Estimating the Redbanded Stink Bug Threat for the 2024 Soybean Growing Season

Jeff Davis and James Villegas, LSU AgCenter Field Crops Entomologists

Article Highlights:

• Due to the 2023 drought and heat stress and freezing temperatures, redbanded stink bug pressure should be down to begin the 2024 season, especially in the Northern Louisiana region.
• Remain vigilant to control redbanded stinkbugs if they are found to be above action threshold.
• Begin sweeping crimson or white clovers during February and March to scout for redbanded stink bug pressure in your area.

This time of year, as we begin to plan for the 2024 soybean season, one of the questions we often get asked is, “How much should I budget for insect control?” For soybeans, the LSU AgCenter recommends budgeting three to five insecticide applications: one to three for stink bug control and one to two for caterpillar control. This gives a general idea for what one should expect, but can we predict what kinds of insect numbers we will have based on last years’ populations and the wintry weather we have experienced this January?

As anyone who grows soybeans in this state knows, redbanded stink bug is the hardest pest to control. Last year, in 2023, we were fortunate to have a low stink bug year, due to the extreme heat and drought. Insects are poikilotherms, deriving their body heat from the temperatures around them. They must regulate their body temperature by finding shade within the plant canopy or basking in the sun. The extreme temperatures we experienced last summer of 100+°F for extended periods not only stressed cattle and crops but insect pests too. Typical upper developmental thresholds for stink bugs are around 99°F, at which point they begin to lay fewer eggs and if temperatures continue to rise, can die. The drought was no benefit to stink bugs either. Stressed soybeans resulted in reduced nutrition and water availability. Overall, extreme weather conditions lowered stink bug populations during the 2023 field season and the populations overwintering.

Cold weather can also help to control stink bug populations. Redbanded stink bug is an invasive neotropical species and cannot cope with colder temperatures as well as our native Southern green, green, and brown stink bugs. In the past, cold spells have reduced, and in some cases, eliminated, redbanded stink bugs in some areas. Previous work by the Soybean Entomology Laboratory at the LSU AgCenter under the direction of Dr. Jeff Davis established the lethal exposure limit for redbanded stink bug to be an air temperature of 23°F. If exposed to 7 hours, 50% of the population died. If exposed to 11 hours, 90% of the population died. This gives us a general idea of what to expect from a cold period during the winter. However, stink bugs overwinter in leaf litter, eaves of sheds, or even under tree bark. This provides shelter and protection. To model the impact of shelter, we reduced the lethal exposure limit by 5 and 10 degrees. Using the Louisiana Agriclimatic Information System (https://weather.lsuagcenter.com/), we counted the number of lethal temperature hours at several LSU AgCenter research stations experienced this January. We chose these stations as they cover the state from north to south. The table below lists the total hours each station has recorded at the lethal exposure limits of 23, 18, and 13°F.

¹Hours at or below each temperature

The farther north the station was, the more hours each accumulated at those lethal temperatures. Based on these observations, we predict redbanded stink bug populations in the northern parts of Louisiana will be significantly reduced this year. Those growing soybeans in the center of the state may see some lower numbers but those growing soybeans in the southern part of the state will not see freezing weather impacts on redbanded stink bugs.

Overall, what will be the threat level of redbanded stink bugs in soybean be this year? Based on the last year’s low populations heading into overwintering due to the drought and extreme heat and the cold lethal temperatures received so far, we predict that the threat from redbanded stink bug early season in the northern parts of the state to be low. If growing soybean in the middle to southern parts of the state, redbanded stink bug will be around but not at levels we have had when the winter has been favorable for their survival. This does not mean we should not be vigilant. An effective way to understand what your populations will be like on your farm is to sweep crimson or white clovers during February and March. If you find redbanded stink bug in clovers, they will be in your soybeans in April and May.

Inoculating Soybean with Bradyrhizobium japonicum for Nitrogen Fixation

David Moseley, Rasel Parvej, and JimWang, LSU AgCenter Scientists

Article Highlights:

• Nitrogen fixation is essential for soybean production.
• Bradyrhizobium japonicum bacteria is required for nitrogen fixation in soybean fields.
• An application of molybdenum (Mo) may be required for effective nitrogen fixation in acidic soils, but it is important to understand Mo can decrease the viability of the inoculant if not applied properly.

Questions have recently arisen regarding inoculating soybean for nitrogen fixation. Nitrogen fixation is a process where soybean plants can utilize nitrogen from the air by a symbiotic relationship with Bradyrhizobium japonicum bacteria and is essential for economical soybean production. An article written by the Science For Success group (A national team of soybean agronomist) published an article on the Soybean Research & Information Network website stating that nitrogen fixation can account for 40 to 70% of the nitrogen requirement for soybean.

Inoculating with Bradyrhizobium japonicum

The bacteria that is responsible for nitrogen fixation in soybean is Bradyrhizobium japonicum. If a field has never been planted to soybean, it is likely the seed will need to be inoculated. The bacteria can survive many years until the next soybean crop, but the common recommendation is to inoculate seed if soybean has not been planted for 3 to 5 years. However, research has indicated that an increase in yield is possible if seed is inoculated in a regular soybean rotation. Adverse conditions can reduce the activity of the bacteria, including drought or flooded conditions. An important reminder is that the soybean plants require specifically Bradyrhizobium japonicum for nitrogen fixation. Other legume plants benefit from nitrogen fixation but are associated with other rhizobium bacteria. For more information on different plants and specific inoculants, see this Penn State Extension publication. When applying Bradyrhizobium japonicum, it is important to remember the inoculant is a live organism. Attention to proper storage and timing of application can help minimize a decrease in viability of the bacteria.

Molybdenum Applications in Acidic Soils

Molybdenum (Mo) is an important nutrient for nitrogen fixation. In low pH soils (acidic soils) with low Mo availability, adding Mo may be necessary. It is important to remember that a Mo application can harm the inoculant. If an application of Mo is necessary, it is important to not combine it with the inoculant unless the application is immediately before planting.

Kicking Around DIRT and Other Irrigation Recommendations

Stacia L. Davis Conger, LSU AgCenter State Extension Irrigation Specialist

Learning from past crop seasons, we should be preparing to irrigate crops well before the expected irrigation season begins. Waiting until the crop needs water will almost always result in a delayed response during critical growth stages. Preparing to apply furrow irrigation should include the following steps:

• Update your flow rate
• Use/update computerized hole selection
• Consider surge irrigation practices
• Select an irrigation scheduling method

It has been well-established that more water is being pumped from our aquifers than is naturally replenished each irrigation year. Drought conditions such as experienced in 2022 and 2023 will exacerbate this issue. While it’s recommended to re-measure flow rates every year, the most critical time to take a new measurement is now, after significant drought.

Computerized hole selection software such as Pipe Planner has made this process much easier to do. If you haven’t used any hole selection software, yet, this is the time to try it out. The primary benefit is maintaining appropriate pipe pressure so that fields water out evenly and pipe does not burst/deflate before the end of the crop season. Even if you have completed Pipe Planner previously, the plans should be reconsidered using the updated flow rate.

Surge irrigation can be accomplished with or without the use of a surge valve. Valves are great because they automate the surge irrigation process, but may be considered an expensive investment for whole-farm implementation. Adjusting management to alternate irrigation across multiple fields, such as breaking up a 24-hour irrigation into three 8-hour or four 6-hour cycles, will accomplish the same goal. Automated pump control can help with managing these switches remotely.

Choosing when to irrigate is the largest variable that affects the economics of irrigation. Irrigating too often can be a waste of fuel and time while irrigating too little can lead to yield loss. Common methods range from using the look-and-feel method to installing soil moisture sensors. As you think about your options, please consider trying out the Drought Irrigation Response Tool (DIRT). This free extension tool can provide sensor-like information but utilizes known weather conditions instead.

Want a quick tutorial on DIRT? Join me on any Friday in March at 9 AM on Teams!

Use this Meeting Link for all five opportunities.

Questions about any of these steps? Please contact your local extension agent or contact me directly at sdavis@agcenter.lsu.edu or (318) 408-0973.

LSU AgCenter Specialists