It’s possible that change could be coming to the pest control arena of the tree care industry. There are potential consequences of change due to the weather, and here are some strategies for adjusting pest control programs as needed.
Many readers are familiar with the classic disease triangle that has guided integrated pest management (IPM) decisions for the past 40 years. When it comes to climate change, it’s wise to expand the triangle to all pests, including nematodes, insects and weeds. Each of these biological organisms depends on weather and climate for their existence.
In the 1980s, a computer software product called Lotus 1-2-3 was popular because it let business owners put their income and expenses into a spreadsheet format and allowed for consideration of changes, such as increased utility costs, decreased account receivables and new categories of expenses. With just a couple of keystrokes, a relatively accurate set of scenarios could be quickly prepared and evaluated. Computer software products are far more sophisticated these days, but the basic premise of “what if” analysis is still pertinent.
In terms of pest control for trees, the same principle applies. No one really knows what the future holds, but gradual changes in temperature, precipitation and humidity are certainly possible.
If conditions are wetter than normal for consecutive years, trees tend to grow more vigorously. Leaf-feeding insects typically respond to this increase in succulent tissues by increasing in numbers in any given region.
Conversely, if conditions are drier than normal for an extended period, trees tend to become stressed and are less vigorous. Trees under extended drought stress respond by producing less shoot growth and leaves with less green color. While humans tend to be attracted to dark green, lush landscape plants and turfgrasses, many insects are highly attracted to yellow hues. The leaf yellowing commonly associated with drought stress make these plants more attractive to certain insects. In fact, certain beetles are specially equipped with infrared receptors that help them locate drought-stressed trees to lay their eggs. Many wood-boring species, including the bronze birch borer and lilac/ash borer, are also attracted to drought-stressed trees.
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.
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 sevenfold to tenfold increase in insect development.
In addition, drought-stressed plants often have higher concentrations of plant sugars and minerals (Mg, K, Ca, Cl) in the leaves and stems, which makes them a better food source and more suitable for insect growth, reproduction and survival.
Plant diseases are also likely to be influenced by climactic changes. In regions of the United States that receive less than 15 inches of rainfall per year, foliage diseases such as anthracnose and apple scab are not common. These diseases generally require several consecutive days of cool, moist weather that facilitates extended leaf wetness for infection to occur. On the other hand, areas that currently receive abundant moisture could also see changes in the incidence of diseases due to the onset of drier conditions.
Pathogenic diseases are not the only sorts of maladies that could develop due to extended dry periods. Dry soils and warmer ambient air conditions could lead to root problems, such as the development of reduced root systems. This is especially problematic in urban areas where room for rooting is often limited.
What to do
As an arborist, your job is to gather all available information and put it to use maintaining and improving your client’s landscape. Balanced nutrition and moisture applications, use of good landscape design principles, moderate mulch placement and soil improvements will go a long way towards reducing the effects of gradual changes in climate and help minimize future pest problems. Consider adding scouting/inspection services for your clients. Timely inspections allow for maximum adaptation to change and optimize your flexibility in dealing with your clients’ pest management issues.
Making well-informed plant selection decisions is also an important component in dealing with possible climatic changes. Take time to seek out plant species and cultivar information from local sources. Unbiased information sources, such as arboretums and botanical gardens, are usually best. Special issue publications from Taunton and Meredith Press can also provide specific recommendations, especially if they contain regional sections. Attend tree, turf and landscape conferences conducted by your local land grant university. 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
• Overwatering—As you are obtaining information about the best plant species and cultivars to introduce, also determine how much water and fertilizer they need to perform well. Monitor (or teach your client to monitor) the soil moisture by probing.
• Mulch excessively—Don’t dump mulch around the tree trunk to form a volcano. As experienced horticulturalists, we cringe when we see mulch piled on the trunk, often extending a foot or more off the ground. Not only does this provide a good location for insects to feed and 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 to effective mulching is to emulate Mother Nature; 2 inches is an ideal depth. A good quote to remember in this regard is from Bruce Fridrich, F.A. Bartlett Co.: “Mulch is a root treatment, not a trunk treatment.”
• Excessive nutrient levels in soil—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, and this is problematic for three reasons. First, abnormal growth leads to a greater concentration of sugars, which make plants more attractive to insects. Second, growth is produced faster than normal resulting in weaker plant tissues. And, third, new growth is produced at the expense of other plant functions; most critical to this discussion, reduced levels of defense and carbohydrate storage which are 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.