2020 Bollworm Management in Cotton and Soybean

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 4^th, and peak around the 15^th. 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.