Tennessee Turfgrass Association – The 411 on Fall Armyworms

November 23, 2022 | Tennessee Turfgrass Association | PODCAST

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TENNESSEE TURFGRASS: Terri Billeisen, Ph.D., Turfgrass Entomology Lab, North Carolina State University

Although they have a wide host range which includes many agricultural crops, fall armyworms are turfgrass insect pests that cause significant and almost immediate damage in both cool- and warm-season grasses. Fall armyworms are a common issue in the southeastern US but during active years, can spread to the Northeast, Midwest, and Southwest (Fig. 1). Nationwide, 2021 was a crazy year for fall armyworms in terms of abundance and distribution. Many observed record numbers of caterpillars and subsequent damage in areas both familiar and unfamiliar with the pest insect. Reports of widespread, severe infestations resulted in extensive media coverage which added to a sense of alarm regarding the outbreak. Like many turfgrass entomology centers, our lab was flooded with calls and emails requesting not only management recommendations but also reassurance that the fall armyworm situation was under control. Uncertainty surrounding pest outbreaks is not only stressful but can cause managers to panic when making management decisions. This can lead to misapplications which stem from common mistakes such as targeting the insect at the incorrect time or life stage, or selecting an inappropriate product or formulation.

As fall armyworm season gets fully underway, it is a good idea to review important aspects of fall armyworm biology and how these affect our approaches to management. We will review different life stages, behavior and ecology and emphasize how that influences the types of products we use and when is the best time to use them. This information should help us improve our fall armyworm approaches, not only during times of outbreak, but for regular management as well.

Description

Fall armyworm adults are small, brown moths (Fig. 2) that are active at night, which makes them difficult to monitor unless you have a sticky trap and pheromone lure (Fig. 3). Similar to other moth species, fall armyworm adults are attracted to lights at night and a well-lit turfgrass area may be more susceptible to fall armyworm outbreaks compared to others. Adult female moths lay their eggs in groups of 100-200 in a clustered mass on the sides of flat, sometimes stationary, surfaces like buildings, fences, signs/sign posts, outdoor furniture or large ornamental leaves. During outbreak years, moths are less particular about their egg laying preferences and will lay eggs on other kinds of moving surfaces like flags, vehicles and smaller ornamentals. Within 3 – 5 days, eggs will hatch and tiny caterpillars (larvae) will drop down to the turfgrass and immediately start feeding. Caterpillars (Fig. 4) can range from ½ – 2 inches and will feed for a few weeks until they pupate. They will remain in the pupal case for a week or two and then emerge again as new adult moths. The entire fall armyworm life cycle typically lasts anywhere from 1 – 2 months, depending mostly on temperature and soil moisture. They will undergo this process several times from June-October (sometimes November) and will die off once temperatures dip below freezing.

Damage and Monitoring

In normal years, fall armyworms generally prefer feeding on finer-textured grasses although we have observed significant fall armyworm feeding in many different grasses and agricultural crops. In residential areas, we have observed that there is a slight tendency to avoid zoysiagrass when other turfgrass species are present but this is not the case during outbreak years or in high population areas. Fall armyworm damage is easily identifiable compared to other turfgrass insects because you can often draw a distinct line in the turf between damaged and undamaged areas (Fig. 5). Unlike other insect pests, you can also easily spot caterpillars “army crawling” across the turf surface during the mid-morning and early evening in the summer (Fig. 6).

During the warmest time of day, you will want to use a soap flush to investigate any areas you suspect as having fall armyworm damage. Soap flushes use lemon-scented dish detergent mixed in water (2 tablespoons soap/gal water) applied to the turf to bring mobile insects like caterpillars, weevils and mole crickets to the surface within a few minutes. When applying a soap flush for fall armyworms, make sure to apply the soap mixture to an area along the border between damaged and undamaged turf to ensure caterpillars are present. As highly mobile insects, caterpillars are rarely present in severely damaged areas and flushing in those locations will often yield few to no insects.

Management

In terms of management, there are a couple of things to keep in mind prior to selecting a product. First, ensure that you are targeting the correct life stage – the caterpillars. Applying a contact product to control the egg, pupal or adult stage is a complete waste of time and product. Caterpillars are the most susceptible to chemical control. Early instar caterpillars (~ ½ in in length) are particularly susceptible to insecticides so it is ideal to target a population as early as possible. Caterpillars are also the most mobile life stage and therefore most likely to encounter treated plant material.

When selecting products for fall armyworm control, there are two different approach: a short-term solution and a long-term solution. The traditional, short-term approach will effectively and immediately control caterpillars causing damage but will have a shorter window for residual activity. This approach utilizes a (preferably) liquid pyrethroid, like bifenthrin, to quickly control caterpillars as they feed on leaf tissue. During the 2021 growing season, we observed a potential option for a preventive approach for fall armyworms in areas of intense pest pressure where an anthranilic diamide (chlorantraniliprole, cyantraniliprole, tetraniliprole) had been applied earlier in the season as part of a white grub management program. Early results indicate that a diamide application can provide more than 60 days residual control of fall armyworms. Residual activity of these active ingredients is part of the current research underway in our turfgrass entomology lab here at NC State and we hope to have more detailed information and additional recommendations for fall armyworm management by the end of this year.

As we progress through this growing season, it is important to remember that fall armyworms have been causing damage in turfgrass for a long time. Like any other insect, fall armyworm populations, and subsequent damage, will fluctuate from year-to-year. When environmental conditions are just right, outbreaks can occur which will result in numbers higher than what we are used to encountering. However, management approaches or recommendations remain the same whether we are managing a routine or outbreak pest population. A crucial component of effective management is to review information from reliable sources, preferably prior to infestation, to ensure management decisions are intentional and not reactionary.

Fall Armyworm FAQs (compiled from inquiries in 2021)

• Where do fall armyworms come from?

O Fall armyworms overwinter most years in southern Texas and Florida and their northern migration can be influenced by weather patterns. We do not have the ability to accurately forecast fall armyworm problems in advance, so timing-wise they can sometimes surprise us with their damage.

• How do fall armyworms move to new areas and is it unusual for them to move so quickly?

O Not unusual at all. Remember, the adult life stage of a fall armyworm is a moth. Moths are capable of flying large distances in a relatively short period of time. Many insects exhibit flying behaviors associated with both localized (short) and long-term migration. Female moths fly to an area with an abundant food source (fresh, green turf, for example), lay their egg masses on the flat side of a building, fence, sign, or large leaf, and when the eggs hatch, the larvae drop to the turf below and immediately start feeding.

• How bad was the fall armyworm season in 2021?

O 2021 had the worst outbreak of fall armyworms on record, in many areas. Fall armyworm populations and damage were on a scale that was very surprising! Areas of the upper Midwest and the Northeast suffered turf damage to both warm- and cool-season turf like they had never seen before. Some of the problems came from populations originating in Texas and some from moths that started out in Florida. These different “sources” potentially exhibit different traits for feeding preferences, like the damage we observed with seedlings and overseedings, and insecticide susceptibility.

• Are fall armyworms more of a problem in certain areas?

O During “normal” years, fall armyworms are more likely to cause issues in turf that has had some mechanical disturbance, like in areas of new construction. They are generally more of a problem in newly-seeded or sodded areas.

• Is there a threshold for fall armyworms?

O Because fall armyworms can cause a lot of damage in a short period of time, it is best to treat as soon as you notice activity.