Abrupt Presence of Fall Armyworms in Double-Crop Soybeans in Kentucky

The larval stage of the fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae) is a voracious defoliator of many plant species. The FAW is a native pest of the New World; however, it overwinters in south Florida or in the southernmost region of Texas (The Rio Grande Valley) in the continental USA. The adults are strong fliers and move northward during the summer months. In double-crop soybeans, fall armyworms can be devastating defoliators affecting plants from the seedling to V4 stages.

FAW started to appear in Kentucky at the end of June to beginning of July, but as they continue their migration pathway to northern areas, they can also have large populations during the fall. This migration covers most states east of the Rocky Mountains and includes several provinces of Canada (for example Ontario, Quebec, Nova Scotia). Fall armyworms were found in Africa 5 year ago; now this insect is a well-established pest in different African countries and is causing severe damage in many crops.

Problem

During the last 2 weeks, there have been reports of FAW outbreaks affecting forages, sorghum, and soybeans (although corn is affected, I have not heard of any damage in this crop). This has been happening in several counties of Central and Western Kentucky from La Center to Bowling Green (Ballard and Warren Counties, respectively). Crop consultants and County Extension agents started to notice this event and warned growers in Lyon and Ballard counties.

Figure 1. (B). Soybean field damaged by fall armyworms. (Photo: Todd Elkins)
Figure 1. (A) Close-up of a soybean plant left without any leaves and a larger than 3/4-inch FAW on the tip of plant (Photo: Todd Elkins)

Small larvae skeletonize the lower leaves; large larvae feed over the whole plant.  Severe damage caused by FAW in a commercial field of double-crop soybeans in Central Kentucky is shown in Figures 1A and 1B. In most cases, severely damaged plants can have a ragged appearance or be left without leaves (Figure 1A).

The pheromone-based trap in Princeton recorded 152, 280, and now 340 FAW moths through the last 3 weeks (July 9 to July 30), indicating that moth flight may not have peaked. Also, egg masses were observed in some crops. Moths in Lexington traps have not been detected.

Biology

Female FAW (Figure 2) deposit egg masses of 50 to 200 eggs per cluster. Clusters are covered with scales (Figure 3). A single female can produce up to 2,000 eggs during its life span. Fall armyworm larvae emerge (Figure 4) and start to feed on plants causing unnoticed defoliation while they are small. However, as they molt to the next developmental stages (Figure 5), their appetite increases, and the defoliating damage is greatly noticed by farmers and scouting agents. The FAW has six larval instar that can be completed in 14 to 30 days, depending on the temperature. Fall armyworm resembles corn earworm and armyworm; however, fall armyworm has a white inverted “Y” mark on the front of the dark head (Figure 5). Pupation occurs in the ground and adults can live up to 20 days.

Figure 2. Female and male fall armyworms (Photo: University of Missouri)
Figure 3. Egg clusters of fall armyworm collected on July 8 and July 30, 2021. Females lay between 50 to 200 eggs; these cluster on leaves, wood poles, screens, or plastic field flags. Scales cover egg clusters as seen in pictures (Photos by C. Whitney and R. Villanueva)
Figure 4. Recently emerged fall armyworm larvae; notice the black head capsule (Photo: Raul Villanueva, UK)

Management

The late vegetative stage of soybeans can support heavy feeding and can tolerate nearly 100% foliage loss during the early vegetative stages before yield loss is achieved. However, if double-crop soybeans are affected in the early seedling stage, the results may be devastating.

Alarming reports have been broadcast in local news about the widespread damage caused by FAW and the lower efficacy of pyrethroids in some fields. This was explained as an increase in resistance by FAW; however, if insecticides were utilized when larvae were larger than ¾ inch, the level of control maybe diminished compared with a greater efficacy against larvae that are smaller.

Insecticides, such as pyrethroids, are effective against this pest, but is known that their efficacy decreases for late larval instars. Early detection of infestations will allow for more effective control of this pest if larvae are larger than ¾ inch in length. In corn, the larvae can form a frass plug in the whorl, and this reduces the ability of insecticides to contact the larvae. A dual mode of action insecticide with systemic capabilities, may be used.

In soybeans Leverage® 360 (imidacloprid + cyfluthrin) at 2.8 fl. oz/A is effective for the 1st and 2nd larval stages; however, Leverage® 360 is not registered for corn in KY. Also, for late stages of FAW larvae (greater than 3/4 inch), probably a dual mode of action insecticide, such as Besiege® (chlorantraniliprole + lambda-cyhalothrin) at 8-10 fl. oz/A, Hero® (zeta-cypermethrin + bifenthrin) at 4-10.3 fl. oz/A; Elevest® (bifenthrin + chlorantraniliprole) at 5.9-9.6 fl. oz/A) may be more effective than single mode of action products.

For all these insecticides, the use of high volumes of water will generally result in better coverage, especially under adverse conditions (e.g., hot, dry) or where a dense plant canopy exists, especially if the FAW migration continues. These types of migratory waves are of short duration and rarely will be duplicated; however, FAW can be present until the fall.

Figure 5. A distinctive, light-colored inverted “Y” mark is present on the head capsule of fall armyworms. Also, pay attention to coloration changes of FAW larvae (Photo: Raul Villanueva, UK)

Insect Thresholds

In soybeans, a threshold for FAW is not well established, but you can use the following: control is required if egg masses are present on 5% of the plants, or 25% of plants are infested with larvae.

Although damage has not been observed in corn, insecticide treatments must be applied before larvae burrow deep into the whorls or enter ears of more mature plants. If sweeping with a 15-inch sweep net, an average of 9 larvae per 25 sweeps would indicate a need for control.

The thresholds used in North Carolina for defoliating insects is 30% defoliation throughout the plant canopy 2 weeks prior to blooming (R1) and 15% defoliation throughout the plant canopy 2 weeks prior to flowering (stage varies) until the pods have filled (R7-R8).

More information:


By Raul T. Villanueva, Entomology Extension Specialist. Research and Education Center, Princeton, KY

Posted in Grains
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