Residual Control of Fall Armyworms

Are you Getting Residual Control of Fall Armyworms from your Insecticide Applications?

By Elijah Carroll, Kendra Carson and Dr. David Held

The fall armyworm (Spodoptera frugiperda), is a significant pest of all turf and pasture grasses throughout the eastern United States. There are several species of armyworms in our area, but this species develops faster (21 days from egg to adult) than the others and has 4–5 generations a year in Alabama. Fall armyworms have a white wishbone on the head and four distinct dots on the end abdominal segment. They do not overwinter in Alabama; however, moths arrive every spring from southern TX and FL and have continuous generations through fall. Outbreaks of fall armyworms are most common in mid to late summer. By then, multiple generations are overlapping leading to large populations that cause significant damage. Fall armyworms feed in large numbers and, in urban lawns, these masses of larvae can easily scalp a lawn in a day. Despite their small size, they can move between lawns in a typical subdivision, between fields, or from rough to fairway in a short time.

The fast generation time, movement and feeding behavior of fall armyworms means that once or twice a year, turf managers must decide how best to respond to fall armyworm damage. For example, do I use a short residual (and less expensive) insecticide, or do I treat with longer residual insecticides as ‘insurance’ against future outbreaks? Newer insecticides have been marketed for extended residual control. Two anthranilic diamides, chlorantraniliprole (Acelepryn) and cyantraniliprole (Ference), are noted for having longer residuals in turfgrass relative to synthetic pyrethroids. Anthranilic diamides are effective by contact and ingestion.

However, our literature review only found two papers concerning residual control of turf-infesting caterpillars with either of these products. One study (Williamson et al. 2013, Int. Turf. Res. Soc. J, 12:1-5) found significant mortality of caged black cutworm larvae on creeping bentgrass treated 88 days prior to exposure with chlorantraniliprole. Another study reported significant mortality of tropical sod webworms on a St. Augustine grass lawn treated 5 weeks prior to exposure with chlorantraniliprole (Tofangsazi et al. 2015, J. Econ Entomol, 108:730- 735). These studies support the claims of long residual control made in the technical bulletins for these products. However, neither study investigated fall armyworms, or differences in toxicity if larvae were exposured by contact, or consumption, of treated grass. Our study addressed these two questions using fall armyworms in short-mown hybrid bermudagrass.

Fall armyworms were released in the center of each plot then allowed to crawl on treated bermudagrass for a few hours.

This study was conducted at the Turfgrass Research Unit at Auburn University from late July to late September using two experiments to determine exposure by contact or ingestion. One field experiment determined the contact exposure of free-range fall armyworms on turfgrass at various times after treatment. Plots of Tifsport bermudagrass were treated with 7 insecticides (Triple Crown, Onyx Pro, Permetrol, Zylam, Provaunt, Acelepryn, and Ference) at label rates. These active ingredients represent synthetic pyrethroids in Onyx Pro and Permetrol; a neonicotinoid in Zylam; an oxadiazine in Provaunt; and diamides in both Acelepryn and Ference. Triple Crown is a mixed product containing both synthetic pyrethroids and a neonicotinoid. Twenty-four hours after application, ten larvae were placed near the center of each plot. After 1–3 hours on the treated plots, the distance traveled by each armyworm was measured and recaptured. Recaptured larvae were placed in a labeled petri dish and taken back to the lab where we assessed mortality following the short-term contact exposure. The same methods were used to challenge the same plots again at 7, 14, 28, and 42 days after application.

A second experiment was used to determine mortality of larvae fed treated grass clippings. Plots of Tifway Bermudagrass were marked and treated with the same insecticides as the field study. For this experiment, we added Dipel and Spinosad at label rates. These products only have ingestion as their mode of action. Spinosad had positive activity against tropical sod webworm in a previous study (Tofangsazi et al. 2015). Twenty-four hours after application, grass from each treated plot was harvested using scissors, placed into a labeled bag, and transferred to the lab inside a cooler. In the lab, grasses from each treated plot were added to five petri dishes with one armyworm each, and kept in a growth chamber. The larvae in the petri dishes were taken out and observed for mortality at 24, 48, and 96 hours later. This procedure was repeated, harvesting grasses from the same plots at 3, 7, 14, 28, and 42 days after application.

Did insecticides reduce fall armyworm movement across treated grass?

Some larvae moved numerically less than larvae on control plots but the differences were not significant. In fact, some armyworms on treated plots traveled farther than larvae on control plots. At 7 days post treatment, armyworms on plots treated with Triple Crown crawled significantly more than armyworms on the control plots. Similarly, armyworms on plots treated with Onyx Pro at 28 days post treatment crawled significantly farther than those on control plots. Both products include a pyrethroid, which can cause hyperactivity in insects. In 1–3 hours, a single fall armyworm can move as much as 10 ft. However, on average most larvae moved 2 ft or less.

Did fall armyworms die after crawling on treated grass?

Yes, depending on the product and the age of residue. At one day after application, larval mortality was 70% or greater after crawling on grass treated with Provaunt, Triple Crown, Ference, and Acelepryn for a few hours. However, only Acelepryn or Ference maintained mortality at or above 70% for 7 or 14 days after application, with Acelepryn being nearly 100%. Mortality was 30% or less for larvae that crawled on insecticide residues older than 14 days. Mortality of fall armyworms fed grasses from treated plots also depended on the age of the residue and type of insecticide. It took about 96 hours of exposure to the grass clippings in petri dishes for the effects to be evident. Only 4 insecticides, Acelepryn, Provaunt, Ference, and Zylam provided significantly higher mortality of larvae relative to larva fed non-treated grass for 1 day after treatment. Of those, mortality of larvae fed grass treated with Acelepryn, Provaunt, and Ference caused significantly higher morality through 7 days after treatment. The diamides, Ference and Acelepryn provide control through feeding through 14 days after treatment. Fall armyworms fed residues of Acelepryn on treated grass provided 65% mortality 42 days after treatment. This was the only treatment still providing significant mortality at 28 and 42 days after treatment.

What does this mean?

The results of this study and previous studies support the published long residual control of turf-infesting caterpillars by newer insecticides. We didn’t take data for 88 days after treatment as in the previous studies, but Acelepryn in this study provided 42 days of mortality above 60% for feeding larvae or 14 days if they are exposed by contact. This was the best performing product in our experiment, and exhibited good residual control in the mentioned study with tropical sod webworms. Ference, the related product to Acelepryn, provided 14 days of control through either contact or ingestion. Provaunt provided high contact mortality when armyworms were exposed to a fresh (1 day old) residue or 7 day residual when armyworms consumed treated grass.

The pyrethroid products in this experiment provide poorer control (less than 50% by feeding or contact) than is noted in other published studies. We think this may be related to research methods. The armyworms in this test were free range and exposed for 3 hours or less. Other studies, cage caterpillars on the grass for 24 hours. In those tests, larvae may be stimulated to move more in cages, simultaneously increasing their contact exposure. In an urban landscape, but perhaps not a golf course, free-range fall armyworms may be able to crawl to non-treated grass or flower beds in a few hours and survive. Considering this behavior and our use of free-range larvae, methods used in previous studies may overestimate the residual control of larvae from products like pyrethroids. If the objective is to minimize call backs in lawn care, you may want to consider one of the products that provide longer residual like chlorantraniliprole. Chlorantraniliprole should provide 2 weeks of protection from fall armyworms crawling across the treated lawn, and at least 35–40 days of protection from fall armyworms and sod webworms that consume the treated grass. It is worth noting that chlorantraniliprole also has a good environmental profile with low to no toxicity against beneficials insects and pollinators.

Elijah Carroll is an Auburn University undergraduate student and research assistant in Dr. D. Held’s turfgrass and landscape entomology lab. Kendra Carson is a research technician in the lab, and Dr. Held is a Professor of Entomology at Auburn University. We thank the Alabama Turfgrass Research Foundation for supporting this and other turfgrass research at Auburn University.