and European Corn Borer
for Control of European Corn Borer
Following is an explanation of Bt-corn and some suggestions for
managing its use. Please consider these suggestions seriously before you
decide to plant or recommend the sale of Bt-corn.
The "active ingredient" of Bt-corn is derived from a
naturally occurring soilborne bacterium, Bacillus thuringiensis
(Bt), that is found worldwide. Bt produces a crystalline
protein (endotoxin) that is toxic to specific groups of insects. The endotoxin
is a stomach poison that must be ingested by the insect, after which the
insect dies. The Bt subspecies kurstaki is toxic to certain caterpillars,
including the European corn borer. After a caterpillar ingests the protein,
the crystal breaks down and releases a toxin that attacks the gut lining.
The insect stops feeding within a few hours, and it dies within a couple
transfer techniques have been used to develop corn plants that contain
the endotoxin-producing gene taken from Bacillus thuringiensis.
The Bt gene has been inserted directly into the corn ge-nome. When
a young European corn borer larva eats plant tissue that contains the
Bt endotoxin, it dies. The presence of the toxin produced by Bt
in corn provides nearly season-long protection against European corn borers.
Bt-corn offers an opportunity to control one of our most economically
damaging corn insect pests without the use of conventional insecticides.
effective against European corn borers?
Research trials to test the efficacy of Bt-corn against European
corn borers have been conducted for several years in many corn-producing
states, including Illinois. Some Bt-corn hybrids have provided
greater than 95 percent reduction in stalk damage caused by corn bor-ers.
However, the effectiveness of Bt-corn varies among hybrids. Because
the level of expression of the Bt protein declines in Cibas
and Mycogens hybrids after pollination, some second-generation borers
may survive and tunnel in the stalks, shank, and ears. The YieldGard Bt
gene protects the plant from corn borers for a longer period of time.
Bt-corn also has yielded well, both in the presence and absence
of corn borers.
control other corn insect pests?
Research results from the western Corn Belt states indicate that several
Bt-corn hybrids offer excellent control of the southwestern corn
borer, an insect that occurs occasionally in southern Illinois. Limited
research suggests that although corn earworms are affected by Bt-corn,
the level of control may not be acceptable. However, Bt-corn affects
the growth and development of corn earworms, so the overall impact of
Bt-corn on corn earworms may be population reduction.
available Bt-corn hybrids are not effective against cutworms and
stalk borers, caterpillars that may be controlled by other Bt-corn
hybrids developed in the future. Additionally, Bt-corn developed
from the subspecies Bt kurstaki does not control aphids, corn rootworms,
or other pests that are not caterpillars.
affect beneficial insects?
Bt-corn developed from the subspecies Bt kurstaki is highly
selective against caterpillars, so it does not directly affect the predators
found in cornfields. In studies conducted thus far, Bt-corn has had no
direct effects on lady beetles, green lacewing larvae, spiders, minute
pirate bugs, and parasitic wasps. Any indirect effects on populations
of beneficial insects caused by the removal of corn borers as a food source
for predators and parasitoids remain unknown.
safe, or does it pose any hazards to the environment?
Bacillus thuringiensis is not toxic to humans, other mammals, birds,
fish, or bees. The use of Bt in formulated insecticides (for example,
DiPel) has been registered for many years. Microbial insecticides that
contain Bt are suggested for control of European corn borers in
seed-corn fields where detasselers will be working. Because of its safety
and the rapid degradation of Bt toxins on plant surfaces and in
Bt-corn as the plants die, Bt is exempt from the need to
establish residue tolerances for all raw agricultural commodities.
Loss estimates suggest that European corn borers cause 5 to 7.5 percent
yield reductions in the United States annually. Bt-corn should
prevent these yield losses. However, because the occurrence of European
corn borers cannot be predicted accurately from year to year, an investment
in Bt-corn is an economic decision that each grower must make.
In some years, the premium paid for Bt-corn seed will not be returned
in yield benefits if corn borers are not present. However, in years when
corn borer infestations are moderate to heavy, the grower who plants Bt-corn
will more than recover his or her investment.
should not plant Bt-corn on all acres. A grower may choose to plant
Bt-corn in fields that are planted early or late to offset the
risk posed by either first- or second-generation corn borers, respectively.
Management and Bt-corn
Corn growers must accept the very real possibility that European corn
borers may become resistant to Bt if Bt-corn is planted
widely and resistance management tactics are not implemented. Intense
selection pressure by any insect-killing agent often results in the development
of an insect population that is resistant to that killing agent. By repeatedly
exposing a lab colony of European corn borers to Bt, researchers
at the University of Minnesota already have created a population of corn
borers that is resistant to Bt.
hybrids are planted widely, European corn borer populations eventually
will develop resistance to this very specific insect toxin. Consequently,
producers who grow Bt-corn should implement a resistance management
plan to slow down the potential onset of resistance. Maintaining "refuges"
where corn borers are not exposed to the Bt toxin may be the most
practical resistance management tactic. In theory, high doses of the Bt
toxin in Bt-corn kill virtually 100 percent of the corn borers.
However, if any borers survive, it is highly desirable to increase the
odds that surviving moths (possibly resistant individuals) mate with moths
emerging from refuges (susceptible moths).
all fields of non-Bt-corn and many other species of plants (including
several crops and weeds) on which corn borers can develop. However, resistance
management will almost certainly require management of refuges, including
entire fields of non-Bt-corn planted adjacent to Bt-corn
specifically to provide a refuge for (and source of) susceptible corn
borers. Other types of management of refuges may be a block of non-Bt-corn
planted within a field of Bt-corn or a designated percentage of
rows of non-Bt-corn throughout the field.
of corn that should be used as a refuge within a field or area is not
known. However, a managed refuge of 5 to 40 percent may be necessary,
depending upon geographical location and the presence or absence of other
refuges (wild hosts and non-Bt crops). Throughout most of the Midwest,
a 25 percent refuge of non-Bt-corn likely will be necessary to
delay the onset of corn borer resistance to the Bt toxin.
Any grower who plants Bt-corn should monitor the fields for evidence
of injury caused by European corn borers. Remember, some leaf feeding
is expected; the borers must eat the leaf tissue before they die. However,
any evidence of unacceptable levels of injury should be reported immediately
to the seed-corn company from which the seed was purchased. Representatives
from these companies are committed to sampling any fields that show evidence
of corn borer injury greater than expected. They should be able to determine
whether the plants affected are expressing the Bt gene or whether
the corn borers have developed some level of resistance to the Bt gene.
If corn borer resistance to Bt occurs, alternate management tactics will
be suggested and all growers in the area surrounding the initial detection
will be notified.
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