Sweet Corn Pheromone Trap Network for Western New York, 2009

This is a seasonal scouting report giving growers in the Western New York area information on the presence of sweet corn pests and recommendations on scouting and thresholds. The report is written by Abby Seaman, NYS IPM's Area Extension Educator for Vegetables.

For 7/14/09

European corn borer numbers are very low this week at all locations.  Larvae being found in tasseling corn now are from the first generation flight.  They have been feeding in the whorl, protected from sprays.  Doing a good job with controlling larvae on the emerging tassel with one or two well-timed applications (see info below) may be the only sprays needed in this between-flight interval.  Keep scouting to be sure there are no new egg masses, though.  Traps are not infallible.

Corn earworm were were caught in low numbers at a few locations this week and catches remain low in Pennsylvania.  Fall armyworms have not yet been caught this season.

Scouting and threshold information:

Split field demonstrations have shown that applying insecticides to sweet corn for worm control before tassel emergence does not produce better results than waiting until the recommended tassel-emergence spray timing.

In fields still in the whorl stage when eggs are deposited, larvae move to the whorl, feed on the foliage and un-emerged tassel, and are found feeding on the tassel when it emerges.

Larvae feeding in the whorl are protected from insecticide applications and mortality will not be as high as at tassel emergence, when larvae feeding in the emerging tassel are exposed to the spray.  Fields should be first scouted for ECB and FAW larvae at early tassel emergence.  For corn borers, look down into emerging tassels for tiny larvae or frass (white to brown material about the size of fine sand), holes through the leaves in straight lines, or "windowpaning" damage, in which tiny larvae leave the clear lower epidermis of the leaf intact as they feed.  For armyworms look for ragged feeding holes and frass pellets the texture of coarse sawdust.

The threshold for ECB and armyworms at tassel emergence is 15% infested plants.
Before any insecticides have been applied, scouting is fast and easy because any sign of feeding is an almost sure sign of live larvae, so it's not necessary to spend time finding the larvae.  After the initial insecticide application, feeding damage may be from a larva that has already been killed, so finding the critter is more important for an accurate estimate of the number of infested plants.

Larvae will leave the tassel as it opens up and no longer provides a moist, protected feeding environment, and move down the plant looking for protected places to feed.  Insecticide applications need to be timed to kill larvae before they bore into a new feeding location where they will be again protected from sprays.  In fields with very uneven development, two applications may be necessary, one when approximately 25-50% if the tassels have emerged, and again after 75-100% of the tassels have emerged, if the field is still over threshold.

Once a field reaches the silk stage, the threshold drops to 5% infested plants. Scout the ear zone (roughly from two leaves above and one leaf below the ears) for ECB egg masses and ECB or FAW larvae.  Egg masses are found most frequently on the underside of leaves near the midrib, and consist of approximately 10-20 flattened eggs overlapping like fish scales. Eggs are white when first laid, turning cream colored after a couple of days, and show the black head capsules of the tiny larvae through the surface of the eggs when within 1 day of hatching (the "black head" stage).  Egg masses can also sometimes be found on the flag leaves of the ears or on the husk itself.  Eggs take approximately 100 base 50 degree days to hatch.   When temperatures are in the 70's during the day and the 50's at night egg masses will take about a week to hatch.  When temperatures are in the 80's during the day and the 60's at night, they could hatch in only 4 days.
Look down into the tops of the silks for newly hatched larvae, and pull the ear away from the stalk slightly to look for larvae feeding between the stalk and the ear.

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