Stress Encourages Pest Problems
It is highly likely that when Japanese Beetles attack a plant in sufficient numbers to be obvious, that the plant was experiencing some degree of stress before the beetles appeared. Hybrid tea roses are the exception. They can be totally healthy and still be a magnet for beetles.
Pest insects target plants that are already struggling for some reason and lack the vigor to fend off their attacks. Researchers are examining the effects on insects of glutathione, a chemical produced by stressed plants. They have found that glutathione is actually beneficial for certain harmful insects--it aids their reproduction and growth, and may even bolster their ability to resist pesticides. So that may explain why Japanese beetles and other pest insects zero in on stressed plants. After you deal with the immediate Japanese beetle problem, try to figure out what might be causing the affected plant to be vulnerable to pest attack.
Some Causes of Plant Stress
Too much or too little sunlight or water.
Inappropriate plant for the climate or local conditions.
Excessive use of nitrogen-rich fertilizer which encourages lush leafy plant growth.
Drastic pruning of trees or shrubs, which stimulates growth of tender suckers.
Use of broad spectrum pesticides which also kill off local beneficial predators.
Transplant shock which weakens plants for a few days to a week.
Other Conditions That Foster Japanese Beetles
Japanese beetles seem to swarm in cycles, like the proverbial locusts. Every 6 or 7 years beetle populations seem to explode and even relatively pest free landscapes are badly damaged. In these years controlling the beetles is critical, so that they do not lay eggs in your yard and come back in droves next year.
Various kinds of birds and beneficial insects that normally live in healthy home landscapes, which have a diversity of plants and wildlife, routinely control Japanese beetle populations. However, where there has been frequent use of broad-spectrum insecticides which kill insects indiscriminately, nature’s first line of defense--the beneficial insects--is eliminated along with the pest insects. Since pest populations rebound faster than those of their predators, the beetles can establish themselves unmolested.