Fall armyworms are an annual occurrence across much of the state. Like many other insect invaders fall armyworm populations are rather light in some years and heavy in others. This year’s invasion across much of the state has been quite heavy, but here in Harris County the reports haven’t been as high as in they have farther to the north and west.
These late season pests can occur in numbers numbers high enough to strip a grassy lawn or pasture bare. They love bermudagrass but also will feed on St. Augustine and some types of zoysiagrass. The good news is that these pests can eat the leaves off of the stolons or runners, but the turf generally just produces new blades in the coming weeks and recovers from the defoliation.
A number of different insecticides will kill armyworms including carbaryl, and a number of synthetic pyrethroids. Organic control options include spinosad and Bacillus thuringiensis (B.t.), but spinosad is the better choice since it lasts longer on the plant than does B.t.
Armyworm larvae may feed for 2-3 weeks before transforming into moths but 80% of the total volume of grass they consume is in their last 2-3 days of feeding! If you have armyworms and are going to treat, it is best to do so early in the infestation while the caterpillars are younger and more susceptible to treatments, and while there is still considerable grass blades left to protect. Treating prior to the arrival of armyworms is not recommended and will likely be of little to no benefit.
For more information on armyworms in lawns check out the following web pages:
Here is a web publication for more information on managing these pests in pastures: