present: aphid skins (or the exoskeletons they shed as they molt) and aphid “mummies,” or parasitized aphid bodies.
Very small insects in the Aphidiinae subfamily of braconid wasps lay eggs in aphid bodies. After the eggs hatch, the larvae feed on the aphid, eventually killing it. The wasp pupates inside the skin of the aphid and leaves as an adult through a hole that it has made in the skin. From there, it may lay eggs in another aphid and continuing the cycle.
This type of naturally occurring biological control is one reason to avoid applying insecticides without a good reason and, when they’re needed, try to find ones that target the pest insect rather than using a product that kills a broad range of insects. Sometimes, populations of pest insects or mites are actually larger where insecticides have been applied, because the beneficial ones that would normally help keep the pests in check have been killed.
When shopping for plants, check them for evidence of pest insects before purchasing them. If you find insect damage on plants in your garden, check to see if living pests are still present. In a home garden, aphids can often be dislodged with a moderately strong spray of water.
Insecticide options for herbs are limited, but some insecticidal oil (including both mineral and plant-based oils) and insecticidal soap products are labeled for use on herbs. These can be effective against soft-bodied insects like aphids, if you can get good coverage of plants and direct physical contact with the insects. Aphids are often found on the undersides of leaves, so getting good enough coverage for direct contact can be challenging. These insecticides are safer for beneficial insects than many other products are, because they lack residual activity.
Some products with pyrethrins are labeled for use on herbs, as well. These are broad spectrum insecticides that have a small amount of residual activity, though the residual activity of pyrethrins is short-lived.
Before using an insecticide, be sure to make sure that it’s labeled for use on the plant(s) on which you plan to use it. Read and follow all label instructions.
Let me know if you have questions.
Contact Mary Helen Ferguson.
Mint leaves distorted by aphid feeding. (Photo by M.H. Ferguson)
Remains of parasitized aphid, with hole left by parasitoid wasp as it exited. (Photo by M.H. Ferguson)
Aphid skins that were shed as aphids molted. (Photo by M.H. Ferguson)