Updated: 10:51 pm, Friday, February 18th, 2005
disappear, we look to resistant plants
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.
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
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.
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
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.
Mark Shour is an
entomologist with Iowa State University Extension, Ames