Western Bean Cutworm Reaching Threshold in Some Fields

Either our own resident WBC populations did fine over the winter or the recent storm fronts brought in the bunch of moths from the US or a combination of the two.  But reports are coming in that fields between Thamesville and Bothwell and near Rodney have reached threshold, well, sort of.

WBC egg mass with newly hatched larvae
WBC egg mass with newly hatched larvae

What is messing up everyone’s ability to determine which fields need to be sprayed is the crop stage.  The current threshold for WBC is 5% of plants with an egg mass, applying insecticide at 95% tassel emergence and when the majority of egg masses are purple or hatching. Though it is easy to find 5% of the plants with egg masses on them, the crop stage is variable and in some cases not close to tasseling.  When WBC larvae first hatch, they move up to the tassel to feed.  If the tassel is not well formed yet and is not emerging in less than a week, the larvae will starve.  They don’t feed on leaves so if there is no tassel or silks to feed on, they will die. So if a field is not going to tassel within 6 to 7 days, that field is not ready to be sprayed even if there are enough egg masses to be at the 5%.  Only focus on fields that will be tasseling within the next 7 days or less to determine if a spray is necessary.  Of course, if the field has been planted to Bt corn containing the Cry1F trait, we don’t recommend it be sprayed, even if threshold has been reached and some feeding damage is expected, due to the potential for increasing the risk to Cry1F resistance.  More details on this is in the recent blog entry titled “Does spraying Bt corn for WBC make sense?

Another complication when making a spray decision is when there are various life stages of WBC in the same field. A call came in today asking what to do in a field that was not going to start tasseling for another 3 days but had both young larvae already hatched and fresh egg masses not due to hatch for another 5 days or more.  In this case, I would recommend they use Coragen (active ingredient: chlorantraniliprole) or Voliam Xpress (active ingredients: chloranthraniliprole and lambda-cyhalothrin) instead of a stand alone pyrethroid like Matador.  Both Coragen and Voliam Xpress have longer residual and should also provide control for both the larvae and the eggs.  A second benefit with Coragen is that it is considered a reduced risk product, having minimal impact on  beneficials including bees.  So it does not have label statements warning against spraying a flowering crop that is foraged by bees.

When spraying any  insecticides not considered to be a reduced risk product for WBC control on corn during tasselling, you must take precautions to protect bees that may be foraging.  Provide beekeepers within 5 km of the site advanced notice of the application to ensure hives can be temporarily protected.  Time insecticide applications to minimize bee exposure.  Insecticide applications in the evening are the safest, unless there is evidence of a strong temperature inversion. Under normal circumstances, spraying after 8 pm allows the spray to dry before the bees are exposed to it the next day. Early morning is the next best time, but spraying should be completed well before 7 am.

Currently registered products, rates and precautions are available here: WBC Products 2014.