Insecticide Termination Guidelines for Cotton and Soybean

As cotton and soybean crops across Mississippi move toward maturity, consultants and producers should begin evaluating when insecticide applications are no longer economically justified. Properly timing insecticide termination helps reduce unnecessary input costs while protecting yield and crop quality. Termination decisions should always consider crop growth stage, pest pressure, and environmental conditions.

Cotton Insecticide Termination

In cotton, insecticide termination is based on both crop development and heat unit accumulation. The key growth stage is NAWF 5 (five nodes above the uppermost first-position white flower), which marks physiological cutout. At this stage, the plant has essentially completed effective boll set, and flowers produced afterward are unlikely to contribute significantly to yield.

Once cotton reaches NAWF 5, accumulated heat units (DD60s) determine when individual insect pests no longer pose an economic threat. Recommended termination thresholds are:

• Tarnished plant bug: NAWF 5 + 350 DD60s
• Bollworm: NAWF 5 + 350 DD60s
• Spider mites: NAWF 5 + 350 DD60s
• Soybean looper: NAWF 5 + 350–400 DD60s
• Stink bugs: NAWF 5 + 450 DD60s
• Fall armyworm: NAWF 5 + 500–550 DD60s

Preliminary graduate research at Mississippi State University indicates that NAWF 5 + 350 DD60s generally corresponds to approximately NAWF 2, although this relationship can vary depending on location and environmental conditions. You can also monitor DD60 accumulations using the Mississippi State University Delta Agricultural Weather Center to make informed termination decisions.

Once the appropriate insecticide termination threshold has been reached, continued scouting for insects is generally unnecessary. Late-season infestations of lepidopteran pests, particularly bollworms and armyworms, may occasionally occur after termination. While these infestations can appear alarming, the harvestable boll population has reached a stage where it is no longer susceptible to economically significant injury. Treating these late infestations rarely provides an economic return and can result in unnecessary insecticide applications.

Soybean Insecticide Termination

In soybean, insecticide termination is determined by reproductive growth stage, primarily between R6 (full seed) and R7 (beginning maturity). Correctly identifying these stages is critical for deciding when additional insecticide applications are unlikely to provide an economic return.

Recommended termination stages by pest include:

• Podworms (corn earworm): R6
• Stink bugs: R6.5 (approximately 7 days after R6)
• Redbanded stink bug: R7, unless prolonged adverse weather threatens seed quality
• Defoliating caterpillars (soybean looper, green cloverworm, velvetbean caterpillar, and saltmarsh caterpillar): R6.5
• Armyworms: R6.5
• Bean leaf beetles: R6.5

Soybean Growth Stage Definitions

• R6 (Full Seed): The seed completely fills the pod cavity at one of the four uppermost nodes with a fully developed leaf.
• R6.5: The seed begins to separate from the pod membrane, typically 7 to 10 days after R6.
• R7 (Beginning Maturity): At least one pod on the main stem has reached its mature color.

For indeterminate soybean varieties, growth stage evaluations should be made using pods from the upper four nodes. For determinate varieties, observations can be taken from any pod on the plant.

Final Considerations

Many of Mississippi’s early-planted cotton and soybean fields are approaching insecticide termination. Following these recommendations can help eliminate unnecessary insecticide applications, reduce production costs, and maximize profitability without sacrificing yield or quality.

Before making a termination decision, consider the following:

• Crop growth stage
• Insect species and population levels
• Current and forecasted weather conditions
• Local DD60 accumulation (cotton)

As always, continue scouting through the appropriate termination threshold to ensure economically damaging pest populations do not develop before insecticide applications are discontinued.