Western bean cutworm causes concern for Ontario corn producers

The western bean cutworm caught Ontario corn growers off guard in 2016. It’s not that they hadn’t seen it before — this pest has been slowly increasing in population over the past eight years.

But it’s the insect’s ability to overwinter and its strong flying ability as adults that helped it to spread further than ever before, wreaking havoc on corn crops throughout much of the province.

“The western bean cutworm has been moving up from the southwestern U.S. and into Ontario, where it continues to move east,” says Art Schaafsma, a professor of field crop pest management at the University of Guelph. “But it is defying conventional thinking on invasive species because, unlike most non-native pests that come and go each year depending on conditions, this one just keeps on increasing in numbers.”

These cutworms lay eggs on the upper leaves of corn at tasselling time. Scouting for cutworm eggs can take time but, with some experience, they are easy to identify. Growers need to look for eggs at the top of the plant and then determine the percentage of plants affected.

Once a week for three weeks after pretassel starts, growers should scout and calculate the percentage of plants with new eggs. Once those figures cumulatively add up to five per cent, an insecticide application is warranted. If you spray too early you won’t get at the eggs. You want to target the newly hatched larvae that are moving about on the plant before they get into the ears, where they are protected from insecticide, Schaafsma explains.

“There is a several day period that is very attractive to the females for laying eggs in pre-tassel corn,” he says. “We set pheromone traps to tell us when the flight is happening. Once you start to see flight, you have a seven- to 10-day window to find and kill the cutworms before the damage becomes serious.”

For most corn growers, it’s not the damage from the cutworms themselves that causes yield loss, it’s the fact that feeding cutworms open up the ear, allowing easy entry for fusarium. Even then, fusarium itself is not the biggest problem, rather it is the DON mycoctoxin that a fusarium infection produces. DON is toxic to human and animal health, making the corn difficult to market.

“The mycotoxin was the big problem in Ontario last year,” says Schaafsma. “But the fusarium would never have done the damage it did without the cutworms paving the way. The high levels of DON in this province meant corn was refused or down graded at ethanol plants.”

There are two ways that fusarium can get into the ear of the corn. One way is when it gets into the silk as it starts to turn brown — a narrow time span that requires just the right moist, warm weather conditions to happen. The second way is when the fungus gets in after the ear is damaged in some way, as from western bean cutworms.

The Alberta experience

In Alberta, fusarium incidence is increasing in corn, but not due to insect damage. Alberta corn growers don’t have to contend with western bean cutworms. They do, however, have to manage for other diseases.

“In Alberta we don’t have any truly catastrophic diseases in corn,” says Michael Harding, research scientist, plant pathology with Alberta Agriculture and Forestry. “Most of our damaging disease happens very early — issues like seed rot, root rot and damping off. Those early fungi can cause big problems for an emerging seedling and ultimately the success of that crop.”

Harding says that these issues can generally be managed with careful seedbed preparation. He says an even bed with moist soil will help the corn seed to explode out of the ground before soilborne fungi can take hold. In addition, the majority of corn production in Alberta is in arid areas that are under irrigation, so growers can control the amount of moisture the crop gets if they spot signs of disease after emergence.

While fusarium is prevalent in Alberta corn fields, it’s not a major production concern for growers as the majority of the infections are in the stalk, not the ear, so they rarely accumulate significant DON levels in the ear or grain. That being said, the prevalence of fusarium in Alberta corn has increased from about 49 per cent in 2010, to 69 per cent in 2016. »

“In addition, the 3-ADON type of fusarium graminearum, which produces higher levels of DON, has been increasing in corn as well, from less than 10 per cent in 2015 to 19 per cent in 2016,” says Harding. “This indicates that while fusarium stalk rot and mycotoxin contamination has not been a major issue for Alberta corn producers in the past, the disease and mycotoxins are becoming more prevalent.”

Using disease free seed with high vigour along with good crop rotation, are the best first steps to prevent the development of many corn diseases, including fusarium. There are also varieties with stronger stalk strength that are better at tolerating stalk rots.

Practicing good agronomy, such as recommended seeding rates, proper fertility and weed control all help deliver a strong stand and healthy plants that are better able to fight off disease. While seed treatments are effective, especially for managing seed rot, damping off, seedling blight and root rot, their efficacy is limited to a two- to three-week window, which may not be long enough if spring conditions are cool and emergence is slow.

In season, there are effective fungicides that work well if applied at the silk stage. In terms of insect management, Ontario growers can seed corn varieties with traits offering resistance to cutworms, and there are several types of insecticides that can kill the cutworms before they hatch. Ideally a tank mix of a fungicide and insecticide will tackle both problems at once for a pest-free corn crop.