You Are Here: Management Updates 2001 Archive July 13, 2001

Date: July 13, 2001
Category: Insects
Subject: Armyworms

We continue to get new reports of armyworm infestations in turf settings, primarily in the western end of the state. In addition, there are numerous reports of severe damage in hay fields, and occasional notes of attacks on corn.

It appears that the worst is over, or will be in a few days, in the eastern part of the state, where the first reports of large caterpillars first surfaced two weeks ago. In many areas of southeastern Massachusetts (e.g., Plymouth County and Cape Cod), caterpillars have finished feeding and have pupated. We anticipate that people in those areas will begin to notice large numbers of moths flying at night. Entomologists in some of the states south and west of us, which experienced similar outbreaks a few weeks ago, have reported that the "nuisance flights" of moths have been a source of great consternation for homeowners. Given that we have observed thousands of caterpillars on some lawns, we can expect thousands of moths to emerge in many locations.

Folks in the western part of the state are still seeing caterpillars, and some areas may continue to sustain damage for a few more days before the "worms" finally begin to pupate (form cocoons) in the lower thatch or upper soil zones. Once the caterpillars are 1.5 inches long, you can figure they are almost done feeding.

Armyworm moths will not cause any damage to plants - they may feed a bit on nectar but will not do any damage. Their sheer numbers may be daunting. At this stage, the moths are vulnerable to movement in wind currents (remember, that is how they arrived in Massachusetts in the first place!). If we are experiencing unsettled weather patterns when the moths emerge, we can hope they will be disbersed to other locations. In any case, there is a good chance that moths will not settle in large numbers in areas that were already attacked earlier this year. (Many lawns that sustained heavy damage still look pretty sparse, and moths will seek more lush settings.)

Just to be on the safe side, homeowners and turf managers might want to monitor for caterpillar activity later in the summer. Bringing out the crystal ball, I would predict that any new caterpillars would begin to appear about two weeks after we see large moth flights. (It takes a while for the moths to develop and lay eggs, and it takes a few days for those eggs to develop before the baby caterpillars emerge.) The baby caterpillars will be very small, perhaps 0.25 inch long, and may appear to be more green than the older caterpillars we observed in late June and early July. In addition, the small caterpillars often move along in a looping gait, rather like "inchworms".

You can scout a turf area for armyworms by parting the thatch gently with your fingertips and looking for the guilty parties. You can also use a soapy flush to irritate the caterpillars and force them to the surface, where you can see them quite easily. Put one or two tablespoons of a lemon-scented dish detergent in one or two gallons of water and pour the frothy mix over an area that is one or two feet on a side. Within a couple minutes, any caterpillars that are in the area will be irritated and will crawl up. Be careful not to use this technique on fine bentgrass or other sensitive turf on a very hot, sunny day, because the soapy water can act like a magnifying glass and you can burn the turf.

If homeowners experience an infestation of armyworms in late July or early August, and they recognize the situation before the caterpillars are 0.5 inch long, they may derive some success from applying an insecticide that is labeled for armyworms and lawns. Once the caterpillars get to be larger than an inch, such materials are markedly less effective. In addition, homeowners can try one of the Bacillus thuringiensis products labeled for use on lawns. Again, BT is much more effective on small caterpillars. Finally, the sprayable formulations of any of these products (i.e., formulations that are diluted in water and sprayed) typically will be more effective than the same product applied as a granule. The liquid version provides more even coverage and makes it more difficult for the caterpillars to avoid contact with the product.

Commercial turf managers in states that permit its use might experiment with Conserve (spinosad), a naturally derived product that is effective against many caterpillar species. Apparently, the material has been effective against medium-sized caterpillars in corn. Conserve is labeled for use in many kinds of turf, but in some states the identification of the caterpillars is critical.

- Submitted by: Dr. Pat Vittum