Armyworm Moth Captures are Abnormally Large.

Doug Johnson, Extension Entomologist, University of Kentucky

Armyworm moth.

Recent moth flight of armyworm (AW), in western Kentucky has been much larger than is normal for this time of year. It is not terribly unusual to see problems with this insect on pasture and forage grasses in late June and July, especially in central Kentucky This year’s much larger AW flight during late July and August in western Kentucky is truly unusual. It is difficult to know what to make of this because it has no precedence in our data set. Nevertheless, the fact is they are here.

Generally, I would expect to be looking for fall armyworm (FAW), Spodoptera fugiperda,at this time of year, but our moth captures for this insect in both central and western KY have been quite low, perhaps because the cold winter and spring pushed their overwintering locations farther south, and/or they were later beginning their annual northward migration. Even so we have seen some localized but significant populations in south-central KY.

  The data we see on the armyworm moth flight of course, describes the movement of adults, which are not the damaging stages. It is the juvenile, caterpillar stage that will appear in September that might pose the threat. Moth flights in the earlier portion of the season followed a relatively normal pattern. Our most common problem with AW comes in May on small grains and that is what we saw this year. Also, there is often a small bump in flight in June and July that usually doesn’t amount to much. Those events were relatively normal this year. What is different this year is that following that relatively normal flight in June and July we have another and much larger flight in July and August that will produce caterpillars in September.

Armyworm Caterpillar

AW has a very broad host range; it eats on many plants but really prefers grasses. On the whole, corn and grain sorghum should be too close to maturity for much damage. However, late planted soybeans and forage crops, most especially newly seeded forage crops could be in some danger.

Producers are advised to keep an eye on very late planted / late developing soybeans, along with forage crops, particularly grasses and most especially newly planted grasses, and grass-alfalfa mixes. There is no established threshold for this pest in these circumstances, but populations around 4-6 worms per sq.ft. probably require treatment.

X-Large Fall Armyworm Caterpillar Population Should Be Past.

The X-large population of fall armyworm (FAW) caterpillars generated by the X-Large population of FAW moths in late August (as indexed by UK-IPM trap counts, See: KPN 1245 Aug 31, 2010) should be past. This does NOT mean that the threat from FAW is over. It simply means that the large pulse of caterpillars generated by the extraordinary number of moths have completed the damaging stage of their life cycle. We do have evidence (especially forage fields having to be treated a second time) that this pulse did cause some producers x-tra trouble.

My best guess is that the FAW caterpillars produced from the X-large moth flight are now in the pupal (resting) stage, from which adult moths will emerge. So it is possible that another large flight could occur this fall, depending upon temperature. The only other recent, large, fall occurring, FAW population for which we have record, occurred on Oct. 5, 2007. This led to large numbers of FAW mostly feeding on volunteer corn (fortunately) in area wheat fields.

Regardless of whether or not we see another large surge of moths, FAW caterpillars will be present until at least the first killing frost and one should continue to monitor crops at risk. By now corn and most soybeans and sorghum are no longer in danger. Grass and alfalfa forages, especially newly established stands are at greatest risk. Additionally, any newly established grass stands including wheat, lawns and grass covers for construction and reclamation should be watched.

Armyworm Flight 2009: No consistent story; but there will be caterpillars!

Doug Johnson, Extension Entomologist
Dept. of Entomology, University of Kentucky

Early captures of armyworm moths appeared to indicate a larger than normal population for 2009. However, after several weeks of monitoring it appears that the capture counts do not indicate an obvious major outbreak situation. This is particularly true of the Princeton, KY trap. We do not yet have enough historic data to know how to interpret the Lexington trap data. However, this does NOT mean that there will not be any problems. [Please see: How to Use Insect Graphs from the UK-IPM Trap Lines at:

http://www.uky.edu/Ag/kpn/kpn_09/pn_090324.html

to review how to use the trap data.]

There will be armyworm caterpillars in susceptible host (corn, small grains, grass forages), as there are every year. In fact if we apply the day degree model to the very first moth captures there are likely caterpillars already appearing in both western and central Kentucky. These will be among the earliest of caterpillars. Remember if you are in a location warmer than where our traps are located then caterpillars will appear earlier. If you are in a cooler location they will appear later. I have received one report of armyworm caterpillars on the heads of wheat (though I have not seen this myself) from a southern Pennyrile area county.

While moth captures in our traps do not indicate a major outbreak year, we certainly have experienced weather that selects for the survival of the caterpillars. Typically armyworm populations do better in cool wet weather. At least in western Kentucky we have certainly seen such weather. Historically, grass forages and corn suffer the most damage from armyworm, but small grains are certainly at risk, particularly if the caterpillars are feeding on the heads.
Scouting techniques and thresholds are available in our IPM Manuals available at:

http://www.uky.edu/Ag/IPM/manuals.htm

If needed, insecticide recommendations are available in our Insect Management Recommendations for field crops available at:

http://pest.ca.uky.edu/EXT/Recs/welcomerecs.html

These publications may also be available at your local County Extension office.

Even though our trap counts do not suggest a big problem, the weather patterns certainly imply that scouting susceptible hosts is still important. I request that anyone that finds an armyworm infestation that requires treatment or should have been treated, please let me know at: doug.johnson@uky.edu.