Date: June 7, 2024 - Included in Issue: 2024.11
By: Christian Krupke and John Obermeyer
Earlier this week, we received reports of armyworm in northern counties continuing to damage wheat. Unfortunately, by the time it was realized, many of the larvae were large and near the pupal stage. Treatment success, with larvae greater than 1.25” is variable. Producers spraying wheat in northeastern counties found this out recently, as last-ditch attempts were made to protect the flag leaves and heads from these ravenous, large caterpillars. The armyworm kept marching on.
How long will armyworm feed? The answer depends on how mature the larvae are in your area. A look back at the literature reveals that in 1914 Indiana experienced a significant armyworm outbreak. Dr. John J. Davis, known to us as the father of Purdue entomology, was inspired to conduct some simple yet valuable life development information on the armyworm that year. The chart below summarizes the approximate number of days for each of the life stages of the armyworm and body measurements:
| Stage | Approximate Days | Approximate Body Length (mm) | Approximate Head Capsule Width (mm) |
| Egg | 6 | – | – |
| Larva: | | | |
| First instar | 4 | 2-4 | 0.4 |
| Second instar | 3.5 | 3-6 | 0.6 |
| Third instar | 3 | 5-10 | 1.0 |
| Fourth instar | 3 | 11-15 (about .5 inch) | 1.5 |
| Fifth instar | 3.5 | 14-21 | 2.4 |
| Sixth instar | 8 | 24-35 (about 1 to 1.5 inch) | 3.4 |
| Total larval stage | 25 | | |
| Pupa | 21 | – | – |
Although the information is over a century old, it’s just as applicable as ever. The question is: are the large armyworms almost finished feeding? When armyworms reach a length of about 1-1/2 inches they still have about 8 days to feed before pupating. During this time period they can consume a tremendous amount of foliage. If they consume their host, they can be seen “marching” from that field to another. Dr. Davis said, “The remarkable voracity of the armyworm during its last larval instar explains its sudden appearance in such enormous and destructive numbers when it is nearly full grown. The amount of foliage eaten in this (sixth) instar was nearly seven times as much as in the fifth instar, and more than 80 percent of all of the foliage eaten during the entire larval period.” In other words, this is when we are most likely to “see” them, as their damage becomes visible even from a distance.
Protect the flag leaf and head from armyworm feeding. (Photo Credit: John Obermeyer)
When is it best to control armyworms? Obviously from the above, one needs to prevent the armyworm from getting over an inch in length. It has been stated by many of you who have been on the “front line” that the big worms are harder to kill than the small ones; the same message we hear for weed control. One might consider treating in the evening, closer to the time when the armyworm leaves their day-time hiding place and climb up plants to feed during the night.
Armyworm are difficult to control when large and they make lousy pets. (Photo Credit: John Obermeyer)
There are a range of products available, with a variety of pyrethroids being a cheap and common option. However, improved control can be achieved with products containing chlorantraniliprole, and this comes with a price premium. There is not one right answer on “which product to choose”. It all depends on how much a producer is willing to spend to attempt to salvage what’s left of the crop.
Mid July armyworm damage to a late-planted and weedy corn field. (Photo Credit: John Obermeyer)
In any event, we are almost done with the larval armyworm story for spring of 2024. Will the armyworm be back later this summer? Moths, from these first-generation larvae, are already flying again (see “Armyworm Pheromone Trap Report,” and notice the uptick of moths in southern Indiana traps). Female moths, laden with eggs, are attracted to lush, dense grasses for deposition. This could be well managed forage grasses or something as small as a patch of ornamental grass in a yard. A handful of times over the years, we have been sent pictures of “back 40” cornfields, planted very late that were completely denuded in late June. A surprise to the neglectful producer! Because of the high moth numbers this spring, the possibility of uncharacteristic, late-season damage from armyworm exists this season. The good news is that armyworm larvae are very susceptible to fungal pathogens and can be wiped out by these plagues when they reach high densities. In short, Mother Nature will probably take care of this pest for the rest of the year. Happy Scouting!
FAQs
Control strategies include pesticides, cultural practices, natural enemies, IPM, and resistance. The IPM and host plant resistance are the most appropriate control strategy. Breeding for multitrait host plant resistance is the best long‐term control strategy.
What causes armyworm infestation? ›
While the exact cause is unknown, there are many theories that all play off of each other. One theory is that cool springs & summers with increased moisture levels are the preferred living conditions for army worms.
What is the best way to eliminate fall armyworm? ›
Spray infested plants with a boiled mixture of plants or apply a mix of ash, sand and tobacco snuff directly to infested whorls. By making best use of farmers' friends and local plants on the farm, nature will help you control the fall armyworm and reward you with a good and healthy crop.
What time of year to treat for army worms? ›
Treat after fall rains: Armyworm infestations are common after heavy rain, especially when it happens in fall. If your area is prone to armyworms, it's a good idea to apply low-impact pesticides as soon as the ground dries after fall rains.
What is a preventative for army worms? ›
Mow and water your lawn regularly, which makes the grass less attractive to the pests. Remove grassy weeds and thatch to make your yard inhospitable for the insects' eggs and larvae. Goatley recommends shutting off outdoor lights by your house at night to avoid attracting moths, which lay eggs that turn into armyworms.
How do you monitor armyworms? ›
Take a weekly sweep net sample in fields that have adequate plant height to monitor for beet armyworm, western yellowstriped armyworm, and alfalfa caterpillars. Divide each field into 4 sections and take 5 sweeps per area with a 15-inch diameter sweep net, for a total of 20 sweeps.
What kills army worms instantly? ›
Neem oil is effective at killing armyworms and other pests. Mix a solution of one to two teaspoons of pure neem oil in a spray bottle with a gallon of warm water and a teaspoon of liquid soap or insecticidal soap.
What is the lifespan of an army worm? ›
After a preoviposition period of three to four days, the female normally deposits most of her eggs during the first four to five days of life, but some oviposition occurs for up to three weeks. Duration of adult life is estimated to average about 10 days, with a range of about seven to 21 days. Figure 6.
What is the natural enemy of armyworms? ›
However, Mantidae and Pentatomidae are known as common fall armyworm predators and have been reported in numerous previous studies (16,22). Entomopathogenic fungus can be effective in controlling fall armyworm populations (2,22).
What is the difference between armyworm and fall armyworm? ›
You can distinguish fall armyworm from other armyworm species by the white, inverted “Y” on the face and four raised bumps in the shape of a square near the end of the abdomen. Fall armyworm – note Y on face and four raised bumps in the shape of a square near the end of the abdomen.
How To Get Rid Of Armyworms
- Birds are the natural predator for armyworms in any stage, including adult moths. ...
- Parasitic wasps may seem scary but are harmless to humans and feed on garden pests like armyworms.
- Ladybugs will devour not only armyworms but also aphids, another common lawn pest.
Armyworms are destructive pests that consume turf grasses, but they'll feed on vegetables and other plants when food is scarce. They eventually become moths, but it's the worm-like larvae stage that cause the most damage.
What is the best killer for armyworms? ›
Knocking out the larvae is only half the battle, as mentioned before they are prolific egg layers. So, applying a long-lasting insecticide like Acelepryn insecticide (that has residual control) is the best way to get rid and prevent armyworms, protecting your lawn.
Do army worms come back every year? ›
Fall armyworms infest just about every state east of the Mississippi, but are the most problematic in the Southeast. That's because where autumn comes early up north, they may be limited to one or two generations per year. In the South, you can get twice as many.
What birds eat army worms? ›
An increase in the number of birds in a yard, such as Starlings, can be a potential sign of an armyworm infestation. While birds eat the caterpillars, they cannot usually eat enough to keep the infestation to a minimum.
What kills armyworms instantly? ›
Spinosad is a bacterial product produced by fermentation and can be used to get rid of armyworms in the landscape and garden. Other common insecticides to use against armyworms include bifenthrin, trichlorfon, deltamethrin, gamma-cyhalothrin and permethrin.
What attracts armyworms to your yard? ›
What Attracts Armyworms. Wet conditions, long grasses, and grassy weeds attract adult moths to your yard, where they will lay all of those eggs.
