Something is Definitely Up with Fall Armyworm!!

Doug Johnson, Extension Entomologist, University of Kentucky

Something is most definitely up with armyworm (Mythimna unipuncta), fall armyworm (Spodoptera frugiperda) or both. I have received information from six counties in less than a week indicating the
presence of large numbers of “worms” in grass situations. These reports stretch from central KY to the Mississippi River. Extension ANR agents in Allen, Ballard, Caldwell, Muhlenberg, Simpson and Taylor counties have reported infestations, some of which are catastrophic, but most thus far are just large enough to be noticed. This is followed by an increase in fall armyworm moth capture in the UK-REC IPM traps (Princeton, Caldwell Co. KY). These counts came in at 285 / trap-week, which is a huge jump from previous weeks. Also, rainy periods in late summer result in increased survival of small fall armyworm caterpillars, which can lead to outbreaks.

 These events could be completely unrelated or could be warning us about large populations of this / (these?) pests. Thus far several of the reports have involved home lawns. But others include pastures, and grass/alfalfa fields.

Fig. 1. Fall armyworm caterpillar.

Fig. 2. Fall armyworm caterpillar.

It is difficult to tell from the reports if this is armyworm or fallarmyworm or even both. In most years, the Purchase and Pennyrile counties suffer most from an outbreak of fall armyworm. However, central Kentucky historically has much larger moth capture of armyworm at this time of year. I have not seen any actual caterpillars and only one photo clear enough to be used for identification. That photo, taken by Dr. Lee Townsend, is (I believe) a fall armyworm. But it is a very light color form, which looks much more, green than the regular darker colors I see in most years. In addition the head capsule is also more like the armyworm color. (See Fig 1 &2) and note the difference in colors.) Regardless of which species is present or if both species is present, or if the species are distributed by section of the state, the outcome of a large population of either of these species may be very similar.

 Both species are “grass loving” insects, but can feed on a broad range of crops. I have already had two calls this week from central KY concerning armyworms feeding on soybeans and alfalfa after a bad grass weed stand was controlled.

 Detect infestations while fall armyworms are small, less than 3/4 inch long or less. Watch for chewed leaves, the earliest signs of an infestation. Caterpillars are up feeding on leaves during the evening or on cloudy days but hide under surface residue or in soil cracks during the day. About 80% of their feeding occurs during the last few days of development so damage can appear to occur overnight when the caterpillars have been present for a week or more before symptoms became apparent.

 • Field corn is probably too far along to be in any danger.

 • Full season soybeans are probably safe as well, o Especially if they are R6 or later.

 • Double crop beans could be problematic, especially if:

      o They are less mature than R6, o the field borders a pasture/hay field, o fields are infested with grass weeds, o and most especially if these grass weeds are controlled using herbicides that kill quickly.

     o These are foliar feeders so % defoliation is used for triggering control. See thresholds in the insecticide recommendations below.

 • Forage fields of mixed alfalfa & grass & pastures could be at risk o Established fields will probably not be killed but a compete cutting may be lost.

    o Treatment may be justified if there is an average of 2 or more worms per square foot. o Malathion is the only insecticide with a 0 (zero) day harvest or grazing interval but control may be limited.

 • By far the greatest risk would be newly established grass or alfalfa/grass stands.

    o These stands could be Killed. o Followed by established Alfalfa/grass mixture which could lose a complete cutting.

    o Historically millet fields have suffered greatly from either of these pests. o Home lawns would typically be in the same danger as grass/hay pastures. A good established stand will probably not be killed, but a new stand certainly could be.

   o Home owners wishing to apply a control should see an insecticide that carries “armyworm” on the label.

 •If you have knowledge of wheat planted as deer food plots, these can be a great sentinel plots. Remember these are ARMYWORMS. If a field is destroyed before the caterpillars have finished their feeding, they will move to another location.

Producers are urged to scout their fields. If insecticidal control is necessary, thresholds and products for use on these crops may be found at: http://pest.ca.uky.edu/EXT/welcome.html or from any County Extension Office.