Stress Encourages Pest Problems
When gypsy moths attack your trees and shrubs in sufficient numbers for you to begin to see damage, it is likely that the plants were experiencing some degree of stress BEFORE the moths appeared. Pest insects prefer to go after weakened plants that are struggling and lack vigor for some reason. How is this so? Researchers are examining the effects on insects of glutathione, a chemical produced by stressed plants, and 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.
Other research shows that increased levels of soluble nitrogen compounds in plants that are either damaged or deteriorating from old age (“senescing”) may enable bugs to thrive when they feed on the nitrogen-rich materials. If your trees and shrubs are being defoliated but gypsy moth levels are low, then the plants were stressed before the caterpillars came, and you should investigate the source of that stress after you deal with the caterpillars.
Some Causes Of Plant Stress
After you deal with the immediate gypsy moth problem, try to figure out what might be causing the trees and shrubs to be vulnerable to pest attack. Some causes of stress include:
Insufficient sunlight or water (or too much water, causing soggy soil)
Soil is compacted and contains little or no organic material
Inappropriate plants for the climate or local conditions
Excessive use of nitrogen-rich fertilizer, which encourages too much leafy plant growth--perfect caterpillar food
Pre-existing injury from other insects such as borers
Drastic pruning of trees or shrubs, which encourages growth of succulent suckers (side shoots)
Shock resulting from transplanting, which makes plants vulnerable to gypsy moths for a few days to a week
A temporary springtime superabundance of gypsy moths that later levels off when natural enemies and diseases catch up.
In the end, you may never figure out what caused the plant’s stress, but it’s always good to think about it when gypsy moths show up. If you eliminate stressful conditions, the plants can more easily resist the worst effects of a gypsy moth outbreak.
Other Conditions That Foster Gypsy Moths
Gypsy moths are usually controlled by various kinds of birds and beneficial insects living in healthy home landscapes that host a diversity of plants and wildlife. 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. Because pest populations rebound faster than those of their natural enemies, the gypsy moths easily reestablish themselves.