Armyworms Creating Battlefield In Massachusetts

The information in the following "frequently asked questions" has been compiled from e-mail exchanges with Dr. Paul Heller (Pennsylvania State University) and Dr. Dave Shetlar (Ohio State University), as well as from three turf insect references:

• Vittum, Patricia J., Michael G. Villani, and Haruo Tashiro. 1999. Turfgrass Insects of the United States and Canada, Second edition. Cornell University Press, Ithaca, NY.
• Potter, Daniel A. 1998. Destructive Turfgrass Insects. Ann Arbor Press, Chelsea, MI.
• Watschke, Thomas L., Peter H. Dernoeden, and David J. Shetlar. 1995. Managing Turfgrass Pests. Lewis Publishers, Boca Raton, FL.

WHAT ARE THE CATERPILLARS THAT ARE COVERING MY LAWN (OR GOLF COURSE, OR ATHLETIC FIELD)? The incredible infestations that are being reported throughout Massachusetts are huge populations of the armyworm, Pseudaletia unipuncta. There are many species of "armyworms," most of which exhibit very typical behavior - appearing in overwhelming numbers over a short period of time, congregating and feeding in densely packed groups that migrate from one site to another. The accepted common name of the species we are seeing on Massachusetts turf is the "armyworm," but some people call it the "common armyworm," to avoid confusion with some of the other species.

WHAT IS DIFFERENT ABOUT THESE CATERPILLARS? Young armyworm caterpillars are pale green and look like inchworms, "looping" across the grass. Full-grown caterpillars are about 1.5 inches long, dark green or green gray on the top (some describe it as navy blue or black). There are two pale stripes along each side of the body and another straw-colored broken stripe down the middle of the back. When the caterpillars are disturbed, they tend to curl into a tight ball. The head of the caterpillar is light brown with a series of intertwining dark lines. The caterpillars are relatively smooth (not hairy, like gypsy moths).

WHERE DID THESE ARMYWORMS COME FROM, AND WHY US? It is not clear whether the armyworms can survive winter conditions in New England, but there is little doubt that the moths can fly into the region in large numbers each spring. We believe that moths arrived on one of the frontal systems that delivered thunderstorms in late May or early June, marking the end of the six-week mini-drought that much of New England experienced. Armyworms often attack corn, but corn had not yet germinated because the soils were so dry. So we suspect the moths sought an alternative and found lawns and other expanses of turfgrass to be very suitable.

Heavy armyworm infestations are not unheard of in New England, but they are quite uncommon. We do not yet know why this year has produced a banner crop, but we think the dry conditions when the moths arrived induced moths to lay eggs in turf, and the subsequent wet conditions may have been beneficial to caterpillar survival.

WHAT CAN WE EXPECT IN THE NEXT WEEK OR SO? AND WILL THE GRASS RECOVER? Armyworm caterpillars tend to move en masse from one location to another. Homeowners whose lawns are currently inundated with caterpillars (variously described as "Medusa's hair, or something from Hitchcock's "The Birds") can expect that caterpillars may persist for several days before they move on to a neighboring site. The caterpillars may be less active in the heat of the day and may migrate short distances to stay in shade. As evening approaches, they begin to move back to the turf areas to resume feeding.

Most damage appears to be relatively superficial - the caterpillars feed on grass blades and upper parts of stems, but seldom feed on the crowns of plants. This means that many turf areas should be able to recover reasonably well if they can be irrigated during the next couple of weeks.

If most caterpillars in an area are an inch or more in length, attempts at controlling populations may not be very successful. But the good news is that large caterpillars will be moving on relatively soon - or will pupate in preparation for emerging as moths.

WHAT CAN WE EXPECT FOR THE REST OF THE YEAR? The armyworm probably completes two generations a year in southern New England but may be able to complete a third generation this year (because the spring, overall, has been quite mild). In most cases, only one generation of caterpillars will devastate a given location. In other words, even though we seem to be blessed with populations of biblical proportions right now, there is a good chance that subsequent populations will move on to other areas - perhaps corn or field crops, or blown downwind. We can hope they are blown out to sea.

SHOULD I TREAT MY LAWN? It is impossible for us to make a blanket recommendation regarding treatment of lawns or other turf areas. Each person has different conditions, expectations and views about insecticides. Application of a lawn insecticide that has armyworms on the label should provide some relief for those who cannot tolerate caterpillar activity - but the treatment window is very narrow. In other words, if you are going to treat, do it soon.

There is some evidence that liquid formulations (once they are diluted in, and sprayed with, water) are slightly more effective, partly because they provide more uniform coverage on the foliage.

For those who are reluctant to use insecticides, keep in mind that many of the armyworms will be moving reasonably soon, or will be pupating. In either case, they should not persist too much longer. Of course, there are no guarantees.

WHAT IS THE LONG-TERM PROGNOSIS? Given that many of the turf professionals in eastern Massachusetts have never seen such an infestation, even in 20 or 25 years of experience, we presume that this is a "once in a lifetime" event. We believe the crazy weather patterns this spring somehow were perfect for armyworm explosions. Areas that have experienced similar plagues of armyworms in previous years normally have not seen outbreaks in subsequent years. Again, there are no guarantees, but at this point it would be difficult to justify preventive applications of insecticides to protect against future outbreaks. Scouting regularly (especially about two or three weeks after we observe large numbers of moths flying) should enable turf managers to determine whether there will be another explosion this year and whether further control actions will be necessary.

CULTURAL RECOMMENDATIONS FOR RECOVERY FROM LAWN CATERPILLAR (ARMYWORM) INFESTATIONS. Turf can usually outgrow light caterpillar infestations when plant growth conditions (temperature and soil moisture) are favorable. Extended weather forecasts for early July in Massachusetts indicate unseasonably cool conditions (high around 80ºF) that are favorable for growth of cool-season turfgrass. Because lawn caterpillars are foliar feeding insects, most turf may recover under such conditions. Furthermore, light applications of fertilizer (0.5 pounds of nitrogen per 1,000 square feet) and ensuring soil water is non-limiting by irrigating insect damaged lawns can enhance recovery. In the end, maintaining a healthy turf with fertilizer and water will maximize recovery so long as heat (and humidity) is not oppressive (>85ºF).

In the fall, if insect damage was especially severe and recovery slow, consider over-seeding with endophytic grasses (insect resistant perennial ryegrass and fine-leaf fescue) and reduce thatch where a surface-feeding insect can hide. Also, soil testing in the fall will provide critical information regarding the nutrient status (phosphorous and potassium levels) to ensure optimum nutrition for maximum recovery from insect damage as well as summer stress. Corrective applications of fertilizer addressing phosphorous and potassium deficiencies (identified by soil testing) should be applied beginning in September during periods of peak shoot growth and turf recovery.