Stressed plants are more susceptible to pests and diseases just as stressed humans are more susceptible to health problems. Often, when a plant is stressed by a nutrient deficiency, it becomes a better source of food for insects or mites. The primary (macronutrients) nutrients needed by a plant are nitrogen, phosphorus and potassium. Secondary nutrients are calcium, magnesium and sulfur. Micronutrients include iron, boron, manganese, zinc, molybdenum and copper and are needed in small amounts. A plant gets these nutrients from the soil.
As part of its defense system, a plant breaks down and mobilizes nitrogen in the face of a deficiency, moving it away from the stressed plant part to conserve it for growth, reproduction or defense in healthier plant tissues. It also does this in the face of other stress factors such as water deficiency, negative environmental impacts or injury.
As nitrogen is mobilized it becomes more available to insects feeding on the plant. The increase in available nitrogen then improves the survival rate of the insects feeding on the stressed plant. This can be a localized and possibly short-lived response or widespread and persistent. With more young surviving, beneficial predators and parasites are less able to contain pest populations (White, T.C.R. Oecologia. 1984).
Potassium is essential for plant growth and metabolism and is critical for plants to resist stress factors, including insects and diseases. The effects of potassium or other nutrients on stress resistance are influenced by the type of plant, the amount and kind of nutrient source as well as the type of insects. For example, in the case of nitrogen, fast release nitrogen sources such as urea can cause insect populations to surge. However, slow release sources do not.
Although we might think that fertilizing the plant to eliminate nutrient deficiencies would reduce feeding by insects and mites, there is a fine balance between adequate nutrient availability and too much. Overabundance of nutrients can actually encourage increased feeding. We often see higher aphid populations on plants in the spring and early summer as new growth, lush with nitrogen, develops.
There is an elegant interaction between plants and insects. One research team calls it an “evolutionary arms race” (War, R.A. et al. Plant Signal Behavior. 2012). Feeding the soil rather than the plant by adding organic matter, appropriate amendments and nutrients after doing a soil test can help keep plants vigorous and more resistant to insect infestations.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.