Armyworms can be classed into two groups, fall armyworms and true armyworms. There are many different species of true armyworms, rice armyworms, beet armyworms, and corn armyworms just to name a few.
Fall armyworm (left), True armyworm-Ohio State University (right)
Western yellow striped armyworn-photo Jack Kelly Clark
True armyworms have whitish stripes down their bodies and fall armyworms have an upside-down y shape on their heads. True armyworms grow to about 5 cm and fall armyworms are a little smaller. Both species are gray to greenish coloured.
Armyworm egg mass-photo Jack Kelly Clark (left), Armyworm moth (right)
True armyworms emerge in the spring and can have 2 or more generations per year. They overwinter as larvae and emerge to feed until they are ready to pupate. In late spring when the pupae hatch into moths, the moths mate and lay their eggs mostly on grains and grasses. They are common most places in North America.
Fall armyworms appear in late summer. They overwinter as larvae in the southern states, feed and then migrate to the north. They can migrate as far as southern Canada. They can be a terrible problem especially in Texas where conditions for these guys are perfect. I’ve seen these pupae, they wiggle in the case and I use the stomp method to get rid of them.
Armyworm pupae in soil-photo Jack Kelly Clark (left)
Armyworm damage-photo University of California (right)
Diet and Damage
These guys can be devastating in large numbers. They emerge quickly in groups and can eat many times their weight in food. They cause defoliation, and can totally defoliate plants overnight. They simply chew the edges of leaves until there iare none left. Fall armyworms feed both day and night whereas true armyworms feed usually in the night. Favourite crops of the true armyworm include grasses, rice & corn but they will attack anything else if they run out of food.
Fall armyworms are different in that they will eat almost anything in sight! Cucumbers, cabbage, all grains, beans and tomatoes are among them. If infestation is very slight then I would choose to leave them to predatory insects.
1) Hand Pick
Go outside at night with a flashlight and pick them off, dropping them in hot soapy water. Fall armyworms can be handpicked in the daytime as well.
2) Predatory Insects
Parasitic wasps & flies and several ground beetles
Use a spray of Bt on the plants at the first sign of these bugs. Spray the ground around the plant and half way up the stems. Bt is a solution of good nematodes, however it must be applied every 3 or 4 days as they die quickly.
4) Destroy Egg Masses
You can destroy them with a cotton swab wetted with only water. Just brush the swab down a leaf where their eggs are and wipe on a napkin that you can later burn. A solution of rubbing alcohol and water will also work.
5) Organic Insecticides
Safer’s soap and other soapy sprays.