Predictions for Round 2 of Fall Armyworm

By  Andy Michel and Kelley Tilmon et.al

“Could Ohio really face another generation of fall armyworm in the next few weeks?” This has been the most frequent question from many of our stakeholders—and rightfully so given the damage we have already seen in forage and turf.  Fall armyworm (FAW) is normally a tropical insect and can reproduce very fast in warm temperatures. In fact, our extension educators found fall armyworm egg masses in the field last week.  Whether or not a new generation of caterpillars will cause damage largely depends on one factor: temperature.

Fall Army Worm Caterpillars

A recent study compared fall armyworm development at different temperatures (see https://www.mdpi.com/2075-4450/11/4/228).  Higher temperatures result in faster growth—at a constant 78.8^oF, FAW can go from an egg to a damaging caterpillar (4^th instar) in as little as 10 days. At colder temperatures (a constant 64.4^oF), the time increases to 21 days.  In addition, only 29% of the caterpillars in the study survived to pupation at the constant colder temperature. A more reasonable, minimum estimate given this time of year could be 14 days from egg laying until the damaging stage (assuming 71.6^oF constant temperature).  So, if egg laying started last week, then we could see larger caterpillars and damage towards the end of this week, if it remains somewhat warm.

Comparing these data to actual field temperatures (which cycle between daytime highs and nighttime lows) is a bit challenging, but we could use the daily mean temperature as a good guide.  If average temperatures are cool enough, this may help limit any damage from another generation.  This map predicts the daily mean temperature forecasted on this Wednesday, September 22.  Most areas of the state (if not all) will be below 64.4^oF, (our magic mortality number).  Thursday may be even colder. This brief cold period is critical as it arrives when many small larvae have hatched but are very sensitive to the cold. Bottom line: as we get colder temperatures, fewer caterpillars survive.

Because predictions are not certain, and we are still checking our adult trapping numbers, we urge everyone to stay vigilant. Watch your fields, especially the previously damaged forage and turf that may still need some time to regrow.  Also keep in mind any early emerging winter wheat or cover crops. Most importantly, watch the temperatures to help determine if any fall armyworm larvae make it through the upcoming cold snap.