Stress Encourages Whitefly Problems
It is generally true that when whiteflies attack a plant in sufficient numbers for you to begin to see damage, then it is very likely that the plant was experiencing some degree of stress BEFORE the whiteflies 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, growth, and may even bolster their ability to resist pesticides. Other research shows that increased levels of soluble nitrogen compounds in the sap of plants that are damaged or deteriorating from old age (“senescing”) may enable pest insects to thrive.
Some Causes Of Plant Stress
After you deal with the immediate whitefly problem, try to figure out what might be causing the affected plants to be 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 leafy plant growth--perfect whitefly food!
Natural aging (senescence) and deterioration of the plant
Drastic pruning of trees or shrubs, which encourages growth of succulent suckers (side shoots).
In the end, you may never figure out what caused the plant’s stress, but it’s always good to think about it when whiteflies show up. If you eliminate the stress in the plant, the whiteflies may be less likely to come back!
Other Conditions That Foster Whiteflies
Whiteflies 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 whiteflies easily reestablish themselves.