UNDERSTAND THE BIOLOGY OF ARMYWORMS TO CONTROL THEM

by: Kathy Flanders
Extension Specialist and Associate Professor, Auburn University


Armyworms invade Alabama pastures and hayfields every year. At best, they damage only a few fields during the summer. At worst, such as in 2009 and 2010, statewide epidemics of armyworms cause widespread damage to Alabama forages. The keys to armyworm control are understanding the biology of the insect and understanding how to best use each insecticide. This article focuses on the biology of the insect. A second article to appear later this summer will discuss control methods in more detail.

Fact #1: The insect that causes the most damage to Alabama forages officially is named the fall armyworm (Spodoptera frugiperda). The adjective "fall" is because the first papers written about the insect were from New York State where damage occurs in autumn. So from here on out I will use the name "fall armyworm." This is to avoid confusion with the "true" armyworm (pseudaletia unipuncta), an occasional pest of fescue, corn, and small grains in April and May.

Fact #2: Fall armyworms are subtropical insects. In extremely mild winters, a few fall armyworms may survive in Baldwin and Mobile Counties. Elsewhere in Alabama they die out after the first hard frost. Each spring, moths fly northward from overwintering grounds in Florida, the Caribbean Islands, and southern Texas. The first moths usually fly into southern Alabama in April. Later generations work their way northward, reaching north Alabama by May, but not reaching New York State until late summer. During the course of the season, local populations of fall armyworm are increased by flights of moths from further south.

Fact #3: If it is a hot, dry summer you can be sure that fall armyworm populations will be higher than usual. In summers with normal or better rainfall a combination of diseases and parasitic insects keeps fall armyworms under control most of the time. In hot dry summers, the natural enemies just can't keep up with the fall armyworms.

Fact #4: Fall armyworm damage tends to occur in short intervals that are about a month apart. Why? It takes approximately one month from the time a fall armyworm moth lays an egg to the time her daughters lay their first eggs. This is all dependent on temperature, of course, since insects are cold blooded. The warmer it is the shorter this generation time. But as a general rule of thumb, in the middle of the summer:

Eggs hatch in two to four days

Caterpillars feed for about two weeks, during which time they pass through six growth periods (instars)

Caterpillars transform into moths during the pupal stage, which lasts about nine days

Adult moths live for about two weeks, but females lay most of their eggs four to nine days after they emerge from the pupal stage.

Fact #5: In epidemic years, the generations of fall armyworms overlap, allowing for almost continuous egg laying, which means that caterpillars of all sizes can be found in a given field. In epidemic years, the monthly intervals when most of the damage occurs are not as noticeable.

Fact #6: Fall armyworm damage seems to appear overnight. All too often, the first warning a cattleman sees is the appearance of brown circular patches where the caterpillars have eaten all the grass. Even worse is to see an army of caterpillars crawling in to a field from an adjoining field, leaving large defoliated areas behind them. Caterpillars tend to hide during the heat of the day, which makes it even harder to find them.

Fact #7: The first damage from fall armyworms is often seen in the same area of the field as in previous years. This is because the female moths are choosy about where they lay their eggs. Some of their favorite places to deposit a mass of eggs are: the underside of fence rails, the underside of tree branches, and lush, well fertilized forage such as bermudagrass and browntop millet. So if you usually see the first brown spots occurring in your best hayfield, under the limbs of the big oak tree, now you know why. Experience in where fall armyworms are first found can help you find the caterpillars before they cause problems.

Fact #8: The good news and the bad news is that most of the damage from fall armyworm is caused over the course of about four days from feeding by the biggest caterpillars (the last instar, which is 3/4 inch long or larger). Bad news if you discover the caterpillars when they are big because damage occurs rapidly and the caterpillars may have burrowed into the ground to pupate before you have time to spray an insecticide. Good news if you can find the armyworms during the 10 days or so when they are younger, smaller, and not yet causing damage. If you find them early, you have more time to react and yet still control them before widespread damage occurs.

Fact #9: It is possible to find the caterpillars when they are small. The easiest way is to try to find them using an insect sweep net. These heavy duty butterfly nets can be pushed or drawn through the grass forage to catch a portion of the insects that are present. Each time the net is moved through the forage is called a "sweep." When damaging populations are present, you will see large numbers of the caterpillars in a sample from 25-50 sweeps. More information about using sweep nets to detect fall armyworms can be found in the Alabama Cooperative Extension System Timely Information Sheet "How to Use a Sweep Net to Find Fall Armyworms in Pastures and Hayfields," found at this link: www.aces.edu/timelyinfo/entomolog/January/jan-20.pdf. A sweep net can be purchased for about $25 from various suppliers of farm and field equipment. In the next article I will tell you more about a joint project between the Alabama Cooperative Extension System and the Alabama Cattleman 's Association. This project will place two to three sweep nets in each county, which can be borrowed in order to scout (look for) the armyworms in your fields. Another way to discover small caterpillars is to keep an eye on your tires if you are driving through a field when dew is present, since the caterpillars tend to stick to the tires. Insect feeding birds, such as cattle egrets can also help you find caterpillars. If they are very interested in a particular part of your field, you should find out why.

Fact #10: Fall armyworm moths usually fly between the hours from dusk until dawn. If you see clouds of moths, with a wingspan of about an inch, check your fields 7-10 days later for fall armyworm caterpillars.

Fact #11: We have a large choice of insecticides that will control fall armyworm in pastures. See the armyworm section in the forage chapter of Alabama Cooperative Extension System ANR-500A, Alabama Pest Management Handbook Vol. 1, www.aces.edu/pubs/docs/A/ANR-0500-A/VOL1-2011/forage.pdf, for the insecticides that are currently legal to use in perennial grass pastures and hayfields. Watch out for grazing and harvesting intervals, which restrict how soon a field can be grazed or harvested following application. Insecticides with longer residuals are a better choice for outbreak years. Some insecticides will only kill smaller caterpillars, so check the comments provided about each insecticide. In the next article, I will discuss how to choose an insecticide for controlling fall armyworms.

You can find much more information about fall armyworms in Alabama Cooperative Extension System Circular ANR-1019, Management of fall Armyworms in Pastures and Hayfields, available from your county Extension office, or at http:/ /www.aces.edu/pubs/docs/ A/ANR-1019/ANR-1019.pdf.







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