The list is long for products & methods of army worm control, get them at farm & ranch store, lawn & garden center, feed store, online pest control places etc...
Depending on area size it can get expensive quick. If
you catch them early & don't want to kill all sorts of non target bugs (including beneficial) I've had good luck with - Bt-k (bacillus thuringiensis kurstaki).
High % pyrethrin / permethrin works, sevin, ortho bug b gone, spectracide triazicide will work.
If you wait too long, don't walk your field everyday looking for infestation or they move into your place in an advanced stage, Ive had good luck with Talstar P, Bifen IT or XTX, Cyzmic Cs, Cyper,
Easier way for me to list out insecticides without typing a full page ---> DIY pest control
There are others not found above.
You can always check with your county Tx A&M Agrilife extension office.
If you own land and don't have an private applicators license you should get one.
One thing is for sure what worked last year will be less effective next year.
No matter what you use apply as close to dark as possible/ very late afternoon. They feed mainly at night & dark cloudy days.
I lost around 230 acres of hay I was counting on to make winter in E Tx between Aug 14 - 18.
They left about 110 un touched.