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.
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.