Photo Courtesy of John Fech.
Improper mulching, such as volcano mulching here, provides a good location for insects to overwinter.

Many factors, including those outside the ones we traditionally think of, can have dramatic affects on the development, spread and vitality of insect populations and severity of pest outbreaks. One of the most influential of these elements is the weather, or, more specifically, drought.

Dry weather and drought

While there are many ways to define drought, most refer to it as a prolonged period without rainfall that causes substantial moisture depletion in soils and plants. Others define it as going a month without a good soaking rain. Regardless, periods of extended dry weather when trees, flowers, turfgrasses and shrubs receive less than half the amount of water needed for healthy growth affects plants in two ways: dieback in the roots and shoots, and a series of events is set in motion that makes drought-affected plants more attractive to insect pests.

Everyday insect/plant interactions

To an insect, a host plant is food. Feeding on a shrub provides the insect with the sugars, proteins and carbohydrates necessary to sustain life. In addition, plants provide water. Insects commonly feed on the succulent parts of turf and the young stems of trees and shrubs to extract moisture.

The basics survival needs for all species on earth consist of food, water and shelter. A turfgrass canopy, the reduced wind speed associated with an evergreen windbreak or the bark of a tree all serve as sites for shelter from extreme heat and wind. Insects also use plants to conceal themselves from a host of insect and vertebrate predators.

A number of physiological changes occur in water-stressed plants that make them more attractive to potential insect pests.

Yellowing leaves

Many insects are attracted to yellow hues. The leaf yellowing that is commonly associated with drought stress makes the plant more attractive to insects. In fact, certain beetles are specially equipped with infrared receptors that help them locate drought-stressed trees to lay their eggs.

Sounds of collapsing plant cells

As cells in plant tissue and stems collapse and water columns break due to drought stress, ultrasonic emissions are produced. Many insects have excellent acoustic sensory capacity and can literally hear drought-stressed plants. Shade tree borers and other beetles are known for being especially well-adapted at honing in on these sounds.

Higher temperatures

Drought-stressed plants are often warmer than their non-stressed counterparts due to higher air temperatures and less evaporative cooling. These plants provide a more favorable thermal environment for insects. When insects develop at optimal temperatures, they grow faster and larger, survive at higher rates and lay more eggs. A few degrees of warmer temperature can result in a tenfold increase in insect development.

Increased concentrations of sugars and other nutrients

Drought-stressed plants often have higher concentrations of plant sugars and minerals (Mg, K, Ca, Cl) in the leaves and stems, making them a better food source and more suitable for insect growth, reproduction and survival.

What to do

This information will help you understand both what to do and what not to do. Balanced nutrition and moisture applications, use of good landscape design principles, moderate mulch placement and soil improvements go a long way toward reducing the effects of drought plants and minimizing future pest problems.

Making well-informed plant material selection decisions is also an important component in dealing with drought effects on insect populations. Seek out plant species and cultivar information from local sources. Unbiased locations are best, such as arboretums and botanical gardens. Attend turf and landscape conferences conducted by your local land grant university, and finally, don’t be afraid to ask questions of curators, horticulturists and landscape designers, especially if they are making a presentation on adapted plant materials for your area.

What not to do

As you are obtaining information about the best species and cultivars of plants to introduce into your clients’ landscapes, also determine how much water and fertilizer they need to perform well. Monitor (or teach your client to monitor) the soil moisture.

Do not dump mulch around the tree trunk to form a volcano. Not only does this provide a good location for insects to overwinter, it also creates a good microenvironment where diseases such as Armillaria root rot can flourish. Start at the base of the tree/shrub, or better yet, 3 inches away from the base, and extend the mulch as far out into the landscape as your client will tolerate. The key with mulch is to replicate Mother Nature. Two inches is a good depth to shoot for.

Be cautious with fertilizer applications, particularly nitrogen. In general, most trees and shrubs require about a fourth as much as turf does. Again, acquiring up-to-date information on the appropriate nutrient needs for each plant species is important. If overfertilized, plants adapted for low levels of nitrogen produce abnormal growth. This is problematic for three reasons:

  • The abnormal growth leads to a greater concentration of sugars, which are more attractive to insects.
  • The growth is produced faster than normal, resulting in weaker plant tissues.
  • New growth is produced at the expense of other functions of the plant. Most critical to this issue is the reduced level of defense and carbohydrate storage, which is used to deter insects.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.