Stress Encourages Pest Problems
It is generally true that when aphids attack a plant in sufficient numbers for you to begin to see damage, then chances are it was already unhappy and experiencing some degree of stress BEFORE the aphids appeared. Pest insects tend to target weakened plants that are struggling and lack vigor for some reason. 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 pest insects. It aids their reproduction, growth, and may even bolster their ability to resist pesticides. So the plant’s misfortune is an aphid’s good fortune.
Because of this phenomenon it is important to try to figure out the source of the affected plant’s stress that made it vulnerable to pest attack. Some causes of plant stress include:
· Lack of sufficient sunlight or water (or, too much water causing soggy soil)
· Inappropriate plants for the climate or local conditions
· Excessive use of nitrogen-rich fertilizer, which encourages too much tender, leafy plant growth.
· Drastic pruning of trees or shrubs, which stimulates growth of succulent suckers (side shoots)
· Use of pesticides against other pests which also kills off the aphids’ natural predators and parasites
· Transplanting shock which stresses plants vulnerable for a few days to a week.
· A temporary springtime population explosion of aphids prior to the emergence of their natural predator insects such as ladybugs.
Other Conditions That Foster Aphids
Various kinds of birds and beneficial insects normally control aphid populations 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. Since pest populations rebound faster than those of their natural enemies, the aphids can establish themselves unmolested.