Corn Earworm in Cotton and Soybean

James Villegas, LSU AgCenter Field Crops Entomologist

Corn earworm feeding on cotton boll (left) and soybean leaf (right). Photo by J. Villegas.

Cotton

Bollworm activity has been steadily increasing across the Mid-South. If bollworm pressure intensifies, a rescue spray may be necessary. Scouting for bollworms varies depending on the cotton variety. For varieties including Bollgard II, TwinLink, and WideStrike, scouting for eggs is recommended. Bollworms prefer to lay eggs near blooms in the canopy, but they can also lay eggs throughout the canopy. Thus, it’s important to check leaves, terminals, and bracts of squares and bolls for eggs. The threshold for treatment is 20 eggs per 100 plants. For newer cotton varieties like Bollgard III, TwinLink Plus, and WideStrike 3, scouting involves searching for feeding injury and live larvae. Bt cotton contains toxins that need to be ingested by the larvae to kill it, hence superficial/minimal feeding indicates that the Bt toxin is doing its job. However, any visible damage, even a tiny-sized hole in a square or a boll, suggests that the bollworm has overcome the toxin. The threshold for treatment on these newer varieties is 4-6% fruit injury of any kind or the presence of 4 larvae (1/8-inch or longer) per 100 plants. If damaged-boll counts exceed 2% and significant numbers of larvae are present and continuing to cause damage, treatment may be warranted.

Soybean

There have been increased reports of corn earworms infesting soybean fields across the state. Corn earworm 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. Monitoring corn earworm infestations is crucial for timely and targeted intervention. Producers are advised to begin sampling for larvae at the initiation of flowering, with particular attention to later-maturing soybean fields. Scouting efforts should be intensified when large numbers of moths are observed or captured by pheromone traps. Sampling for larvae using sweep nets is recommended. The suggested treatment threshold in Louisiana, based on sweep net sampling, is 38 worms per 100 sweeps (9-10 worms per 25 sweeps). Treatment is most effective during the flowering and early-pod development periods (R2-R5). However, treatment is generally not recommended prior to flowering. Before any treatments are applied to the field, consider the crop value and the cost of insecticide application. To achieve optimal control, producers should target mid-size corn earworm larvae, as small larvae may be difficult to reach, and large larvae may have already caused economic losses and are ready to pupate. Pyrethroid applications may be less effective due to insecticide resistance, so alternative insecticides should be considered.

More information about CEW on soybean can be accessed at the LSU AgCenter.

Visit the 2023 Insect Pest Management Guide for control recommendations.

Louisiana Soybean Crop Progress and Condition Update – July 2023

David Moseley, LSU AgCenter Soybean Specialist

Soybean Crop Progress

The USDA-NASS survey indicated 2% of the Louisiana soybean crop was planted by March 19, 2023. This year, I walked fields planted during the first week of March. According to the five-year average, soybean planting in Louisiana usually does not begin until the fourth week of March. Favorable planting weather allowed the corn crop to be planted quickly which led to an earlier than normal beginning to the soybean planting season. Unfortunately, a freeze on March 20^th caused some early planted soybean plants to be killed. There were some fields that survived the freeze. The planting progress remained ahead of the five-year average until the week ending on May 28, 2023.

On June 30, the USDA estimated the Louisiana soybean acres to be 1.12 million acres. This estimate is down 140,000 acres from 2022 and down 50,000 acres from the beginning projections for 2023.

Figure 1 shows a field that was ready for harvest by July 10, 2023. According to the USDA-NASS survey, the only year soybean acres were recorded to be harvested in July was in 2017. In 2017, 2% of the crop was harvested during the week ending on July 30.

Figure 1. A soybean field in lower Catahoula Parish ready for harvest on July 10, 2023.

A new factsheet, Desiccating Soybean for Harvest in Louisiana (lsuagcenter.com), was recently published by the LSU AgCenter.

Soybean Crop Condition

As of July 16, 77% of the Louisiana soybean crop was rated good to excellent. The rating was the highest during the weeks ending on June 4 and 11 with 91% of the crop rated good to excellent. Due to hot and dry weather, the rating began to drop and was as low as 71% good to excellent by July 2. There were some areas catch a rain which helped the rating improve to 79% good to excellent by July 9. On July 2, 17% of the soybean crop was rated as very poor to poor; however, by July 16, only 3% was rated as poor and 0% was rated as very poor. In several fields, I have seen symptoms of manganese (Mn) deficiency after applying glyphosate. More information on Mn deficiency can be found in the article Identifying and Correcting Manganese Deficiency in Soybean (lsuagcenter.com).

LSU AgCenter Agronomic Crops School

David Moseley, LSU AgCenter Soybean Specialist

An Agronomic Crops School will be held at the Dean Lee Research and Extension Center on August 3, 2023. The event will begin at 4:00 PM at the State Evacuation Shelter which is adjacent to the LSUA campus. Figure 1 shows the featured speakers. A sponsored meal will be provided. For more information, contact Dr. Daniel Stephenson at dstephenson@agcenter.lsu.edu or 318-473-6520.

Figure 1. Dean Lee Research and Extension Center Agronomic Crops School information.

LSU AgCenter Specialists