When to Spray Late-Season Bollworm
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Collapse ▲Another bollworm flight is on and, from the looks of it, there are many more moths in the system than the previous flight. This shouldn’t be surprising, since this insect undergoes multiple generations a year and can quickly build up numbers in each successive generation. Should cotton growers treat or not this late in the year?
The answer to that question is simple for some fields. There is solid research supporting the fact that just before fields reach cutout (at 5 nodes above white flower [4 NAW is cutuout]), cotton is safe from bollworms once an additional 350 DD60s are reached. Under average weather, this will occur at 2 1/2 weeks following cutout. So fields matching these conditions will not need sprayed.
Growers should also consider bollworm biology. Small larvae need to establish on tender squares before they can penetrate bolls. As a result, more mature fields are not only bad for larval establishment, moths do not prefer to lay eggs in them. Furthermore, even if moths do lay eggs in these fields, the larvae do not establish well (see figure). Starting mid-August, bollworm begin to overwinter in the soil in their pupal stage. This tendency to overwinter will only increase into the fall.
Data from 2016 and 2017 show heavy egg lays in late August but little larval survival (red x’s).
As always, there are exceptions. Late-planted cotton is more susceptible for moths to lay eggs in and for larvae to establish. Furthermore, some years, this generation does establish and can persist well into the fall. This does not look like one of those years. However, growers with late-planted cotton (late-May or June) should be vigilant and consider a spray of Prevathon or Besiege based on eggs. I recommend waiting it out on earlier planted fields, since larvae are not likely to establish.