I recently noticed that the filet beans that I had planted had germinated well but all of the first leaves had holes in them. On some of the plants, at least 50% of the leaf surface was gone. I couldn’t see any insects at the time so I was pretty sure that it wasn’t caused by a caterpillar. (If a caterpillar has been chewing a plant, you can usually find it somewhere on the plant.)
Have ruled out caterpillars, I remembered the old entomology adage – “bugs suck and beetles chew.”
True bugs are insects that fed by using their mouthparts to suck the juice from a plant. The true bug everyone in PA is talking about is the brown marmorated stink bug. This pest doesn’t bite or chew; instead it inserts its tube-like mouthparts to suck the sap of a plant. The brown marmorated stink bug can’t chew the leaves of an apple tree but it can suck the juice from a growing apple which will cause the mature fruit to be disfigured.
On the other hand, beetles are insects that chew plants and usually create holes in the leaf tissue. A beetle that every gardener in PA knows about is the Japanese beetle. These insects can turn a rose leaf into a skeleton in no time because they’re chewing insects.
Well back to the beans – since I saw holes I knew that something was chewing them and that something was probably a beetle. After a little online search, I found the culprit – the bean leaf beetle.
This 1/4 inch long beetle ranges in color from yellow to red and may or may not have spots. It overwinters as adults in leaf litter and other protected areas, emerging in the spring to feed and reproduce. The female beetle lays her eggs at the soil surface near beans and other host plants. The larva feed on the roots of the plant and after about three weeks build a little cell in the soil where they pupate. A week later, adults emerge and start the cycle again.
The damage to the beans is worse than I’ve ever seen. I bet that’s because of the very mild winter we had – a mild winter means that more of the beetles survived. The good news is that this beetle is easily controlled by spraying the plants with an insecticide containing pyrethrins, an organic substance that’s extracted from a particular kind of chrysanthemum. One spray of a pyrethrin based insecticide and the new leaves have no holes and look great.
I’ll have to keep an eye on the plants in the coming weeks, but if I see more holes in the bean leaves, I now know that they’re caused by bean leaf beetles and they’re easily controlled.