Facts You Need to Know to Scout and Manage WBC in Corn

What Moths Prefer– Moths prefer laying their eggs in corn before it tassels.  After the crop is in tassel, it will prefer neighbouring fields that are less advanced in crop stage OR they move to dry beans and other host crops (like snap beans, sweet corn etc).  Pick the tallest and healthiest looking fields that are not quite tasseling yet.

Where Eggs are Laid- Eggs are laid on the top 4 or so leaves, especially on those leaves that stand erect or just have or are about to unfurl from the whorl.  Eggs are usually on the upper (hairy) surface of the leaf.

What WBC Eggs Look Like – Egg masses can range in size from smaller than a dime to bigger than a quarter.  Each individual egg is shaped like a tiny cantelope.  Initially they are pearly white when first laid but turn tan and then purple just before hatch.  Click here to see a picture of how an egg mass changes colour as it ages over time.

How to Scout – Select the appropriate fields based on information above.  Inspect 20 plants in a row.  Inspect the upper leaves for both egg masses and young larvae.  If any are found, make note of the age of the egg and when it might hatch.  If young larvae are found, make note of their location on the plant.  After scouting those 20 plants, cut across the field and pick another 20 plants in a row.  Continue this until you have inspected 20 plants in 5 areas of the field.  That gives you a total of 100 plants inspected per field.   Eggs will be laid in patches in the field so you may or may not come across one of these hotspots during your scouting attempts.  Change up the areas of the field you monitor so that you are more likely to hit one of these hotspots in one of your scouting trips.

Threshold in Corn – If 5% of the plants (or 5 plants if you have scouted all 100 as suggested above) have egg masses on them, the corn needs to be sprayed.  Spray may not be required in those fields containing Cry1F (Herculex or SmartStax) as this type of Bt provides the crop with some protection (similar to one spray of insecticide).   However if populations are high, Cry1F fields may also experience damage.

Spray Timing – Time the spray application to immediately after the majority of the eggs have just hatched.  This will be one or two days after the eggs have turned purple.  This will ensure that you are targeting the smallest larvae which are the easiest to control.  These small larvae will be close to the top of the plant near the tassel unless there are already silks on the plant.  If silks are present, then the larvae will be attracted to the silks and may be hanging around the ear.  You must control the larvae before they enter the ear and are protected from the spray.

Chemical Control Options: Both Matador and Decis are registered on corn for western bean cutworm control.  See the 2010 supplement for Pub 812 that is now available at  http://www.omafra.gov.on.ca/english/crops/pub812/sup/p812suptoc.htm for label rates and recommendations.

Next Blog:  Update on this weeks moth catches, when peak moth flight is expected and dry bean management recommendations.

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