You Are Here: Management Updates 2001 Archive July 10, 2001

Date: July 10, 2001
Category: Insects
Subject: Armyworm Frequently Asked Questions - Part II

The following update is provided with the help of Deb Swanson (Plymouth County) and Mary Owen (turf specialist), who have been handling countless calls and visiting field sites. Their descriptions of the circumstances in the eastern part of Massachusetts have been invaluable in piecing together what is happening with the armyworm in turfgrass.

What is the situation with armyworms now?
Armyworms (Pseudaletia unipuncta) have been active throughout Massachusetts for about two weeks, now. The first reports of heavy infestations came from turfgrass settings (home lawns, golf courses) in southeastern Massachusetts but subsequently, armyworm infestations have been reported in turfgrass, hay and alfalfa fields, corn, and other vegetables in most regions of the state.

In most locations, caterpillars were already half an inch long or longer by the time activity was first noticed. They are efficient feeding machines, and grew quickly to 1.25 to 1.5 inch long. Those who first observed caterpillars two weeks ago have reported that the caterpillars are no longer active. They have pupated, and armyworm moths should be emerging fairly soon. (The moths are a dull tan color and are night fliers. Some areas that experienced similar armyworm "explosions" earlier this year in other parts of the country have reported that thousands of moths emerged in some areas, creating a nuisance in their own right.)

While armyworm caterpillars are normally described as being "nocturnal" (active at night), there have been numerous reports of caterpillars feeding throughout the day, particularly on cloudy days. In some cases, armyworms have been moving throughout the day, staying along a shade line or migrating to protected areas during the hottest hours of the day and returning to turf or fields at night.

We can expect this infestation to begin to decline very soon in most locations, if it has not already begun to do so. As a guideline, once most of the caterpillars are 1.25 inch long, you can expect those caterpillars to complete their development (and stop feeding) within a week, and probably sooner than that. However, from descriptions we have received, there appear to be a few locations where caterpillars are still a little bit smaller, and may continue to be problematic.

Why are we seeing dead or dying caterpillars in areas that have not been treated?
Some turf managers have reported situations where numerous caterpillars on a home lawn have turned black and died, even though that lawn had NOT been treated with an insecticide. We do not know why this is happening, but two possible explanations could be considered. First, dense armyworm populations (reports of as many as 100 to 150 caterpillars per square foot in southeastern Massachusetts) are under considerable stress, and it is hard for each individual to find enough grass (or hay or whatever) to feed on. So it is possible that naturally occurring pathogens, which ordinarily might not be "strong" enough, are killing some of the caterpillars. Another possibility is that caterpillars may have migrated from a neighboring lawn that had been treated previously.

Is it too late to apply insecticides to control the caterpillars?
While there are some insecticides available to homeowners, and additional products are available for commercial applicators, it is probably too late to treat infestations in most locations.

Once caterpillars are 0.75 to 1.0 inch long, they are much less susceptible to the insecticides, and it becomes difficult to justify applications at that point. Commercial vegetable growers or dairy farmers are facing a very different situation, and should consult their Extension specialists for recommendations.

Keep in mind that in almost all locations of the state, caterpillars are very near the end of their development, and will be out of sight very soon. Once they finish feeding, they will pupate (form a cocoon) in the thatch and emerge as night-flying moths, dispersing to other locations. So for most people, an insecticide application at this time probably is not economically viable or justifiable environmentally.

What can we expect - when will caterpillars show up again and is there anything we should do in preparation?
Armyworms usually complete two or three generations in New England, but the good news is that normally an area that is heavily infested in early summer is not similarly attacked later in the year. Usually the moths that emerge will migrate or be blown downwind to new locations. Colleagues at other state universities have told me that they have never seen a given location attacked a second time in a year. So while there are no guarantees, it is most likely that once we have weathered the current onslaught, we should be able to relax a little bit.

We can expect to see large numbers of armyworm moths flying very soon in the southeastern part of the state, where the infestations were first reported. In areas where caterpillars are still apparent, moths may not start flying for another week or ten days. In any case, the moths are night fliers and may be a nuisance in areas where they are attracted to street lights or homes. The moths will not cause any damage to turf or anything else, but of course they are looking to mate and lay eggs for a new generation of caterpillars.

About two weeks after the peak moth flight, we can expect to see small caterpillars emerging from eggs. At this point homeowners can start monitoring their own lawns for caterpillar activity. The small "worms" are hard to see without some help, but you can flush them to the surface of a lawn with a soapy flush. Mix one or two tablespoons of a lemon-scented liquid dish detergent in one or two gallons of water. Get it nice and frothy, and then pour the mix over an area one or two feet on a side. Caterpillars (along with earthworms and other interesting things) will be irritated and wriggle to the surface, where you can take an inventory. Keep in mind that small armyworm caterpillars often are more green in color than the older stages, and tend to "loop" along, resembling inchworms.

We expect that most areas which were hit in June will NOT be similarly devastated in July or August, but monitoring with a soapy flush might put people at ease.

Can we use BT against these caterpillars?
Bacillus thuringiensis (BT) is a bacterial pathogen that causes diseases in various insects. There are several strains of BT, each of which is relatively specific. The kurstaki strain is effective against several kinds of caterpillars, and is used to suppress gypsy moth populations in suburban settings. The kurstaki strain is also used occasionally to suppress cutworm or webworm populations in turf, with varying degrees of success.

While we have not located any field data on the effectiveness of BT against armyworms in turf, we believe it could be at least somewhat effective. However, BT is much less effective against large stages of caterpillars, so at this point it is too late to use BT to treat armyworm-infested lawns. If soap flushes reveal small (less than a half inch) caterpillars later in the summer, BT could be considered as an alternative at that time.

Will my lawn recover?
Based on descriptions from Mary Owen and Deb Swanson, and based on my own observations, I expect that many of the lawns that have been attacked by armyworms will recover reasonably well. The caterpillars, while numerous, generally have been feeding on the blades and stems of the grass plants, and the growing points, which are hidden in the crown of the plant, have remained intact. If you are able to keep the lawn irrigated during the next couple weeks, and give those growing points a chance to recover, the lawn may well bounce back. It would probably be helpful to raise the mowing height of the lawn mower for a few weeks, as well, to give the grass every opportunity to recover.

Unfortunately, there appear to be a few lawns that were even more severely damaged, and they may not recover quickly. Such areas can be overseeded in later summer or early autumn. We would urge you to consider using turf varieties that contain endophytes (fungi that grow inside the plants and produce natural products that are toxic to certain insects, including webworms and some armyworms). These endophytic cultivars, available in some fescues and perennial ryegrasses, also tend to be more tolerant of drought conditions.

- Submitted by: Dr. Pat Vittum