Jeffrey A. Davis

Pests have always been a yield-limiting problem soybean producers have faced in Louisiana. These pests can rob soybean farmers of both quantity and quality of grain. However, it was not until 2000 that a new invasive species arrived that significantly changed Louisiana soybean production forever.

Louisiana soybean growers have historically battled a diverse insect pest complex consisting primarily of native stink bugs (Southern green, green and brown stink bugs), three-cornered alfalfa hopper and bean leaf beetle. In addition, Lepidopteran defoliators, such as green cloverworm, soybean looper and velvetbean caterpillar, are also serious pests that don’t overwinter in Louisiana but invade from southern Texas and Florida every spring. However, these are not invasive species. The federal government defines an invasive species as nonnative to an ecosystem and whose introduction causes economic and environmental harm.

Before 2000, the redbanded stink bug had never been an economic threat to Louisiana and was not found in the Midsouth. However, since its arrival in 2000, the redbanded stink bug has rapidly spread across the state, reaching all soybean growing areas by 2006. It is now the dominant stink bug species in Louisiana soybeans year after year, constituting 59% to 72% of the total species found during the growing season. Thus, each year, soybean producers treat up to 95% of their soybean acreage for redbanded stink bug with a yield loss plus cost of insecticide treatments reaching $40 million.

What makes the redbanded stink bug the most important stink bug pest to Louisiana soybean? Its high tolerance to insecticides with only a few products providing sufficient control, its capacity to cause greater damage to soybeans than any other stink bug, and its quick reproduction. Since its arrival, soybean producers have increased their insecticide applications from one to two per year to as much as three to five applications per year. Losses in soybean quality and oil content have increased and delayed maturity, a syndrome by which the plants retain green leaves, green stems or green pods when they reach maturity, has also increased.

To combat this invasive stink bug, LSU AgCenter entomologists have studied the biology and behavior of the redbanded stink bug, modified their monitoring and action thresholds, and recommended tank mixes of insecticides to better combat this pest while monitoring for insecticide resistance. This has resulted in a set of best management practices:

• Monitor fields early (prior to pod set) and often (every five days).
• If redbanded stink bug numbers reach 16 adults and/or nymphs per 100 sweeps, apply an insecticide application immediately and rotate modes of action to prevent resistance.
• Resistance monitoring indicates acephate applications of 0.5 pounds per acre do not provide sufficient control. Minimum applications of acephate should be at 0.75 pounds per acre.
• The redbanded stink bug spends time in the lower to mid soybean canopy. To ensure insecticide applications reach this location, higher spray volumes are recommended with slower ground speed.
• Spray only when necessary to conserve natural enemies.
• Avoid susceptible soybean varieties.

The redbanded stink bug is not the only invasive species threat to soybean. The kudzu bug was initially discovered in northeastern Georgia in 2009, then moved through the southeast and arrived in Louisiana in 2013. Range expansion appears to be facilitated by an abundance of kudzu, a plant that covers approximately 7 million acres in the southern U.S. Like the redbanded stink bug, the kudzu bug can reduce soybean yields, though not to the same extent as the redbanded stink bug. Within Louisiana, the kudzu bug is an occasional minor problem that is easily controlled. In fact, nature provides the best control through unfavorable temperatures combined with Beauvaria bassiana, a naturally occurring fungus that infects insects.

To add to Louisiana soybean farmers woes, another new invasive stink bug found its way to Louisiana in 2021. The brown marmorated stink bug is a recently introduced pest from eastern Asia and was first identified in Pennsylvania in 2001. It has since spread across the U.S., covering 42 states from the east to west coasts. In June of 2021, we confirmed a reproducing population at a property in Iberville Parish. Unlike the redbanded stink bug, the brown marmorated stink bug can damage corn and soybeans and feeds on numerous vegetables and fruits. AgCenter agents use pheromone traps to monitor current populations and range expansion. Many insecticide options are available for control.

AgCenter entomologists can protect soybeans from the next invading species by maintaining a sustainable soybean integrated pest management program that uses biological, cultural, physical and chemical tools to regulate pest populations while minimizing environmental risks and by generating the necessary data on pest density, impact, biology and control tactics. Only through continued vigilance and research can the AgCenter provide the necessary information to predict and manipulate pest population dynamics in Louisiana soybean fields.

Jeffrey A. Davis is a professor in the Department of Entomology.

(This article is in the winter 2022 issue of Louisiana Agriculture.)

The redbanded stink bug. Photo by Jeffrey A. Davis

Studying the redbanded stink bug involves comparing the usage and efficacy of sweep nets and drop cloths in determining redbanded stink bug population densities in late-stage soybeans. Photo by Tyler Towles