During the past 30 years, changing crop rotation practices from corn and small grains to corn and soybeans and changing harvesting methods from picker and ear storage to combine and shelled grain storage have dramatically reduced overwintering populations of European corn borer in the Corn Belt. Reduction of over-wintering populations might be important. However, research in Iowa and Minnesota shows that European corn borer survivorship is highly dependent on weather conditions during mating, egg laying, egg maturation, and early larval development.

Table 9 presents information showing the effects of drought stress and inundation on the survivorship of first-generation larvae on a susceptible corn hybrid. Drought stress caused relatively consistent mortality; inundation caused periodic high early mortality. Drought stress caused 74.6 percent mortality by the eighth day after hatch. However, inundation of the larvae by 1 1/8 inches and 7/8 inch of rainfall on days 0 to 2 and 4 to 6, respectively, caused 87.5 percent mortality by the sixth day. Regardless of the precautions taken before planting, climatic variables play a major role in determining the number of European corn borer that survive (Table 9).

Table 9. Average number of larvae per plant and cumulative percentage mortality for first-generation European corn borer caused by drought stress and moisture inundation in Iowa (W. B. Showers).

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                     Drought Stress                Moisture Inundation
Observation   -----------------------------   -----------------------------
days after    Number        Percent larval    Number        Percent larval
hatcha        larvae/plant  mortality/plant   larvae/plant  mortality/plant 
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0             20.0          0                 20.0          0
2             15.7          21.5              6.5           67.5
4             9.0           55.0              6.5           67.5
6             8.7           56.7              2.5           87.5
8             5.1           74.6              2.5           87.5
12            4.25          78.8              1.25          93.8
14            3.5           82.4              1.25          93.8
17            2.5           87.6              1.25          93.8
20            1.8           91.1              0.8           96.0
22            0.9           95.6              0.5           97.5
24            0.6           97.2              0.45          97.8
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aAll leaves of 20 corn plants examined/observation.