UMass Extension Turf Program


July 31, 2001

Armyworms - What's Next?

While most areas are reporting much less activity from armyworms at this time, many people have been asking what they can expect to happen next. This update is provided, along with a disclaimer ... since it has been at least 30 years (and probably closer to 60 years) since we have experienced an invasion of armyworms similar to the one we have had in 2001, there are many things we do not know about armyworms. So we are trying to anticipate their activity, based on the experience of colleagues in other parts of the country.

We have received several reliable reports of locations where heavy infestations of armyworm caterpillars were attacked by parasitic insects or by a fungus. While we do not know exactly which species of beneficial insects were involved in the parasitization, we do know that parasitic wasps and parasitic flies have been observed emerging from caterpillar cadavers. My assumption is that these parasitoids were already present in the area, and took advantage of the huge populations of caterpillars, adapting to the bounty that surrounded them. We don't know the life cycles of the parasitic wasps or flies that have been noted, so we don't know whether they will have a second generation this summer that coincides with a second generation of caterpillars... but we can hope they do!

Meanwhile I saw caterpillars that had been attacked and killed by a naturally occurring fungus. This fungus might be an opportunist, but it undoubtedly had an impact on armyworm populations in some areas. While we don't know the life cycle of this fungus, either, we can assume that the number of fungal spores in the environment is much higher than it was before the first invasion. Perhaps if weather conditions are suitable, the spores will provide some level of natural reduction of caterpillar populations during the rest of this growing season.

Some areas in eastern Massachusetts may already be experiencing the development of small caterpillars. (I received one unsubstantiated report, from a reliable source, of small caterpillars, less than a quarter inch long, showing up in fields in the Dracut-Tewksbury area.) Other areas of eastern Massachusetts should be on alert now - this would be a good time to start watching for large numbers of moths flying in the evening, and to start monitoring for caterpillar activity. (See the description of a soapy flush in an earlier update.)

We still do not know much about why armyworm invasions occur in one spot and not another. One possible explanation is that moths are carried in on frontal systems, and are deposited in downdrafts associated with storms. If we have a continuation of the current relatively calm weather pattern, moths may not be moved long distances. In other words, they may remain fairly close to the areas from which they emerged.

Turf managers and homeowners will want to monitor their lawns or turf, to ascertain whether they have small caterpillars showing up. (Remember that the small stages often will "loop" along with a gait like an inchworm, and may be lighter and greener than the larger stages.) If do find large numbers of the small caterpillars, you will have several control alternatives to consider. However, do not panic if you find a few caterpillars in the lawn or athletic field. Sod webworms have been active for a few weeks, but are much less damaging "pound for pound" than armyworms. Golf course managers are very accustomed to managing black cutworms, another caterpillar. The take home message - not every caterpillar in turf is an armyworm!

In terms of predictions, I suspect most turf areas that were hit hardest in July will NOT be hit hard again in August, in part because those areas are still looking a bit sparse. The moths will be seeking lush sites to lay their eggs. Meanwhile many agricultural fields may look more attractive now than they did in May - good news for the homeowner in suburbia, bad news for the corn grower or dairy farmer!

Submitted by: Dr. Pat Vittum


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