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Management Updates: Aug 23, 2017

Turfgrass Ants, Sod Webworms, White Grubs
Aug 23, 2017

Over the past several days, I have observed several things on my own lawn that might be of interest to some of you.

Turfgrass ants - Ant mounding has become apparent in some of the drier areas of my lawn. While ants seldom cause damage at lawn heights of cut, they can be quite a nuisance on fairways or greens of area golf courses. (The ants push up mounds that can be abrasive to mowing equipment, can dull the blades, and can suffocate the turf underneath them.) Often overlooked, however, is that ants are usually good predators of other insects, including the eggs and small larvae of white grubs and caterpillars.

If a golf course superintendent believes that treatment is necessary, there is a short term approach and a long term approach. The short term approach involves applying a pyrethroid or chlorpyrifos (the latter is only registered for use on golf courses). These products will kill many of the ants that are on the surface, foraging or defending the colony, and this will reduce the evidence of mound building. However, the chemicals will not reach the queen, so she will continue to produce prodigious numbers of eggs and the colony will persist. To truly impact the colony, the superintendent must use an ant bait registered for use on turf against ants. The baits are much slower, so you will not see an immediate knockdown in mounding activity, but the effect will be much longer lasting once the queen is killed. If you use a bait, do not apply before rain, and do not irrigate for at least 24 hours after application.

Sod webworms - I have noticed large numbers of sod webworm moths flying around my lawn over the past couple weeks. While there do not seem to be quite as many as I saw last year, they are quite noticeable. (The moths are rather tubular in shape, as they fold their wings tightly around their bodies, and they tend to fly haphazardly just above the canopy, often in late afternoon or evening.) I did not notice any damage from the caterpillars that hatched last summer. Indeed I do not usually receive complaints about sod webworm feeding activity in New England.

Nevertheless, sod webworms (there are many species, at least some of which have two generations per year with adults laying eggs in late summer) are capable of damaging turf. In this case the caterpillars feed through late summer into mid fall, and then spin a little silken "hibernaculum", like a silk blanket, that protects them from desiccation, and hibernate through the winter. During that hibernation, they replace most of the water in the tissues between the cells with a liquid that acts like anti-freeze, preventing cell damage when the insect encounters freezing conditions. They emerge in mid spring and resume feeding for a few weeks before pupating and emerging as adults in late spring or early summer. So if you see new caterpillar damage in the spring in New England, it probably is caused by one of the species of sod webworms that overwinters as caterpillars.

Sod webworms appear to be perfectly happy feeding on home lawn-maintained turf, as well as athletic fields, golf courses, parks, and cemeteries. So if you believe you might have had some damage last fall, or you are noticing an unusually large number of moths flying this year, you can apply a pyrethroid sometime in the next week or two. The pyrethroid will stay in the thatch and kill the caterpillars as they hatch out of the eggs.

White grubs - While poking around in my lawn last week, I found several white grubs (a mixed bag of species), all of which appeared to be right on target developmentally. In other words, they were young third instars, and clearly they were feeding quite actively. The timing of egg-laying and development seems to have followed the "normal" pattern this year, in part because we have received timely rain throughout the summer in the Connecticut River valley.

If you applied chlorantraniliprole between mid April and early June, you should be in very good shape. And if you applied a neonicotinoid during the ideal timing (when the adults were laying eggs) in early summer, you should also achieve very good levels of control. The exception for both approaches is if you have very thick or dense thatch. Either condition can make it difficult to achieve good contact of the insecticide with the grubs.

Finally, now would be an excellent time to scout for white grub activity. As mentioned, the grubs are getting quite large but because we have had consistent rain events, the turf is still growing quite well and masking grub activity. But as the grubs get larger (and if we experience a period of a week or more without rain), grub feeding damage will become apparent. The best curative approach for white grubs is trichlorfon, but it cannot be used on school grounds in Massachusetts (and several other states in the Northeast). This restriction is at least in part because it is a very soluble chemical so it is more likely to leach or run-off than most other grub insecticides.

Submitted by: Dr. Pat Vittum