December, 1995
Field Crops 28.4-5
Fighting European Corn Borer with B.t. Corn:
a Preliminary Evaluation
Joe Lauer and John Wedberg, Agronomist and Entomologist
What is B.t.?
Using B.t. products to control European
corn borer
Using B.t. corn to control European corn borer
B.t. corn evaluation in Wisconsin during
1995
Development of European corn borer resistance
to B.t.
European corn borer, Ostrinia nubilalis (Hubner), is a major pest of corn in North
America. Yield losses of 2-7 percent for each borer on a plant can result from feeding
at various stages of plant growth. Corn yield reduction from European corn borer
is estimated to exceed $50 million in Illinois and $1 billion in the U.S. annually.
Feeding results in physiological disruption of the plant leading to lower plant
yield as well as physical lodging and ear drop of the plants. Effective use of insecticides
to control European corn borer requires scouting and a knowledge of treatment thresholds.
Applications should be immediate and a relatively narrow application window exists.
Many insecticides are currently registered for European corn borer control. A new
tool in our "arsenal" of weapons for fighting European corn borer is B.t.
corn.
What is B.t.?
B.t. is an abbreviation for the bacteria, Bacillus thuringiensis, which is found
in the soil. B.t. forms a crystal protein that is toxic to caterpillars (lepidopterans),
beetles (e.g. corn rootworm and Colorado potato beetle), and aquatic flies (e.g.
black flies and mosquitos). After B.t. is eaten by the insect, the crystal dissolves
to release a toxin that attacks the gut lining. Feeding stops within a few hours.
The insect gut wall breaks down within 24 hours. Bacterial spores germinate and
invade the body cavity of the insect. The insect dies from toxins attacking the
gut wall, by a general body infection (septicemia) which is present within 48 hours,
and food deprivation. Over 70 different toxins are formed in B.t. crystal proteins.
The activity of the toxin in an insect depends on gut pH, the presence of enzymes
and reducing agents, and the presence of binding sites on cell membranes.
Using B.t. products to control European
corn borer
The first use of B.t. products against European corn borer occurred in Europe in
1929. Currently, many commercial formulations of B.t. products are registered. B.t.
products are effective against first generation European corn borer, because the
larvae concentrate in the leaf whorl of the corn plant and the product is easy to
apply. B.t. products perform poorly against second generation larvae and are generally
not labeled for use. B.t. products are safe not only for applicators and detasslers,
but also birds, amphibians, fish and aquatic invertebrates. Most beneficial insects
are not affected by applications aimed at European corn borer. Use of B.t. products
requires early detection and scouting and prompt treatment is critical. B.t. activity
is relatively short with UV light and rainfall posing the biggest challenge to persistence.
Using B.t. corn to control European corn
borer
In 1993, Ciba-Geigy reported successful B.t. transformation of elite corn hybrids
using the "gene gun." Nearly every seed company has substantial efforts
underway to incorporate genes into elite corn hybrids. Incorporation of the B.t.
toxin into a corn plant overcomes many difficulties associated with insecticides
and B.t. products. B.t. corn offers season long protection against all generations
of European corn borer. B.t. corn offers potential for cross protection against
other caterpillar pests attacking corn such as sod webworm, cutworms, hop vine borer,
stalk borer, armyworm and corn earworm. Insecticide applications are eliminated
which also eliminates scouting costs and toxicity potential to nontarget species
due to drift, poor timing, and availability of applicator or product.
B.t. corn evaluation in Wisconsin
during 1995
Our objectives were to compare B.t. corn hybrids with adapted high performing non-B.t.
hybrids, and to evaluate B.t. corn performance under economically significant European
corn borer infestation.
Experiments were established near Arlington, Hancock, and Lancaster, WI during 1995.
Plots were 10 feet by 25 feet. Typical cultural practices used by farmers in the
surrounding area were used for plot establishment. Plots were planted on 15 May,
11 May and 17 May and harvested on 18 October, 16 October, and 14 October for Arlington,
Hancock, and Lancaster, respectively.
The experimental design was four replications of a randomized complete block in
a split-split plot arrangement. Main plots were hybrid. Three groups of hybrids
were evaluated: 1) hybrids with the B.t. gene included Northrup King 1032BT, Northrup
King 1033BT, Ciba 1134E, Ciba 3206E and Ciba 3208E; 2) closely related "isolines"
that did not have the B.t. gene included Northrup King 397, Northrup King 777, Ciba
1134X, Ciba 3206X and Ciba 3208X; and 3) standard hybrids that have performed well
at these locations included DeKalb DK512 and Golden Harvest H2387. Split-plots were
European corn borer inoculation. Each plant in the middle two rows of a four row
plot were infested with first and second generation European corn borer. Approximately
100 egg masses on waxed paper were placed on each plant at V7, V9, VT and R1. The
control plots did not have any egg masses placed on the plants. Split-split-plots
were the presence or absence of the B.t. resistance gene (+ B.t. gene, - B.t. gene)
in the corn hybrid.
The corn production year of 1995 saw two events which had an impact on corn yields.
During the middle of July extremely high temperatures were observed throughout most
of Wisconsin. This may have interfered with pollination, especially in areas that
did not receive moisture until later in the season. Secondly, a severe European
corn borer infestation was seen throughout the state.
Agronomically, there was less stalk breakage with the B.t. hybrids compared to the
standard and isoline hybrids (data not shown). Grain moisture of the B.t. hybrid
was not significantly different from the standards and isolines. Overall, the presence
of European corn borer in the plots inoculated decreased grain yields about 11 bushels
per acre at Arlington, 20 bushels per acre at Hancock. Lancaster grain yield was
not affected by inoculation.
Averaged across all locations, grain yields of the standard hybrids yielded 158
bushels per acre (Figure 1). When the standard hybrids were inoculated with European
corn borer four times during the season, grain yields decreased to 146 bushels per
acre. B.t. corn hybrids, whether inoculated with European corn borer or not, yielded
similarly to the uninoculated standard corn hybrids. Yield of isoline hybrids was
147 bushels per acre when uninoculated and 134 bushels per acre when inoculated.
Since there was greater than normal European corn borer pressure during 1995 and
there was no treatment where European corn borers were completely controlled by
insecticides throughout the season, we do not know how the standard and isoline
hybrids might have performed in the absence of European corn borer. Based on the
three locations of data and the European corn borer pressure expressed during 1995,
B.t. corn hybrids performed similarly to adapted high performing hybrids. This was
observed at both Hancock and Arlington, while at Lancaster it appears that other
problems, i.e. pollination problems may have affected results.
Development of European corn borer resistance
to B.t.
Widespread use of B.t. corn could lead to B.t. resistance in the European corn borer.
Development of resistance is favored because corn is the predominant host of the
European corn borer. Since B.t. is always expressed in B.t. corn and all feeding
stages of the European corn borer occur on corn there is essentially no escape.
European corn borer can produce multiple generations per year which usually means
faster expression of resistance. Resistance on a local scale will likely evolve
because there is limited dispersal of the European corn borer; typically corn borer
moths will fly less than two miles. Development of B.t. resistance has already been
seen in the Indian meal moth and has been duplicated in the laboratory with the
European corn borer.
Development of resistance to B.t. may be delayed because untreated areas can provide
a source of susceptible moths to dilute the buildup of B.t. resistant genes in the
European corn borer population. The European corn borer can live on more than 200
species of plants, so alternatives to corn as a host exist. Not every field of corn
will be planted to B.t. corn so geographical mosaics of B.t. and conventional hybrids
will occur. Toxins vary among seed companies and hybrids, thus more than one toxin
could be produced in a hybrid. Toxins could be used in conjunction with other forms
of resistance developed by plant breeders such as DIMBOA and stalk strength.
Maintaining the effectiveness of B.t. corn after it reaches the marketplace will
be the responsibility of seed companies, educators and farmers. Education about
the limitations of this new technology and the potential development of insect resistance
must be communicated to farmers along with management recommendations to insure
that B.t. technology is successful in the long run.