Does spraying Bt corn for Western bean cutworm make sense?

Author: Jocelyn Smith, University of Guelph, Ridgetown Campus

Growers in the high WBC risk areas (sandy soils) saw heavy infestations of WBC in 2013 and many were surprised by the survival and amount of damage by WBC on Bt corn. Bt corn hybrids that express the Cry 1F insecticidal protein such as the Herculex® and Genuity® products are expected to have some control of WBC, however, the only Bt event that has close to 100% control of WBC is the Vip 3A protein found in Viptera® products. In the early years of WBC establishment in Ontario (2008-2010) we estimated that Cry 1F would provide 70-90% protection from WBC, however, since 2011 we have noticed less effective control in Ontario with these products and similar observations have also been made in Nebraska, raising concerns about resistance development or decreased tolerance of WBC to Cry 1F.

Since WBC has been considered mostly a secondary corn pest in North America compared to European corn borer or corn rootworm, insect resistance management (IRM) plans have not been similarly emphasized or enforced. Now that we know WBC are overwintering in Ontario and are not sufficiently controlled by some Bt corn products, we should be seriously thinking about IRM for WBC. If WBC become resistant to Cry 1F, this only leaves Vip 3A and foliar insecticides as effective control options. Although yield loss to WBC is highly variable, their feeding damage may increase the risk of ear mould infection and accumulation of mycotoxins such as deoxynivalenol (DON, vomitoxin) and fumonisins (Fig. 1).

Figure 1. WBC feeding damage and mycelial growth on SmartStax corn hybrid, 2013. J. Smith
Figure 1. WBC feeding damage and mycelial growth on SmartStax corn hybrid, 2013. J. Smith

Recommendations for effective WBC IRM:

  1. Use a hybrid with the Vip 3A event
    • Will cause high mortality of WBC larvae
    • No foliar insecticide application will be necessary
  1. Use a Bt-corn hybrid that does not control WBC (i.e. no Cry 1F or Vip 3A) and apply a foliar insecticide when populations reach threshold*
    • Majority of population will be controlled by the insecticide if properly timed

Practices that are NOT recommended for WBC IRM:

  1. Use a Bt-corn hybrid that expresses Cry 1F without being pyramided (stacked) with Vip 3A
    • Population will continue to be exposed to Cry 1F selection pressure
    • Most of these products are refuge-in-the-bag (RIB) which is not a good refuge option for ear feeding corn pests. With a 5% RIB, there is almost no effective refuge for a larva that feeds on an ear that has likely been cross-pollinated by neighbouring Bt plants.
  1. Use a Cry 1F hybrid and apply a foliar insecticide
    • Any insects surviving the insecticide application are still exposed to Cry 1F, increasing the chances of resistance development.

*Managing WBC with foliar insecticides

  • Scout for egg masses on the upper surface of leaves close to the whorl prior to tassel emergence (Fig 2).
  • Scouting should take place weekly from mid-July until mid-August
  • Scout 20 plants in 5 areas of the field
  • Threshold: 5% of plants with an egg mass
  • Apply insecticide at 95% tassel emergence and when the majority of egg masses are purple or hatching (Fig 3).
  • Registered insecticides:
  • According to CFIA: If required, the refuge may be treated for control of lepidopteran pests with non-Bt insecticides. If the refuge is treated, the combined trait corn product must be similarly treated.