We have received a higher amount of calls this season than
any other concerning this pest than any other this year. We are fairly confident that if you pick any
field in the area, you will likely be able to spot one of these nasty worms
chomping away in the cobs.
Life Cycle:
Western Bean Cutworm has originated from the southern and western
United States, and has over the years flown its way across the Midwest (in its
moth form) and made it to Ontario about 6-7 years ago. The female moth first laid its eggs on the
upper leaves of pre-tasselled corn, to hatch 7-10 days later into these tiny
worms. The worms make their way down the
plant and into the cob, where they begin to feed and grow as the grain develops. Once it has reached its final instar (full
size) it leaves the cob, makes its way down to the ground, where it burrows
into the soil to overwinter and emerge as a moth again in the spring. The one saving grace for most of Ontario, and
has kept this pest under control for the most part, is that because they burrow
into soil to overwinter they tend to target sandy farms because it is much
softer soil and easier to burrow into. In
the North Middlesex area, there is a very small amount of
sandy ground. In Southwestern Ontario, there are hot
spots where there is heavy cutworm infestation in largely sandy areas, such as
Bothwell, Strathroy, and out in Norfolk.
This year however
there seems to be more feeding than usual. It is not nice looking, I know, however it likely is not as
bad as you might think it is. In sandy
hotspots, you can get as multiple worms in every cob, which can do a
significant amount of damage and destroy a crop quickly. Thanks to the Herculex Bt trait in all of our
hybrids we have sold this season we are generally ok with the low pressures
we have seen. The Herculex trait, for
Western Bean Cutworm, is labeled to have about 70-80% control of Western Bean
Cutworm under low pressure. This means
if there is a worm or two in a cob, they will generally munch away at the tips,
however will not continue to feed on the whole cob because it doesn’t like the
Bt protein it tastes. But, if there are sevral worms per cob, that
combination of feeding can destroy it pretty quickly. Another concern with this pest is the fact
that the feeding damage it causes can act as an entry point to ear molds, such
as Giberella, which can put it at greater risk for higher Vomitoxin levels.
The threshold to spray for the cutworm occurs when 5% of
plants have an egg mass present; the Herculex Bt gene is not enough to keep the
worms from doing little damage. You must
scout for them in July, just before tasseling time, inspecting the upper leaves
for egg masses. If you find 1 in 20, or
5 in 100 plants have an egg mass, you should consider spraying just after the
eggs hatch. Don’t wait too long, as the
worms travel down to the cob in a few days, at which point they are protected
from the insecticide. The egg masses
first appear milky white, but begin to turn purple about 2 days before
hatching. As soon as you see purple egg
masses, get ready to spray in a few days.
So why is the pressure higher than normal this year? Well, like many of the other problems we have
seen this year, the theory behind this year’s higher than normal cutworm
pressure (outside of hot-spot areas) is the late planting season. Every year, OMAFRA collects cutworm trap data
to figure out which week through the summer has peak WBC moth flight. Since sands are generally planted earlier
than the rest of the province, it is believed that because they were planted on
time they tasseled earlier than peak flight timing. So, the moths flying looking to lay eggs pass
over the already tasseled corn on the sands to look for the next best thing –
the delayed corn that hasn't tasseled yet elsewhere, across the non sandy areas
of Southwestern Ontario.
What can you do about it? Not much at this point now, spraying it useless as the worm has made it to the cob and is protected inside the husk. The best thing you can do is prioritize the worst infested fields for harvest to prevent the spread of any Vomitoxin infection. Also, at harvest you could adjust the combine to blow any light weight, infected kernels out the back.