Virginia Turfgrass Council – The 2021 Invasion of the Fall Armyworm in the U.S.

June 8, 2022 | Virginia Turfgrass Council | PODCAST

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Virginia Turfgrass Journal – Dr. J. Kevin Mathias – Retired Turfgrass Instructor and Advisor, University of Maryland Institute of Applied Agriculture

In 2021 unprecedented damage from the fall armyworm occurred in numerous areas of the U.S. including locations in Maryland, Pennsylvania, and Virginia. Many turfgrass managers struggled with the severity and the speed with which the damage occurred to their facilities or their clients’ lawns. The goal of this article is to provide information about the fall armyworm and what we might expect in 2022.

I have been involved in consulting work with a new sod producer in central Virginia over the past two years. I visited his facility on August 17, 2021 to inspect his fields and to discuss weed control options for the coming fall and spring months. During this visit, the tall fescue-Kentucky bluegrass fields were in good condition and showed no damage from any type of pest. However, I received a panicked phone call on Sunday, August 22 to ask if I could come by on Monday to take a look at a field that was turning brown.

During my inspection, there were large areas in one of the production fields that had severe brown patch damage. As I was showing the classic brown patch lesion to the owner, we also started to find small (1^st to 3^rd instar larvae) and very active caterpillars that were present in other areas of the field. I had never seen the fall armyworm caterpillar during my 38 years as an entomologist and instructor at UMD and I was at first taken aback as to what type of caterpillar we were looking at. I knew it was not a cutworm or sod webworm caterpillar and through a process of elimination came up with an armyworm. Since the caterpillars were quite small, I was not able to identify it as the fall armyworm until I had an older specimen that had the classic markings of fall armyworms.

Because of the number of fall armyworms that we were finding, well over 12 per square foot, and the previous damage from brown patch, I recommended that he treat immediately for the fall armyworm to prevent any future damage from this insect and provide time for the field to recover from brown patch. The sod grower started to treat his fields on Monday afternoon with bifenthrin and was not able to get all of his fields treated due to equipment limitations on Wednesday, August 25. It was not until Thursday August 26^th that he was able to continue to treat his remaining fields. On Thursday afternoon he started to see damage to the edges of a 24-acre field, and by Friday morning the field was completely defoliated.

Damage was first observed on Friday, August 27^th. Complete defoliation of this field occurred in less than a 24-hour time period.

I returned on Monday, August 30^th to assess the severity of damage caused by the fall armyworm. I have never seen this level of insect damage and was stunned to view the carnage. At an estimated value of $10,000 to $12,000 dollars per acre and a 24-acre field, the sod producer was looking at roughly a quarter million-dollar loss.

Close inspection of the field did show new shoot growth emerging, however it was not consistent throughout the field. My recommendation was to apply a .5 pound of soluble nitrogen per 1,000 ft² and   allow two weeks to see if any reseeding may be required.

During this site visit the owner also showed me a section of this field that was not damaged nor treated and was seeded a year earlier with a different seeding mix of tall fescues and Kentucky bluegrass. The line effect between resistant and susceptible turf to fall armyworm feeding was stunning and begged the question of what was causing the difference. Was it a cultivar difference, age of crop, or an endophyte difference?

I returned to the field on September 23^rd to see how much recovery occurred over the previous three weeks. There was considerable turfgrass loss in certain areas of the field, +90% loss, while other areas of the field may have had 30-40% loss. The fields were overseeded on September 17. The amount of loss was surprising for an insect that is a leaf defoliator and I had expected to see a greater survival of the turfgrass stand. It is possible that severe defoliation may have exposed the crown to environmental stress and subsequent severe turfgrass loss. The overseeding operation was critical in restoring the harvestability of the sod crop for the Spring 2022 season.

Biology of the Fall Armyworm

The fall armyworm is considered a tropical insect unable to survive winter conditions (temperatures that drop below 32 F) in most of the continental U.S. except for areas in southern Texas and in southern Florida. During the spring and summer months the moths begin to fly north into the southern states such as northern Florida, Georgia, Alabama, Mississippi, and northern Texas. In these areas the fall armyworm may undergo 3 to 5 generations and can cause significant damage not only to sod crops but to corn, sorghum, and other field crops. As summer progresses the moths continue to move northwards east of the Rockies and into the mid-West from overwintering sites in south Texas. Fall armyworm moths coming from south Florida travel north and east of the Appalachian Mountains through South and North Carolina and into the Mid-Atlantic area of the country. There is overlapping of both the Texas and the Florida overwintering sites as they fly north as we move into mid to late August and September time periods. The distance the moths can fly is dependent on tropical storm fronts moving in a south to north direction.

The fall armyworm undergoes complete metamorphosis: as a pupa, its overwintering stage, to the adult stage (sexual dimorphism), then as eggs, and finally the destructive caterpillar stage which consists of six instars. Most of the feeding damage is done by the larger caterpillars (5^th and 6^th instars). During the summer months the fall armyworm can complete its life cycle in 30 days while in the spring and fall months it may take 60 days.

Researchers have also identified two strains of the fall armyworm which look identical but have slightly different preferred hosts, susceptibility to certain insecticides, and timing of their northward migration. The two strains are known as the “corn strain” and the “rice strain”; the rice strain prefers to feed on rice, pasture grasses, and turfgrasses while the corn strain prefers to feed on larger grasses such as corn and sorghum.

Why did we see so much damage by the fall armyworm in 2021?

A number of entomologists have speculated that a perfect storm existed for the movement of large populations of the fall armyworm from southern states up into areas that normally never see fall armyworm activity or damage. Based on blog and scouting reports by ag specialists and entomologists, large populations and multiple generations of fall armyworms were being reported in Texas, Alabama, Mississippi, and Florida by late May-June. Environmental conditions during this time period were ideal for this expanding fall armyworm population and severe damage was being seen at this time in southern states. For perspective, the last time this level of armyworm movement and destruction occurred was some 50 years ago in the mid-1970’s. With expanding populations of the fall armyworm developing in the southern states all that was needed was a way to move the migrating moths northward. Several major storm systems coincided at the ideal time to move overwhelming populations further north. On average, moths can fly up to 60 miles within a 24- hour period. However if they can catch a major storm front they can easily increase their movement into the hundreds of miles.

What can we expect in 2022 with regards to fall armyworms?Odds are that the level of damage we experienced in 2021 will not occur in 2022. However, there are a few things to consider in planning for the 2022 season. In the southern states there is a network of pheromone trapping data for the fall armyworm that can be found online. This type of data can be used as an early warning system to determine if control actions may be needed. An example of the type of information one can acquire is shown in the following photo.

Also, the preferred hosts for fall armyworms are bermudagrass, tall fescue without endophyte, and creeping bentgrass. Since bermudagrass has extensive rhizome and stolon growth it can recover from defoliation damage by the fall armyworm. Tall fescue and creeping bentgrass turf will require closer monitoring since they appear to be more susceptible. Zoysiagrass, fine fescues, Kentucky bluegrass and perennial ryegrass show more resistance to the fall armyworm.

There are a number of insecticides that can be applied for control of the fall armyworm. Since the fall armyworm can cause severe damage within a 24- hour time period vigilant monitoring and quick action will be needed to reduce turfgrass damage. Application of contact-type insecticides will perform best when applied early in the day or later in the afternoon based on fall armyworm feeding activity. The following table lists the products that are labeled for fall armyworm. Be sure to read and review the label of these insecticides before application.

Table 1. Recommended insecticides for control of fall armyworm.

What explanation is there for the difference observed in the sod field?

Crop or Plant Age

Based on plant age there have been observations that seedling turf is more prone to damage by the fall armyworm. However, a one-year-old crop is not considered a seedling plant.

Cultivar Differences

The tall fescue cultivars of Montana, Tonto, and Toltec which showed minimal turfgrass loss were not in the 2012 NTEP study nor on the current MD-VA Turfgrass Cultivar Recommendation List. Thus, it is difficult to trace any field performance data on these tall fescue cultivars and whether these cultivars have the tall fescue endophyte. The cultivars in the heavily damaged field did have the endophyte present in them based on data from the 2012 NTEP report. The cultivars and the percent endophyte infection are as follows: Titanium (85%), Raptor III (97%), and 4^th Millennium (100%). Also, these cultivars are on the MD-VA Turfgrass Cultivar Recommended List. However, PCR (polymerase chain reaction) testing in 2021 of live field samples showed no or minimal presence of the tall fescue endophyte. The viability of these grass fungal endophytes is dependent on a number of environmental conditions ranging from field conditions during seed production to seed storage conditions. It appears that these tall fescue cultivars lost the viability of the endophyte and ultimately resistance to the fall armyworm.

Effect of the Tall Fescue Endophyte

The role of fungal endophytes in plant resistance to leaf feeding insects has been reported in the literature. There are several different techniques that can be used to determine the presence of fungal endophytes from staining techniques to PCR testing. Initially a staining technique, rose-bengal stain, did not provide conclusive evidence of endophyte presence.

A total of four samples, two from the damaged field (-1 and -2) and two from the undamaged field (+3 and +5) were submitted in early November, 2021 to Dr. Hannah Rivedal at the USDA Ag Research Service in Corvallis, Oregon for PCR testing. Each of the samples submitted contained multiple shoot systems that were randomly selected within each field area. Dr. Rivedal then collected 5 tiller samples from each of the four samples and performed nucleic extraction on these samples for PCR testing. The PCR test was designed to determine the presence of several mycotoxins related to fungal endophytes and the table below shows Dr. Rivedal results.

Table 2. The results of the PCR testing are shown below.

The fields that were not damaged, samples (+3 and+ 5) had the endophyte while sample -2 which was the field area heavily damaged had no endophyte. Sample -1 had only Loline present suggesting a lack of sufficient mycotoxin production to reduce armyworm feeding damage. At the time of this writing, results for the other two mycotoxins, ergot alkaloids and the indole diterpenes has not been reported.

These results strongly implicate the tall fescue endophyte for imparting host plant resistance to the fall armyworm. This further supports the development of tall fescue cultivars with the fungal endophyte to reduce insecticide applications for not only fall armyworm but other leaf feeding insects. However, there needs to be a recommended methodology to ensure the viability of the endophyte from initial cultivar development to field applications. Endophyte-enhanced resistance is a major cornerstone in Turfgrass IPM programs and should play an even larger role in pest management programs when municipalities impose restrictions on traditional based insecticides.