MINNEAPOLIS, Minn. – All summer long, corn rootworms have been hiding in the soil, feasting on plant roots and potentially costing yield. Now that fall is here, the larvae have hatched, and the beetles are emerging. This is when growers can really assess their acres, determine the scale of their problem and make management plans for the next season.
“Corn rootworm is a very challenging pest, and if you think about the weather we experienced this last year, we experienced a lot of challenges getting our crops in the ground. With corn planting, some was planted early, but more of it was planted later than normal. I think that may have extended when the corn rootworm larvae hatched and turned into adult beetles,” said Troy Bauer, BASF field technical representative.
The typical emergence of corn rootworm beetles has historically been late July. Growers would be able to scout fields for the beetles in August and through September. This gave the grower the option to try to control the adult population with insecticides, potentially reducing the number of eggs laid in the soil for future crops.
Over the years, there has been later and later emergence of the corn rootworm adult beetles.
“It seems to be a learned behavior on their part,” Bauer said. “From the biology of corn rootworm, we know it consists of one generation per year. Those females will come out, they'll lay eggs in corn fields, and the eggs will over winter and hatch the next year.”
The larvae are the ones that do the significant damage to the crop – feeding off the plant’s roots. This reduces the plants ability to take in nutrients, and it also affects standability, which can lead to lodging later in the season.
The adult beetles can also reduce yield. Depending on when they emerge, they can feed off the corn silk and reduce pollination.
“We estimate that western corn rootworm cost growers over a billion a year in yield loss and in terms of trying to control the insects,” he said. “We've used insecticides on corn rootworm ever since the late 40s.”
But rootworms are notoriously difficult to control. As early as 1959, there were reports of rootworms being resistant to the soil applied insecticides used to control them. Even the adult beetles have developed resistance to insecticides that were applied after emergence.
The most common method of control has been to rotate crops. By planting soybeans, or another type of plant that the rootworm larvae cannot feed on, that generation cannot survive or lay eggs for the next year.
“In certain geographies, corn rootworm actually developed a strategy to overcome that as well,” he said. “In Illinois, they’ve documented corn rootworm adults laying eggs in soybean fields in anticipation of corn being planted the following year.”
In other areas, Minnesota and northern Iowa, the northern corn rootworm larvae have developed an extended diapause stage. Meaning the eggs do not hatch the next year, but they stay dormant for two or more years, waiting for corn to be planted. Then the larvae hatch.
The other method of control is using traited corn with in-plant protection, like Bt corn.
Unfortunately, both Iowa State and Pioneer have documented corn rootworm resistance to the in-plant protections – as early as 2009 for Iowa State and Pioneer last year depending on the Bt trait.
“As you can see, corn rootworm has adopted a lot of strategies to overcome our control practices with them – they're very resilient,” said Bauer. “Our number one recommendation is growers need to get out and scout their fields, throughout the entire field, to determine the amount of root feeding presence.”
This means digging up roots and really taking a look at them.
They also need to watch fields this time of year to determine adult beetle populations and see how many are flying around.
Then the grower can make management decisions for the following year, such as what corn varieties to plant, seed treatments and insecticides to use. Maybe some acres will need to not be planted with corn the next season.
“The thing to really think about is a single adult female, if it's the northern corn rootworm, can lay up to about 1,000 eggs per adult. If it's a western corn rootworm female, they can lay up 1,800 eggs,” he said. “There can be a lot of eggs in the soil, so farmers need to get out and scout their fields, see what type of pressure they have and then manage accordingly.”