People who garden or keep houseplants are bound to encounter insect pests eventually. As I mentioned in an earlier post about fungus gnats, keeping track of the insects in your plants and gardens is the first step in managing the problem. Fungus gnats are easy to monitor using yellow sticky traps. Some pest, however, are harder to find. Sometimes damage on the plants themselves is what alerts you to a problem.
This happened to me last week with my wood sorrel (Oxalis Acetosella) houseplant. I noticed that its newest mature leaves were crimped. And the older mature leaves had clear patches on them.
The crimping is caused by early damage to young leaves by thrips and aphids. In order to determine which problem I had, I gently shook the leaves over the table. Juvenile thrips and live aphids were crawling around on the tabletop! And not only live aphids, but a whole bunch of their dried molts. Aphids, like other insects, shed their exoskeletons periodically in order to grow larger. This process is called molting. The presence of aphid molts tells me that they have been there for a while.
The reason that it took me so long (too long!) to spot this infestation is that there is a delay between when the damage occurred and when it becomes visible. The crimping in the leaves is caused by thrips or aphids feeding on very young leaves. As the leaves grow, the damaged areas cannot, distorting the leaves. This problem is certainly not specific to wood sorrel. In fact, crimping and other distortion from thrips is a big problem for many crops. Strawberries, for example, can have serious damage from thrips. When thrips feed on strawberry flowers, you cannot see it at the time. But it causes fruits to mature in irregular shapes or never fully ripen. The frustrating part is that you don’t know that there was damage to the flower until the fruit is already formed!
So keep your eyes open for any sign of insect damage. Shaking the plants gently onto a white sheet of paper is a good way to see what is on them.