Photo Credit: 
Randy Mitchell | For The Chronicle
A group of fall armyworm caterpillars (Spodoptera frugiperda) crawl along in a container. These caterpillars can vary in appearance and coloring.
A group of fall armyworm caterpillars (Spodoptera frugiperda) crawl along in a container. These caterpillars can vary in appearance and coloring.

The "armyworms" which are currently infesting certain areas of Southwest Oklahoma aren't really worms, they're caterpillars — caterpillars of the fall armyworm moth (Spodoptera frugiperda).

Entomologists at Oklahoma State University report that fall armyworm caterpillars are potential turf pests in late summer and fall. Large numbers are capable of consuming all above-ground plant parts, and they are capable of killing or severely impeding the growth of grasses.

This species has a permanent range in areas of warmer climate. However, a portion of fall armyworm moths migrate northward each year as summer approaches.

Something I learned recently that seemed quite fascinating to me is, although a certain number of moths migrate north, the offspring of these moths do not migrate south. The northward migrant population just dies when it becomes too cold for them to survive.

I found that odd because it didn't appear to me as if the expansion contributed to the survival of the species.

However, last week, I spoke with Dr. Eric Rebek, a professor of entomology and state extension specialist at OSU, about Spodoptera frugiperda, and he explained it to me.

"It is true that the populations that migrate north don’t survive ultimately, but the species survives because populations that remain in their native range (i.e., don’t migrate) continue to reproduce year-round," he said. "So, while it doesn’t intuitively make sense for a species to move to areas where they won’t establish in the long-term, organisms regularly test the limits of their range expansion and succumb (or not) to the elements."

Spodoptera frugiperda is native to tropical regions in the western hemisphere, from the United States to Argentina. The only places it normally overwinters successfully in the U.S.A. is southern Florida and southern Texas, according to entomologists.

The species which expands north into Oklahoma comes from Mexico and South Texas.

Rebek indicated that this year is a big deal when it comes to the fall armyworm.

"Right now, we're definitely seeing a huge outbreak of these critters," Rebek said. "I've only been in Oklahoma for about 15 years now, (and, while) talking to a few old-timers, they said this is the worst fall armyworm season they've seen in about 20 years. So, there's just something special about this year — and I couldn't tell you what that is — that's been a contributing factor to this explosion in their numbers."

And the caterpillars are more prevalent in some areas, while nearly non-existent in others.

"There are certain areas that are infested more than others," Rebek said. "My yard, I've not really seen much activity at all, but when I get closer to campus at OSU in Stillwater, particularly at the botanic gardens right now, there's a pretty heavy infestation of fall armyworm there. Sometimes, they just end up in a particular area very different one mile away from another (area). They may find more food, more suitable habitat, something that they're preferring that they will just kind of settle in that spot, and not others. And so, you can see drastic differences even in the same town in terms of lawns or parks or golf courses that are infested."

How they get here

Rebek said during migration, the moths are often carried along by wind currents from the south.

"It's not directed movement on their own," he said. "They're kind of at the mercy of the prevailing winds, and they just kind of settle out accordingly when they drop out from there. I mean, they're flying, they're not just getting blown in the wind, but the wind definitely has a big role in pushing them northward (more so) than they could under their own power."

End in sight?

As mentioned previously, these moths and caterpillars will die off when it becomes too cold for them. But how cold must it be?

"That first good killing frost, that's when the caterpillars will cease activity, they die off," Rebek said. "Because they're cold-sensitive, it ends in their demise once the frost starts hitting."

But when does the first killing frost occur? Well, I'm going to be cautious about predicting that, but I'm going to guess some time in November, perhaps the latter half of the month.

Last year, in a column about ragweed for another publication, I wrote that experts say that ragweed dies off after the first frost. I also wrote, "And the first frost of the area typically occurs around Halloween, according to the Old Farmer’s Almanac."

Well, a frost did occur around that time, and a lot of ragweed did die off, but some persisted into early December! I found that quite surprising.

Appearance

Providing a description of Spodoptera frugiperda is tricky because both the moths and the caterpillars can vary greatly in appearance.

The caterpillars grow to about 1.5 inches long and are marked with green, brown or black colors arranged in stripes, with darker stripes along the sides (see photo). Their body color can range from green to brown or black, and they have a distinct stripe along each side of the body (see photo).

The top of each abdominal segment is marked with two pairs of black dots from which stiff hairs arise. The front of the dark head capsule is marked with a pale colored upside-down “Y” (see photos).

OSU entomologists report that "male moths have dark gray front wings mottled with darker and lighter splotches. There is a prominent pale, diagonal marking near the center of the front half of each wing and a prominent white spot at the extreme tip (see photo). The front wings of female moths are dull gray brown with only small, inconspicuous markings (see photo). The hind wings of both sexes are white with a slight purplish sheen. The wing spread is about 1.5 inches.

Range

As mentioned previously, the fall armyworm does not overwinter in Oklahoma, but the moths of which re-infest the state each year after migrating northward from Texas or Mexico. Migrating populations usually reach Oklahoma by late June.

Reproduction

Each female is capable of laying about 1,000 eggs in masses of fifty to several hundred, and larvae are present by early July.

Rebek reports that larvae develop through six instars. One generation of fall armyworm can develop in about 18-28 days, depending on temperature. In Oklahoma, there are 2-3 generations present from late July through late October.

After feeding for two to three weeks, larvae dig into the soil to pupate. A new generation of moths emerges about two weeks later. There are several overlapping generations extending into October or even November in some years.

Moths are attracted to lights and may lay masses of eggs on non-host plants, walls, clothes on lines, etc.

The eggs are pale gray, laid in masses, and covered with grayish, fuzzy scales from the body of the female moth.

Food

Rebek said that fall armyworms are surface-dwelling "climbing cutworm" caterpillars. They prefer to eat grasses, and often go unnoticed until they become large, and their feeding damage becomes evident. Fall armyworms tend to prefer bermudagrass, but they also feed on tall fescue and other turfgrass species.

They feed at night, early morning, late evening or on cloudy days. During the heat of the day, they will hide either underground, or under debris such as leaves.

Feeding behavior

While I haven't seen many fall armyworm caterpillars around my home, I found a residence just northeast of the town in which I live where they are prevalent. From what I've observed on several mornings, these caterpillars follow the shade. As the sun rises each morning, I've watched as dozens follow the line of shade and eventually wind up in a wooded area.

Another behavior I observed is, when I captured a few caterpillars and placed them in a container, I learned they weren't very friendly to each other. In fact, they were outright aggressive, often biting each other when one was too close to another.

I later found out that these caterpillars will cannibalize each other, with large larva often eating smaller ones. Yikes!

Testing

According to entomologists, fall armyworms can be detected through close

examination of the turf, or by using a soapy water flush. A soapy water flush involves mixing one tablespoon of lemon-scented dish soap per gallon of water and pouring the solution over several small areas of damaged turf. If present, larvae should be visible within 30 to 60 seconds as they become irritated by the soapy water and leave their hiding places.

If three to four larvae are found per square foot, treatment may be warranted in commercial turf or golf courses. Homeowners should carefully consider the need to control fall armyworms, according to entomologists.

Odds and ends

• I mentioned the dreaded ragweed earlier. Just FYI, mid-September is peak ragweed season. As I'm writing this, the ragweed pollen count is high, and I can't breathe! And the ragweed pollen count is predicted to remain high for the foreseeable future.

 

Randy Mitchell is a freelance writer and photographer. He has been an avid birdwatcher, nature enthusiast and photographer for more than 40 years. Reach him at rnw@usa.com.

RANDY’S NATURAL WORLD