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Thursday, June 21, 2012

What is going on with European corn borer?

Two years ago many extension and university researchers were ready to write off European corn borer (ECB) because of a more or less steady decline in the number of adults caught and general reduction in the amount of damage over the previous ten years.  There were several ideas promoted about what had caused the decline.
This year we have caught modest numbers of adults but in some local areas the amount of infestation is very high.  Why?
-Joe Ingerson-Mahar

What those causes were seem less important now since last year we had large adult populations and this year we seem to have very high larval populations at least in sweet corn around the state.
As a reminder, the majority of ECB in the state has two generations a year (and sometimes a partial third generation).  One strain of ECB has only one generation a year but if it occurs in New Jersey it is rather rare.

In 2011, while we had larval damage it did not seem extreme, despite large numbers of adults.  This year we have caught modest numbers of adults but in some local areas the amount of infestation is very high.  Why?

Most of our prediction of the damage potential of ECB comes from catching the adults, that is, the moths.  While this is fairly easy it can be wildly inaccurate.  The adult is two stages away from the crop damage.  The adults lay eggs in small egg masses and then larvae, caterpillars, hatch from the eggs to begin feeding on their host plants.  A lot of bad things can happen to the ECB in this process.
Bad weather may inhibit or prevent the moths from laying eggs.  Eggs that are successfully laid are susceptible to predators and egg parasites.  So, too, the larvae are susceptible to predators and bad weather.  So even though we start with large numbers of adults, there may be very little damage.
Conversely, if weather conditions are good, predators and parasites are low in number or non-existent, then there may be a high survivorship of eggs and larvae.  From a small number of moths a lot of damage may occur, and that seems to be the situation this spring/early summer.

The best way to determine what the potential damage will be is field scouting, where a person can look at individual plants, subsample the field, and come up with a percentage of infestation indicating whether an insecticide application is needed.  Lacking that, we can rely on the adult captures.  This will help us to know when moths are flying, when peak moth activity is and when most of the larval activity will occur.   We can use this information to better time insecticide applications, but the number of adults may not produce an accurate picture of potential damage.

What the second generation will be this year remains to be seen.

Joe Ingerson-Mahar