By Mark Shour
Iowa State University Extension
In the past few years, landscapers have seen the loss of several insecticides previously used to manage insect pests. Active ingredients such as acephate, chlorpyrifos, diazinon, dimethoate and lindane have vanished from supply shelves. The challenge of minimizing insect damage to valued plants, however, continues. Let's look at other avenues of insect pest management.
Initial plant selection speaks loudly for the success of a given plant at a site. Knowing the soil texture and pH, amount of sunlight, average rainfall, how well water drains from the site, USDA Plant Hardiness Zone, expected soil-compacting sources, and distance from buildings and hard surfaces are among the factors to consider when selecting a tree species. If you take this process a step further, consider natural resistance.
In general, there are three mechanisms by which plants resist insect attack: tolerance, antixenosis and antibiosis. Tolerance centers on the plant's ability to recover from any insect damage or to replace damaged plant tissues during growth phases following the insect attack.
Antixenosis (also called nonpreference) is the ability of a plant to repel an insect. This can involve volatile chemicals given off by the plant, which causes the insect to turn away before it contacts the tree. Other times, defensive chemicals are picked up by the insect once it lands on the plant or begins feeding on it. In other cases, antixenosis entails hairs on leaves or stems, surface waxes, copious sap flow or toughness of the tissue.
Antibiosis is plant resistance that directly affects the insect's survival. These chemical or physical defenses are either a normal part of plant growth or they are induced by the feeding of an insect. The result is longer development, fewer eggs laid or direct insect mortality.
Research has identified some tree species, cultivars and varieties that are pest resistant, as well as ornamental. One example is birch trees and the bronze birch borer. Twenty years of research at Ohio State University revealed that paper birch (Betula papyrifera), grey birch (B. populifolia 'Whitespire') and river birch (B. nigra) had high survival rates (80 percent), while neighboring European white birch (B. pendula), mountain birch (B. pubescens), monarch birch (B. maximowicziana) and Asian white birch (B. platyphylla) had been completely destroyed by this boring pest.
Another example involves crabapples and the Japanese beetle. Research from Purdue University found the following crabapples to have high resistance to adult feeding and to apple scab: Bob White, Louisa, Prairifire, Red Jewel and Sargent. Similar studies are reported in the literature for other tree species and this defoliating pest.
Searching the Internet and current periodicals will result in additional examples of trees that resist insect pests. Alternatively, landscapers can keep notes on those plants that do not seem to be bombarded by common pests (aphids, beetles, caterpillars). After a period of time, plant selections can be modified based on this list of apparently pest-free species.
Here are 15 trees and shrubs suitable for Iowa that are seldom attacked by insects. Make sure the site characteristics match well to the plant requirements.
* Abies concolor - White Fir
* Cercidiphyllum japonicum - Katsura tree
* Cladratus lutea - Yellowwood
* Ginkgo biloba -Ginkgo
* Gymnocladus dioicus - Kentucky Coffeetree
* Maackia amurensis - Amur maackia
* Phellodendron amurense - Amur Corktree
* Cotinus coggygria - Smoketree
* Exochorda racemosa - Common Pearlbush
* Myrica pensylvanica - Northern Bayberry
* Rhodotypos scandens - Black Jetbead
* Viburnum dentatum - Arrowwood Viburnum
* Viburnum prunifolium - Blackhaw Viburnum
* Viburnum rhytidophyllum - Leatherleaf Viburnum
* Viburnum sieboldii - Siebold Viburnum