Corn earworm feeding on foliage (A; Photo by J. Villegas) and pod damage (B; Photo by Scott Stewart, University of Tennessee).

Recently, there have been increased reports of corn earworms infesting soybean fields across the state. Corn earworm (aka bollworm or soybean podworm) can cause occasional but severe damage to soybean. The primary damage caused by corn earworm occurs during the larval stages when the insect feeds on the soybean flowers and pods. This feeding activity leads to direct yield losses as pods abort, or the larvae consume or injure developing seeds. The most severe damage typically occurs in the latter reproductive stages of soybean growth. Typical symptoms of corn earworm damage to soybean include flower feeding injury, entry holes in the pods, seed consumption, and the presence of frass (insect excrement) near the feeding sites. Severely damaged pods may also become susceptible to secondary infections by fungi and bacteria, further compromising seed quality.

Sampling and Monitoring

Monitoring corn earworm infestations is crucial for timely and targeted intervention. Corn earworm moths are particularly attracted to soybeans during the R1-R5 growth stages, as adults prefer to lay their eggs on flowering soybeans. As soybean fields mature, they become less desirable for corn earworm colonization. Producers are advised to begin sampling for corn earworm larvae at the initiation of flowering (R1), with particular attention to later-maturing soybean fields. Scouting efforts should be intensified when large numbers of moths are visually observed during sampling or when the number of moths captured using pheromone traps has increased. Sampling for larvae using sweep nets is recommended, with 25 sweeps per location at four or more different locations within the field. Larger fields should have a higher number of sampling sites, especially those fields exceeding 50 acres. Additionally, it is essential to document the presence of other pests, such as fall armyworm, which may contribute to pod feeding or leaf-feeding caterpillars such as soybean looper, green cloverworm, or velvetbean caterpillar.

Treatment Timing and Thresholds

Determining the appropriate treatment for corn earworm infestations depends on the growth stage of soybean plants. Treatment is most effective during the flowering and early-pod development periods (R2-R5). However, treatment is generally not recommended prior to flowering. The suggested treatment threshold in Louisiana, based on sweep net sampling, is 38 worms per 100 sweeps (9-10 worms per 25 sweeps). Before any treatments are applied to the field, consider the crop value and the cost of insecticide application.

Insecticide Use Strategies

When insecticide treatment is required, it is important to target mid-size corn earworm larvae for optimal control. Small larvae may be difficult to target due to their location within flower clusters or terminals, while large larvae may have already caused economic loss. Also, late instar (large) larvae will only feed for a few days, drop from the plant, and pupate in the soil. Insecticide resistance monitoring has revealed high levels of pyrethroid resistance in corn earworm populations in Louisiana, making pyrethroid applications less effective for control. Alternative effective insecticides for corn earworm control in soybeans include chlorantraniliprole, indoxacarb, spinetoram, and spinosad. Against low corn earworm infestations, a tank mix of a pyrethroid and 0.5 lb of acephate can potentially provide effective control. However, producers should be cautious, as only 2.0 lb of acephate can be applied per acre per season, and it may be advisable to save acephate for stink bug applications.

Insecticide Active Ingredient and Trade Name	Amount formulation per acre	Amount active ingredient per acre
chlorantraniliprole Vantacor (5)	1.2 – 2.5 oz.	0.047 – 0.098
lambda-cyhalothrin, chlorantraniliprole Besiege	5-8 oz.	premix
Methoxyfenozide, spinetoram Intrepid Edge	4.0-6.4 oz.	premix
bifenthrin, chlorantraniliprole Elevest	5.6 – 9.6 oz.	premix
spinosad Blackhawk (0.36)	1.7 – 2.2 oz.	0.038-.049
indoxacarb Steward (1.25)	5.6-11.3 oz.	0.055-0.011
Npv*** Heligen	1.0-1.6 oz.	---
bifenthrin, z-cypermethrin Hero (1.14)	4.0-10.3 oz.	0.04-0.1
acephate Orthene	12-16 oz.	0.75-1.0
esfenvalerate Asana XL (0.66)	5.8-9.6 oz.	0.03-0.05
carbaryl Sevin (4)	24-32 oz.	0.75-1.0
lambda-cyhalothrin Warrior II (2.08)	0.96-1.60 oz.	0.015-0.025
gamma-cyhalothrin Declare (1.25)	0.77-1.28 oz.	0.0075-0.0125
z-cypermethrin Mustang Maxx (0.8)	2.8-4.0 oz.	0.0175-0.025
beta-cyfluthrin Baythroid XL (1)	1.6-2.8 oz.	0.013-0.022
cyfluthrin Tombstone (2)	1.6-2.8 oz.	0.025-0.044
bifenthrin Brigade (2)	2.1-6.4 oz.	0.033-0.1

*** NPV (Heligen) is specifically effective against corn earworms. It is recommended to apply the virus when the larvae are small. It is important not to apply the virus if the majority of the larvae are already large or if the infestation levels exceed the treatment threshold.

Written by

Dr. James Villegas
Assistant Professor - Extension and Research
Field Crops Entomology
LSU AgCenter
Email: jvillegas@agcenter.lsu.edu | Mobile: 225-266-3805