Tyler Towles, Huang, Fangneng, Wilson, Blake, Stout, Michael J.
Tyler Towles, Blake Wilson, Michael Stout and Fangneng Huang
The 2021 crop growing season presented many challenges for Louisiana producers, but none as remarkable as the fall armyworm (Spodoptera frugiperda) infestations that occurred beginning in late June and extending through August. Although the fall armyworm has been a major pest of maize and other crops across North and South America for many years, problems with this pest were more severe and widespread in 2021 than almost any other year in memory. In addition, over the past five years, the fall armyworm has invaded virtually the entire continent of Africa and much of India and southeast Asia. Thus, this insect now represents a major threat to global food security.
Female moths of this pest species lay egg masses, which can include hundreds of eggs, and each female can lay multiple egg masses, meaning that one adult female could potentially lay thousands of eggs. The landscape could theoretically be infested overnight assuming suitable hosts are present. Identification of the larval form can be challenging during early growth stages but becomes much easier as growth progresses. Most often, fall armyworm larvae are identified by the inverted “Y” between the eyes on the head capsule (Figure 1).
One factor leading to the fall armyworm being a major issue in 2021 is the timing at which the pest arrived in Louisiana. The fall armyworm is a tropical species that overwinters in southern Texas and Florida, meaning that migration must occur before the species arrives in Louisiana. Typically, following the first spring generation further south, migration northward begins, leading to the fall armyworm reaching Louisiana well after crop establishment. However, in 2021, the fall armyworm showed up earlier than expected. This led to many issues in crops including rice, soybean, sugarcane and forages. Bt cotton varieties and Bt corn hybrids, which are genetically engineered to be resistant to insect pests, are widely adopted across the state and play a major role in controlling fall armyworms; therefore, the species did not cause noteworthy impacts on the two crops. While it is difficult to accurately explain the unusually early arrival of fall armyworms in 2021, it may be due to warmer temperatures and timely spring rains in southern Texas that bolstered the insect populations.
Two host strains of fall armyworms exist — the rice strain and the corn strain. These strains are genetically similar, but host feeding preferences vary between the two. The rice strain prefers grasses including rice, johnsongrass and bermudagrass. In contrast, the corn strain prefers hosts including corn, grain sorghum and cotton. Historically, the rice strain has been controlled effectively using pyrethroids, which are cheap and widely available. However, the corn strain has developed resistance to the pyrethroid insecticide class, requiring substantially more expensive chemical control measures to achieve acceptable control.
Sugarcane is not a preferred host for fall armyworms, but infestation of sugarcane by fall armyworms is not uncommon. Infestations most often occur in newly emerged plant cane in the late summer or early fall (Figure 2). This feeding is rarely considered to be economically important because fall growth is often killed back by winter freezes. Typically, when fall armyworms start to reach high population levels in Louisiana in early summer, sugarcane plants are entering the grand growth phase and are much less susceptible to feeding. Due to the early arrival of the pest in 2021, infestations occurred in April when other crop hosts including rice and corn were being planted. Most sugarcane producers opted out of insecticidal applications to avoid decimating beneficial insect populations. However, insecticidal applications were warranted in some situations.
Pastures and turfgrasses were also heavily infested by fall armyworm in 2021. Entomologists received calls from forage producers that pyrethroids were not providing satisfactory control, unlike as in previous years. Many producers needed follow-up insecticide applications to achieve acceptable control in pastures leading to increased inputs and decreased profits. It is unclear if inadequate control is a result of pyrethroid resistance development in the rice strain or a combination of the two strains occurring concurrently.
Soybean acreage in Louisiana was similarly affected by intense fall armyworm pressure in 2021. Soybeans planted during the recommended planting window experienced heavy defoliation in some areas, while soybeans planted late due to poor planting conditions and excessive rain were eaten as they emerged from the soil (Figure 3). Fall armyworms began showing up before soybean podworm (Helicoverpa zea) and soybean loopers (Pseudoplusia includens), which are common yield-limiting pests in soybeans. One major factor that made fall armyworms a challenge in soybeans is that they typically do not reach treatable levels in most cases. However, in 2021, many producers’ insect control budgets were drained before the arrival of more consistently damaging pests because unexpected additional applications were needed beforehand for fall armyworms.
Rice acreage in the southern portion of the state was largely unscathed by fall armyworm populations mainly because the Dermacor X-100 seed treatment, which is used to control the rice water weevil (Lissorhoptrus oryzophilus), also provides good control of the armyworms. Damaging infestations did occur in nontreated rice planted in midsummer to serve as forage in crawfish ponds. Unfortunately, control options that are safe for crawfish are limited, so little could be done to manage these infestations. However, portions of rice acreage in northeast Louisiana were significantly affected by the pest due to the lack of the Dermacor seed treatment (Figure 4).
Upon receiving complaints of pyrethroid failures for fall armyworm control across various crops, foliar insecticide trials were deployed in infested soybean fields to test the efficacy of various products against the pest. Collections of fall armyworms were made from the Macon Ridge Research Station, in Winnsboro, Louisiana, and sent to the U.S. Department of Agriculture for strain identification. The collected larvae were identified as the rice strain. Pyrethroid insecticides providing seemingly inconsistent results ultimately led LSU AgCenter entomologists to seek emergency exemptions for alternative control options.
Tyler Towles is an assistant professor at the Macon Ridge Research Station, Winnsboro, Louisiana. Blake Wilson is an assistant professor at the Sugar Research Station, St. Gabriel, Louisiana. Fangneng Huang is a professor, and Michael Stout is a professor and head, both in the Department of Entomology.
(This article appears in the winter 2022 issue of Louisiana Agriculture.)
Figure 1. The inverted “Y” on the head capsule is one key characteristic used for identification of the fall armyworm. Photo by Blake Wilson
Figure 2. Late-stage fall armyworms defoliated sugarcane acreage in southern Louisiana. Photo by Blake Wilson
Figure 3. Late planted seedling soybeans were eaten down to soil level by fall armyworms moving off grass hosts that occurred alongside the crop. Photo by Hank Jones