Fall Armyworms in Mississippi Crops

I am seeing more and more fall armyworms in most crops here at the DREC in Stoneville and have been seeing a lot of moths flying around.  Fall armyworm can be a challenging pest to find and an even more challenging pest to manage in some crops.  The damage that they cause and their management varies greatly from crop to crop.  The reason for this is due to their preferred feeding sites on different crops and the presence of host strains.

Most of you probably know that there are two different strains of fall armyworm.  The grass or rice strain prefers to feed on grasses and is very easy to control with most insecticides.  The corn strain feeds on corn, grain sorghum, cotton, soybeans, and peanuts and is more difficult to control with insecticides.

Fall armyworm tends to be a late season pest in most crops. The reason for this is that it does not go into diapause during the winter like we see with other caterpillar pests such as bollworm.  Fall armyworm  generally overwinter in southern Florida and southern Texas and migrate into Mississippi from those areas similar to what we see with soybean looper.  As a result, fall armyworm populations tend to be higher in years following a mild winter because they are able to overwinter further north than normal.

Becasue the damage they cause and their management can be so different from crop to crop, I will go through most of the crops seperatley.

Cotton

Fall armyworms can occasionally be a significant pest of cotton in Mississippi.  The dual gene cottons generally provide good control of fall armyworm, but I have been seeing some larvae and damage in our Bollgard II plots this week.  In cotton, fall armyworms lay their eggs in clusters on leaves and the bracts of squares and bolls low in the canopy.  When they hatch, the small larvae feed by grazing on the bracts and on the surface of bolls (see picture below).  Larger larvae will bore into the base of bolls and squares.  Most of the time fall armyworm damaged bolls can be distinguished from bollworm damaged bolls by the location of the entrance hole.  Bollworms will enter through the boll wall near the middle to top of the boll; while, fall armyworms will enter near the bottom of the boll (next to the peduncle).

Fall armyworms are very difficult to control in cotton because they are low in the canopy and it is difficult to get any insecticide to them.  In general, we tend to see significantly less fall armyworm damage in cotton that has been sprayed with novaluron (Diamond or Mayhem).  Other than that, most of the catterpillar specific insecticides will do a good job if you can get it down in the canopy where they are feeding.

Peanut

I have been seeing a few fall armyworms in peanut as well.  The fall armyworms that we are seeing in peanuts now are the corn strain and are more difficult to control than the ones we were seeing several weeks ago.  The reason for the difference is that the fall armyworms we were seeing in peanuts earlier in the season were moving off of grass that was recently sprayed with a herbicide.  We were able to easily control them with a pyrethroid.  Fall armyworm feeding in peanuts is primarily going to be on foliage, so thresholds can be based on a combination of numbers of larvae per foot and percent defoliation.  When sprays are made this time of year, the caterpillar specific products tend to provide the best control.  Also, remember that when spraying peanuts, water is the cheapest thing you can add to the tank and more is always better.

Grain Sorghum

In heading grain sorghum, the population will most likely consist of nearly 100% corn strain.  Damage is similar to bollworm damage and the insecticides used to control bollworms will usually do a good job on fall armyworm as well.  The only exception is pyrethroids.  They will have little activity against fall armyworm in grain sorghum.

Rice

A few fall armyworms are still out there in some of the later planted rice.  In rice, they will almost always be grass/rice strain.  I haven’t heard of many treatments going out in Mississippi lately.  In general, it takes a lot of fall armyworms to cause yield losses in heading rice, but they can blow up fairly quick.  Especially in fields that have not been sprayed for rice stink bug.  If treatments do need to be made in rice, the pyrethroids should do a good job when used at the higher rates.  In heading rice, higher rates are needed to ensure good coverage.

Soybeans

Fall armyworms usually are not a significant problem in soybeans, but they can occur occasionally.  During the early season we see some grass strain fall armyworm moving off of grasses that have been sprayed like we see in peanuts.  This time of year, if fall armyworms do occur in soybeans they should be treated as corn strain and caterpillar specific insecticides should be used.

Fall armyworm feeding in soybean will mostly be cosmetic foliage feeding, but they will graze on pods some as well.  Pod feeding is not very common, but it can occur.  It will be interesting to see what happens in some of these fields I’m hearing about that may be double cropped behind corn.  We had some soybeans planted around the first of August last year and they had heavy fall armyworm and beet armyworm pressure almost as soon as they came out of the ground.  We had to make multiple pyrethroid applications for three cornered alfalfa hoppers during the seedling stage that probably made them worse.  The point is, in these very late beans we will likely see some things that we are not used to seeing in April and May planted beans.  Keep in mind that management decisions for one insect will likely influence other insect pests.

There is no way we can cover everything about fall armyworm in one article so if you see something unusual or if you need additional information, feel free to give any of us a call at any time.