First, check out this article for a good background of fall armyworm, with some pictures.
Organic options are limited, and fall armyworms can do a great deal of damage in a short time. Take quick action when you notice fall armyworm or damage. Talk with your neighbors to find out about infestations as early as possible. If you have a neighborhood group on social media, it can be a valuable resource.
Look for these active ingredients in the organic section of your pesticide dealer.
Option one – Keep the lawn as healthy as you can. Water once a week but not more. Too much water favors turf diseases. Healthy lawns will typically recover from foliar damage of fall armyworm; the lawn will be unsightly for a while.
Option two – Bacillus thuringiensis kerstaki is commonly called Bt. It is a disease of caterpillars. If you notice the caterpillars when they are small, it can do a good job. As the caterpillars grow Bt will become less effective. Bt will not hurt birds, bees, your pets, your kids, etc. Be sure to apply fresh product. Bt is a living thing.
Option three – Neem is a natural product of a tree. It will kill caterpillars, but it will likely require multiple applications. Again, it is important to treat when the larvae are small. High concentrations can harm aquatic life, but risks to bees and other non-targets is relatively small.
Option four – Pyrethrins are squeezed out of chysanthemum flowers (pyrethroids are synthetic analogs). Pyrethrins are broad spectrum, so they can kill many kinds of insects, including bees. They are less toxic to birds and mammals; do not allow your pets or children in the treated area until it is dry. Pyrethrins are quite toxic to fish. Pyrethrins break down quickly; multiple applications may be required.
Option five – Spinosad comes in two forms. The organic option comes from a soil bacterium. The inorganic form also comes from the bacterium, but petroleum derivatives are also added. Spinosad is very effective against caterpillars; it is quite toxic to bees until dry. Keep children and pets off of treated ares until residues have dried.
So, what to do. Keep a close eye on your turf; fall armyworm are very abundant this year.
Consider the amount of damage already done to the lawn. Spraying after the grass has been eaten does no good. If most of the lawn is already destroyed, option one may be your best choice.
If you catch the infestation when the larvae are small, you may be able to resolve the problems options two or three while minimizing risks.
If the larvae have gotten larger, you may need to use options four or five to protect the lawn. Be aware that you are also increasing the risk to nontarget organisms