Worm Invasion Devouring Lawns

Keith Fontenot  |  9/16/2010 11:20:19 PM

Lawn damaged by tropical sod webworms.

Moth that lays eggs that hatch into the larvae (worms) that feed on the turf.

Larvae of tropical sod webworm feeding on grass.

Just about anywhere you drive in and around Evangeline and surrounding parishes these days you will notice many lawns that look just terrible. These lawns have large areas of tan to brown colored grass, which seems to continue to expand and enlarge very quickly. In many cases these areas appear to have changed almost overnight or over a very short period of time. St. Augustine and carpet grass lawns appear to be the most severely affected but these invaders are not overlooking the other grasses such as centipede and bermuda.

In most cases we are seeing Tropical Sod Webworms, and to a lesser extent Armyworms in the lawn, devouring the grasses. The lawn problems we are seeing with these two worms is on the largest scale we have ever seen, or heard of for quite some time.


How do you know if it is tropical sod webworm damage?

The damage looks unusual and shocks most homeowner because it appears to happen overnight. The lawns may have a few chewed up circular areas or browned out areas that exceed 1000 square feet. At first glance you think it is caused by disease. However, after close inspection of the turf you will see that the leaves have heavy chewing damage. The young larvae feed along the mid-rib trough of the leaf.  Mid-sized larvae chew notches in the edges of leaves and older larvae will completely strip off the leaves, similar to the damage caused by armyworms.  It is the notched, ragged edges on grass blades that are the tip off.  Additionally, the worms leave trails of a silky web as they crawl through the grass. You can spot these “webs” in the morning when dew is on the ground. Like armyworms, watch for birds feeding in the area.

Armyworms feed at midday but sod webworms prefer to feed at night. One thing you will find is that just looking carefully through the grass during midday may not turn up any webworms. However, if you make up a 2 gallon solution of soapy water, and pour it in a transition area (damaged turf “brown area” transitioning to good green turf), the armyworms present in the area will go crazy. Tropical sod webworms will, at least, start moving, making them easier to spot as they move out of the sod and thatch.


Tropical sod webworm life cycle

Moths are 0.5 to 0.75 inches long and have snout-like projections on their heads. The moths can be seen fluttering over the turf and in flowerbed areas nearby. We have also seen some on carports and porches where the lights were on at night, the moths were on the bricks of the house in the morning. This short-lived moth does not feed on the turf but is responsible for laying the eggs in the grass that hatch into the larvae (worms) that feed on the turf.

The eggs hatch in seven to 10 days and the larvae begin feeding on the turf. The larvae (worms) can get up to one inch long and are light green to gray green with dark spots on their body. Some almost have a translucent or clear to light yellowish appearance to part of their body. There is no characteristic inverted Y shape on the head of webworms like you see with armyworms. The tropical sod webworm completes their development from egg to moth emergence in five to six weeks. The species over-winters as larvae in the soil. We could get three to four generations in a season.


How to control tropical sod webworms

The good news is that control is possible and the same insecticides that kill armyworms work well on tropical sod webworms. Insecticides labeled for use include but are not limited to:

Insecticide (common trade names)

Bifenthrin (Talstar, Quali-Pro Golf and Nurs-ery)
Carbaryl (Sevin, Carbaryl)
Chlorantraniliprole (Acelepryn)
Halofenozide (Mach 2, Macho)
Trichlorfon (Dylox)
Permethrin (Permethrin)


What can be done to help the lawn grass recover?

The combination of worm damage, hot temperatures, and drought may kill off lawns that were already weakened by winter cold damage, disease, low mowing, and overall low fertility. However, recovery is likely for lawns that were in good shape prior to the worm damage. Lawns will start to recover once the worms have been killed.

Fertilizing with nitrogen containing fertilizer will help lawns recover quickly but it is risky for another reason this late in the summer. Normally, it is not recommended that you fertilize lawns in Louisiana with nitrogen containing fertilizer past August 15. Applying nitrogen fertilizer this late may lead to increased incidences of another serious problem, brown patch disease. If you decide to fertilize the lawn with a nitrogen containing fertilizer be willing to go back with fungicides for brown patch disease as part of the overall treatment.

As an alternative, if you can use a fertilizer without any nitrogen, such as a 0-8-8, or something like a 0-13-13, the phosphate and potash will assist your lawn turf in surviving this winter and be in better shape for the spring surge of growth when you can come back in with some nitrogen containing fertilizers.

Otherwise, raise the mowing height to three inches and maintain adequate soil moisture.  I have looked at a lot of lawns over several years, and very few in our area are being cut at a height of 3 inches!  Many times the lawn is being scalped and we always hear the homeowner say, “I don’t want to have to cut grass all the time”. In fact, if you cut the lawn higher it will grow thicker, form a more dense turf, look better, and withstand many of the stresses such as insects, drought, heat and disease much better. Also, as an added benefit, you will not be mowing as often.

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