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The Armyworm Saga Continues

May 25, 2001
The armyworm outbreak of 2001 has been one for the record books. According to most observers who have spent more than a couple of decades in Illinois agriculture, this has been the worst armyworm outbreak they have ever seen. The numbers of reports of serious damage to pastures, grass hay fields, no-till corn, wheat, and both corn and soybeans adjacent to wheat fields or pastures have been innumerable. People in towns in rural areas have seen armyworm larvae invade yards, stripping the turf to the ground and defoliating gardens. Reports of 30 to 70 or more armyworm larvae per square foot have not been uncommon. Most reports during the first couple of weeks came from southern and central Illinois. More recently we have become aware of infestations as far north as Kankakee, Knox, and LaSalle counties.

Armyworms on wheat heads. (Photo courtesy of Robert Bellm, Crop Systems Educator, Edwardsville Extension Center.)

Many people who have called on the telephone or sent e-mail messages have asked the same questions about armyworms, among which are the following:

· Why have armyworms been so bad this year?

· How much longer will armyworms be around?

· How will the recent cool temperatures affect the armyworms?

· Are natural enemies slowing armyworms down any?

· Will we have to deal with armyworms any more this year?

· What insecticides are effective against armyworms?

I'll attempt to address all of these questions in this article. In addition, when I discuss insecticides, I'll offer strongly worded advice about which insecticides can be used for different crops.

Why have armyworms been so bad this year? As I indicated in the previous article, we probably will never know all of the reasons why the armyworm outbreak occurred this year. It's apparent that a lot of adults flew into the Midwest from southern states this year, and it's just as apparent that environmental conditions have been conducive for armyworm survival. Until recently, the parasitoids and pathogens that often suppress armyworm populations have been most noticeable by their absence. So the combination of a major moth flight, favorable conditions for survival and development, and the low impact of natural control factors have made life pleasant for armyworms this year. Some of the older literature indicates that cool, wet spring weather the year following a drought increases the potential for an armyworm outbreak and that high temperatures are not favorable for armyworms. I've always had great faith in the older literature, but these forecasts have not panned out this year.

How much longer will armyworms be around? This question is not always easy to answer. When people report finding armyworms that are 1-1/4 to 1-1/2 inches long, we can assume that the larvae are fully mature, or nearly so, and will finish feeding within a few hours to a couple of days, depending on how recently they molted to become the sixth instar. An armyworm larva has six instars and requires about 3 to 4 weeks to complete development, depending on the temperature. The sixth instar usually requires about 7 days to complete development, and this instar consumes about 80% of all the foliage eaten during larval development.

In search for answers to this question, we checked out a couple of articles published in the scientific literature in the early 1960s. Both of the articles, which were published by Canadian entomologists, included information about the lengths of time required for development of the six (occasionally seven) instars. Citations of these articles are:

· Guppy, J.C. 1961. Life history and behaviour of the armyworm, Pseudaletia unipuncta (Haw.) (Lepidoptera: Noctuidae), in eastern Ontario. Canadian Entomologist 93: 1141­1153.

· Pond, D.D. 1960. Life history studies of the armyworm, Pseudaletia unipuncta (Lepidoptera: Noctuidae), in New Brunswick. Annals of the Entomological Society of America 53: 661­665.

Guppy (1961) measured the durations of the larval instars in field rearing cages from 1957 through 1959. He examined 25 to 30 groups of 50 to 100 larvae each to obtain the data. However, because he reared them in the field, he provided no information about the effects of different temperatures on the duration of each stage. He reported the following mean duration of development of the six instars in the first generation: first instar--4.8 days; second instar--3.3 days; third instar--3.3 days; fourth instar--3.8 days; fifth instar--4.4 days; sixth instar--10.3 days. The duration of the sixth instar ranged from 7 to 13 days during the 3-year study.

Pond (1960) reared armyworms in an insectary and a greenhouse, and measured average maximum, average minimum, and mean temperatures to determine the effect of different temperatures on duration of development. He reported the following ranges of duration of development at mean temperatures that ranged from 62.7 to 72.6°F: first instar--12.8 to 5.5 days; second instar--6.2 to 3.16 days; third instar--5.7 to 2.71 days; fourth instar--7 to 3.5 days; fifth instar--7 to 4.19 days; sixth instar--8.57 to 5.94 days. He also reported some seventh instars (only at the lower temperatures) that ranged in development from 9.4 to 11 days. In general, for all instars, duration of development decreased as the temperature increased.

How will the recent cool temperatures affect the armyworms? Related to the preceding question, cooler temperatures will slow down armyworm development, so the larvae will persist for a longer time. At this point, a good spell of hot weather will get us through this first generation of armyworms more quickly.

Are natural enemies slowing armyworms down any? During most years, a slew of predators, parasitoids, and pathogens suppress armyworm populations, usually keeping them below economically damaging levels. However, the impact of these natural enemies has been minimal thus far this year. But all is not lost. Matt Montgomery recently encountered some diseased and parasitized armyworms (some were both diseased and parasitized), suggesting that natural enemies might begin to help us out a bit. Several types of viruses, a microspori-dium, and fungi infect armyworm larvae. Infection of a few larvae with pathogens could result in an epizootic that wipes out armyworm populations, as we experienced a few years ago when a virus caused armyworm populations to "crash," seemingly overnight. Tachinid flies deposit eggs on armyworm larvae, usually on the thorax near the head so the armyworm can't bite the eggs off. Some parasitic wasps also parasitize armyworms, most notably the braconid wasp, Cotesia marginiventris. A combination of these natural enemies could eventually have some impact on the current and future generations of armyworms.

Armyworm larvae killed by pathogens. Note the egg of the tachinid fly on the specimen on the left. (Photo courtesy of Matt Montgomery, Sangamon/Menard Extension Unit Educator.)

Eggs of a tachinid fly on an armyworm larva. (Photo courtesy of Matt Montgomery, Sangamon/Menard Extension Unit Educator.)

Will we have to deal with armyworms any more this year? As suggested in previous paragraphs, the armyworm completes two, and occasionally three, generations in Illinois. Therefore, we will have to watch for armyworms again about 1 month from the time pupation occurs. Our experience over the years suggests that the second generation is far less threatening than the first generation. However, stranger things have happened. The bottom line is that grass pastures, grass hay fields, and corn adjacent to grass waterways or grassy field margins will be at risk when the second generation occurs. We now are well aware of the suddenness and severity of an armyworm attack, so we will have to maintain a vigil. A quote from Fundamentals of Applied Entomology, edited by Robert E. Pfadt, says it all: "Shrewd growers inspect crops regularly for armyworms as well as for other insect pests."

What insecticides are effective against armyworms? The answer to this question depends on the crop (what is registered?) and who is reporting. Many people have been spraying pyrethroids (especially Pounce and Warrior), and most people have been satisfied with their effectiveness. Others have applied malathion or Sevin to pastures, with variable results. I have heard reports of excellent-to-very-poor results from an insecticide application, regardless of the insecticide used. It's possible that the larger armyworms are tougher to kill than the smaller larvae. Interestingly, the Warrior label does not seem to include the "true" armyworm on its label for corn, although other species are listed. For the other species listed, a footnote indicates "For control of first and second instars only." The "true" armyworm is listed on the Warrior label for wheat and wheat hay, but no such footnote about first and second instars is printed. I'm not certain what this means, but it contributes to some of the confusion about insecticides for control of armyworms.

Now for the strongly worded advice. We recognize that there are not many options for control of armyworms in pastures and grass hay fields. So far as we can determine, malathion and Sevin are the only chemical insecticides labeled for control of armyworms in these types of fields. In the past, several other insecticides (for example, Lannate and Penncap-M) were labeled for control of armyworms in pasture. But for whatever the reason, these sites are no longer listed on the labels of insecticides other than malathion and Sevin. Pounce and Warrior are not labeled for use in pastures and grass hays. There are no exceptions. Application of either of these products, or others not labeled for use in pastures or grass hay fields, is illegal. Also of note, the harvest interval (required time between application of an insecticide and harvest of a crop) for Warrior on wheat is 30 days. Let's be cognizant that agriculture is under a lot of scrutiny, so we should avoid doing anything foolish.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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