In the middle of the hot summer, insect populations build to increasingly damaging levels. Pest control becomes harder this time of the season, and two key little-known issues arise for herbivorous insect control.
Drought: Plant stress due to heat and drought can lead to higher herbivore numbers, especially if that stress is intermittent. For example, let’s say you have a shrub, and aphids really like that shrub. You do not water it regularly, and the plant lives outside in the heat of the summer. The weather has stayed dry for a while, and your shrub is starting to stress. Sap feeders like aphids do not usually respond well to drought, because plant sap is powered by turgor pressure, and the sap must move for them to eat. No rain, no food. However, leafy plants produce more nitrogen in times of stress. Your dry, stressed plant produces tons of nitrogen and then (viola!) it rains. The aphids suddenly have everything they need to produce tons of baby aphids.
Fertilizers: Nitrogen fertilizers are generally applied to improve plant growth and help them cope with biotic and abiotic stresses. However, although maintaining plant health is the first line of defense against herbivore attack, trees that are over fertilized are often more vulnerable. This occurs by improving plant nutrition for herbivores. Just as nitrogen is essential for plant growth, it also increases herbivore growth. Therefore, plant tissue that is high in nitrogen due to fertilization makes herbivorous pests grow faster and reproduce more. Fertilizer also benefits herbivores by reducing plant defenses. This happens because nitrogen stimulates rapid plant growth which requires all the carbon fixed through photo-synthesis to be used for plant tissue instead of defensive chemicals. Therefore, plants do not build chemicals toxic to herbivores or chemicals that attract natural enemies such as parasitoids to help defend themselves. Therefore, be sure to calibrate fertilizer application equipment, and do not apply more fertilizer than is recommended or necessary to maintain plant quality.From: Steve Frank, NCSU Extension Entomologist