2020 Bollworm Management in Cotton and Soybean

Whitney Crow, Extension Entomologist
By Whitney Crow, Extension Entomologist, Angus Catchot, Extension Entomologist, Jeff Gore, Research and Extension Entomologist and Don Cook, Research Entomologist July 17, 2020 16:35

2020 Bollworm Management in Cotton and Soybean

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This last week there have been reports of bollworm eggs starting to show up in Mississippi Cotton. Generally speaking, in a normal year the bollworm flight would begin around July 4th, and peak around the 15th. The wet spring pushed the start of corn planting back, hence the delay in movement from corn to cotton. Unlike last year, corn in most areas seemed to silk in a relatively narrow window, therefore we hope to see flights that peak versus the extended continuous egg lay that was experienced last year.

So what should you consider when deciding on a management strategy:

When to treat cotton?

  • Technology: 3-gene cotton (Bollgard 3, Widestrike 3, Twinlink Plus) vs. 2-gene cotton (Bollgard 2, Widestrike, Twinlink). The 3-gene cotton varieties are going to provide better control of bollworm than 2-gene cotton. This does not mean that foliar insecticide applications will not be needed at some point in the future on 3-gene cotton.
  • Resistance: In recent years, there have been numerous escapes in 2-gene cotton due to resistance to the Bt cry proteins. 3-gene cotton has the addition of Bt Vip, which currently has shown no issues.
  • Thresholds: The threshold that is recommended is dependent on the technology. ONLY USE the egg threshold on non-bt or 2-gene cotton.
    • The threshold for 2-gene cotton is 20% of plants with at least one egg anywhere on plants. Plant terminals can be scouted for eggs, but the threshold for eggs in the terminal should be lowered to around 10% of plants if you are only looking at terminals. You can also utilize the damage threshold, if that’s what triggers first.
    • The threshold for 3-gene cotton is 4% larvae (>1/8 inch) or 6% damaged fruit (squares and bolls).

When to treat soybeans?

  • Threshold: Mississippi’seconomic threshold for bollworms in soybean is 9 1/2 inch long larvae per 25 sweeps. Or you can utilize the dynamic threshold (table below).

  • Remember when determining your control cost to include both the application and insecticide cost.
  • For example, $8 soybeans using a $15 control cost ($10 insecticide cost plus $5 application cost) would result in 8.3 larvae per 25 sweeps.

 Control Options: Scouting and spraying on threshold is critical. More than one spray may be needed in a lot of situations.

  • Acephate plus a pyrethroid is no longer recommended for bollworm control.
  • Pyrethroids are no longer recommended for bollworm control in soybean or cotton. Widespread tolerance provides inconsistent and poor performance.
    • This does not mean zero control will be achieved with a pyrethroid. It just means that 7 out of 10 applications that were targeted at bollworm management in the Delta have resulted in poor control or even control failure.
  • The best control options: Intrepid Edge (5-6 oz for soybean, 7-8 oz for cotton), Prevathon (14 oz for soybean, 14-20 oz), or Besiege (7 oz for soybean, 7-10 oz for cotton).
    • For cotton, the best control will be provided by 20 oz of Prevathon or 10 oz of Besiege. When treating eggs in cotton with a diamide, early is always better than late.
    • For soybean, lower rates can provide acceptable control when they are applied in a timely manner.
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Whitney Crow, Extension Entomologist
By Whitney Crow, Extension Entomologist, Angus Catchot, Extension Entomologist, Jeff Gore, Research and Extension Entomologist and Don Cook, Research Entomologist July 17, 2020 16:35
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