Re-Alert: Fall Armyworm, Part II? - Joe Boggs Dave Shetlar Curtis E. Young

Participants in the BYGL Zoom Inservice on Tuesday morning had a lengthy discussion about whether we’ll experience a sequel to the Fall Armyworm (FAW) (Spodoptera frugiperda, family Noctuidae) saga in Ohio. Points were pondered and expectancies examined; however, the consensus was straightforward: never bet against insects!

We made no predictions in our first Alert titled, “Fall Armyworms March Across Ohio,” which was posted on August 30, partly because caterpillars were still producing damage. So, we focused on control and recovery. Also, we are navigating uncertain territory when it comes to predicting what will happen with the development of a subtropical or tropical insect such as FAW in a temperate region like Ohio.

We do know that FAW eggs, caterpillars (larvae), pupae, and adult moths cannot survive winters in Ohio. The outbreak occurred after the moths rode the winds into Ohio from sub-tropical zones in the southern U.S. and Mexico. Some of the moths were mated females with eggs (= gravid females), so they were poised to quickly initiate the damaging caterpillar stage.

We also know that FAW caterpillars pass through six instar stages with their size and appetite increasing with each molt. The 1st, 2nd, and 3rd instar caterpillars are small and secretive making them difficult to detect in turfgrass. They commonly hideout during the day and feed at night. In fact, the 1st instar caterpillars tend to feed within the leaf folds.

Soap solutions such as those used to drive burrowing caterpillars like black cutworms (Agrotis ipsilon) from their daytime burrows aren’t particularly helpful for exposing small caterpillars, especially the first four instars. FAW caterpillars don’t burrow. The main problem with detection is that early instars are simply too small to easily spot beneath the turfgrass canopy during the day without a very close “on-hands-and-knees” examination of the turfgrass.

Also, the small caterpillars typically don’t consume entire turfgrass blades. They feed along the edges allowing some blades to remain green while others simply turn brown and shrivel. The overall damage is a mixed bag and can easily be mistaken for summer moisture stress or other turfgrass maladies.

The elevated damage caused by the 4th instar caterpillars begins to reveal the presence of FAW in turfgrass. The damage accelerates during the 5th and 6th instar stages when the voracious caterpillars mow down entire turfgrass blades. That is why the FAW caterpillar damage in Ohio seemed to appear “overnight.” The caterpillars had been with us for some time, but they hadn’t produced noticeable damage until the final instar stages.

Of course, armyworms are so named because of their habit of moving en masse to greener pastures once they’ve depleted their food supply. It is not uncommon for the caterpillars to move from field crops into nearby turfgrass or even vice versa. The caterpillars will continue eating turfgrass to the crowns until there is no more food or they complete their development, whichever comes first. If the plant food is exhausted, the armyworms will become meat-eaters with the larger caterpillars eating the smaller caterpillars to complete their development.

Predictions: It’s All About Temperature

A scientific paper published in 2020 and titled, “The Effect of Temperature on the Development of Spodoptera frugiperda (Lepidoptera: Noctuidae)” [seeSelected Referencesbelow] provides key data regarding predicting whatmayhappen next. The paper includes a table thatshows the relationship between the speed of FAW development and ambient temperature.

The higher the temperature, the faster the FAW development. On the other hand, FAW’s sensitivity to low temperatures meansthe race is onas to whether another generation of armyworms can develop into the damaging 4th, 5th, and 6th caterpillar instarstagesbefore they’re doomed by cold temperatures. The table shows that FAW larval mortality rises to 71% at an average temperature of only 18 C (64.4 F).

Temperatures in late-August were remaining at or slightly below normal. However, temperatures have been trending upward since then to remain above normal. In fact, long-range forecasts indicate that temperatures may remain above normal throughout the rest of September.

Another factor that may favor another round of armyworms is the large number of pupae observed at or around the time we posted our first Alert. Elevated temperatures have led to the rapid development of new adults emerging from heavily infested turfgrass. Mating occurred quickly and the females began laying eggs. Indeed, adult emergence and egg-laying are ongoing.

Once again, we can turn to the 2020 paper for information helpful in predicting “what’s next.” The aforementioned table shows that at 22 C (71.6 F), FAW development from eggs to adults can be completed in around 42 days.

Of course, the most crucial developmental points are the time required for the caterpillars to develop into the most damaging4th, 5th, and 6thinstar stages.  The table shows that at 71.6 F, the 4th instar caterpillars can appear once eggs are laid in around 16 days; 5th instars around 19.5 days; and 6th instars around 25.5 days. On the other hand, caterpillar mortality will be 37% at this average temperature.

So, does this answer the million-dollar question regarding whether we’ll see another round of damage by FAWs? While we have said that you should never bet against insects, there are three very important things to consider. First, the paper cites data collected by holding caterpillarsin a laboratory at a constant 71.6 F. What happens in the field is no doubt going to be different with a probability that developmental speed may take longer.

Second, we don’t know if the population density of this developing generation will match the caterpillar densities that developed from the southern moths blown north into Ohio. Keep in mind that a percentage of the caterpillars will not make it. The researchers observed acaterpillar mortality rate of 37% at 71.6 F.

Finally, temperature-based larval mortality isn’t the only factor that may affect the reappearance of late instar FAW caterpillars. Heavybird predationwas observed in several locations in Ohio with pupae being a favorite bird food meat item. Indeed, we learned that observing large flocks of birds pecking at turfgrass is a good way to locate a possible armyworm population.

Food Preference Observations on the 2021 FAWs

Entomologists are noting that there are some differences in the current FAWs attacking Ohio turf and crops. First, many professionals are finding that turf-type fall fescues (Festuca arundinacea) appear to be a preferred grass and the FAW larvae appear to be able to withstand the toxins normally produced by fescues with endophytes.

This population of FAW also relishes creeping bentgrass (Agrostis stolonifera) that is commonly used across Ohio on golf courses. However, they focus their attention on the taller bentgrass on golf course aprons and roughs while ignoring close-cut bentgrass on collars, tees, greens, and fairways. Presumably, this is because of exposure to predators given that the caterpillars don’t burrow and there’s nowhere to hide, as well as less canopy to consume.

Surprisingly, some Kentucky bluegrass (Poa pratensis) lawns and athletic fields are escaping damage and it appears that this population of FAW may have difficulties eating and thriving on some of the Kentucky bluegrass cultivars. This has been observed with the black cutworm, a pest that rarely reaches outbreak levels in turf. Black cutworm larvae can withstand the toxins produced by endophytic turfgrass cultivars, but the larvae can’t survive on Kentucky bluegrass! The Ohio FAW populations appear to prefer alfalfa, wheat, and oats, but it largely didn’t show up in the ears of field corn, like in previous years.

Finally, a review published in the journal “African Entomology” showed that FAW caterpillars have been recorded on host plants belonging to 76 plant families, but they most commonly feed on plants belonging to three families: Poaceae, Asteraceae, and Fabaceae. FAW in Ohio ignored creeping Charlie (Glechoma hederacea, family Lamiaceae) and only nipped yellow nutsedge (Cyperus esculentus, family Cyperaceae) after grasses were depleted. On the other hand, the caterpillars did very little feeding on crabgrass (Digitariaspp., family Poaceae) and common dandelion (Taraxacum officinale, family Asteraceae).

Don’t Jump to Conclusions: Monitor and Diagnose!

Predictions on paper should never take the place of observing what’s actually happening. Turfgrass that was affected by the first round of damage should be closely monitored. Keep in mind that FAWs do not destroy the crowns which are the meristematic growing point of turfgrass. In essence, the turfgrass suffered an extremely low mowing.

We’re observing a healthy recovery of the damaged cool-season turfgrass. This is the time of the season when we see rapid growth of cool-season turfgrasses. On the other hand, another round of FAW feeding damage will delay a full recovery and could significantly stress the turfgrass. 

However, don’t become lazy with your diagnostics! It’s importantto separate FAW damage from symptoms produced by other turfgrass maladies. During the FAW outbreak, it was common for such things as drought stress as well as turfgrass diseases such as brown patch, gray leaf spot, or even rust to be mistaken for FAW damage. Unfortunately, it was also not uncommon for needless insecticide applications to be made based on the misdiagnosis. Remember that treatment without an accurate diagnosis is malpractice!

Treatment Options

We want to stress thattreatments should not be preventive! Don’t make an application unless there is a clear target. Insecticide applications must be weighed against environmental consequences as well as possible impacts on overall pest management strategies. For example, the collateral damage caused by indiscriminately killing beneficial insects can lead to other pest outbreaks. However, if FAWs are clearly in the crosshairs, there are effective treatment options.

Turfgrass managers had some success with pyrethroids such as bifenthrin (e.g., Talstar) during the first round of FAW. However, we also received reports from the agricultural markets that pyrethroids are not working well, so alternative chemistries should be considered.

FAW populations often develop resistance to insecticide categories that are extensively used in the agricultural markets. Since our populations arrive from more southern regions, some moths may have arrived here in Ohio after their ancestors had been exposed to several applications of pyrethroids, carbamates, and/or organophosphates.

If you did not see a rapid kill of a FAW population after the application of a pyrethroid during the first round, consider using an alternative. The diamides such as chlorantraniliprole (e.g., Acelepryn) or tetraniliprole (e.g., Tetrino) have excellent caterpillar-killing abilities.  Both are registered for turfgrass usage and can be used at their lowest label rates for curative caterpillar control. Two combination products that contain a neonicotinoid plus a pyrethroid and seem to overcome any resistance are Aloft (clothianidin+bifenthrin) and Alucion (dinotefuran+bifenthrin).

Acelepryn is also the active ingredient in some over-the-counter granular grub control products (e.g., GrubEx) as well as several professional use products. Although the insecticide will not directly protect the turfgrass blades, the caterpillars will be killed as they contact the insecticide while crawling beneath the canopy. However, keep in mind that water must be applied to release the insecticide from the granules. Although some of the active ingredient may be released by heavy dew, it’s still best to follow label directions with irrigating after the applications.

Finally, azadirachtin-containing products are effective for control of all types of turfgrass-infesting caterpillars. Azatin O, Azaguard, and Neemex 4.5 are three such products and each is certified organic (OMRI).  Note thatAzatin XL is not registered for turfgrass use.

These alternative insecticides are often difficult to find in over-the-counter outlets, but none are restricted-use insecticides except for Aloft GC or certain formulations of pyrethroids that are used on golf courses. Those that are not restricted use can be purchased by homeowners through internet vendors, but you will need the proper equipment to apply these commercial products, and the ability to properly calculate rates used in small, non-commercial sprayers and spreaders.

Products based on various strains of the naturally occurring bacteriumBacillus thuringiensis(Bt) have also proven effective against FAW in agricultural sites. However, it’s important to use products with armyworms on the label. All strains are not equal. Also, Bt is most effective against early instars which means close monitoring is critical for effective timing.

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