A turf issue: Armyworms vs. monarch caterpillars

Oklahoma homeowners weigh lawn care against butterfly, bird impacts

Fall webworm. (Frank Peairs, Colorado State University, Bugwood.org)

For the CCOF

Save the yard or save the butterflies? That is the question.

Facing hoards of hungry caterpillars, some good some bad, many Oklahomans are for the first time questioning backyard pesticide use.

“I’ve been busy on Facebook,” said Sandy Schwinn, volunteer conservation specialist for Oklahoma with the national MonarchWatch program. “I bet I’ve had 50 questions a week about ‘what worm is this and what should I do?’”

Fall armyworms are unusually plentiful this year and they are wiping out lawns nearly statewide. Southerly winds bring in the coastal-states moths every year, said Oklahoma State University entomologist Eric Rebek

Some years a variety of environmental factors combine to create a year of plenty, he said. It’s hard to know exactly what factors make the difference in any given year. As the name suggests, Armyworms lay their eggs in masses and when they hatch, they are a destructive lot.

“Each female can lay about 1,000 eggs,” Rebek said. “They get the name because they basically march en masse, like an army, and eat pretty much everything in their path they find palatable.”

The good news is the armyworms do disappear, eventually.

“They are a cold-sensitive species so the first hard killing frost sort of resets the clock every year,” he said.

The good news is the armyworms do disappear, eventually.

“They are a cold-sensitive species so the first hard killing frost sort of resets the clock every year,” he said.

Concern for monarchs

But this also is the time songbirds flock and prepare to migrate, and when favored caterpillars appear. Of highest concern are the caterpillars of monarch butterflies. Monarchs are dwindling in numbers and considered for listing under the Endangered Species Act.

Schwinn said monarchs migrate through Oklahoma during their great move south later in autumn. Early migrants and summertime resident monarchs produce a crucial hatch of butterflies in August and September. These are migrants that fly south to Mexico, overwinter, and return next spring. Resident monarchs were more plentiful this summer, so eggs and caterpillars are more plentiful as well, she said.

“You have to be careful because the same pesticides that kill armyworms will kill any other caterpillar,” she said.

The particular strain of fall armyworms on the ground now prefer Bermuda grass, but they also like zoysia grass, and also hit tall fescue, rye and even some cultivars of bentgrasses, Rebek said.

Risks to the homeowner’s yard vary. Bermuda and zoysia grasses establish an underground network of rhizomes, so even though the armyworms make it look dead, the issue is short-term.

Monarch caterpillars. Kelly Bostian/KJBOutdoors

“It’s a temporary aesthetic issue for Bermuda lawns,” he said. “You can keep it watered and wait it out and it will recover. People with fescue and rye those kinds of turf are much slower to recover because they don’t have those underground runners. In heavy infestations, they can entirely wipe it out and you can lose the lawn.”

Pesticide options

A laundry list of broad-spectrum backyard pesticides available at stores will hit armyworms. The university always encourages appropriate, only necessary, use and always according to Environmental Protection Agency guidelines, he said.

The American Bird Conservancy warns against using any insecticides that contain neonicotinoids, which is banned in European countries and has been shown to be a danger to pollinators as well as songbirds.

Pesticides that use Bacillus thuringiensis (Bt)—also used in mosquito larvae control—are targeted at small caterpillars of all kinds. Studies have shown that pesticide is not harmful to birds, Rebek said.

“Whatever you’re treating your yard with you don’t want to get it on the milkweed or any flowering weed that butterflies might land on and come into contact with it,” he said.

The natural approach

Stephanie Jordan, the pollinator outreach coordinator for Okies for Monarchs, said the statewide group encourages people to think hard before using any pesticides. The best control method is wild birds, which can use a plentiful protein source.

“Your yard is a natural system, too, and when you take out one part you’re taking more than you realize,” she said. “To the extent that someone can allow for some of that damage to happen and realize it’s not permanent that’s best.”

People who have planted gardens specifically to attract monarchs are best off just letting nature run its course, Schwinn said.

Some programs, such as the Oklahoma Association of Conservation District’s Yard-by-Yard Community Resiliency Project and the National Wildlife Federation’s Certified Wildlife Habitat programs, only certify yards that entirely eliminate the use of chemical pesticides.

Both Jordan and Rebek said birds in the yard sometimes are the first indicator of a coming infestation. It’s not a bad idea to just let the birds take care of it. The birds can use the protein this time of year.

“We often find that if a bug of some kind arises, just give it time and another bug or predator comes along to take it out,” Jordan said.

Natural sources of control that can be purchased include buying nematodes, ladybug larvae or lacewing larvae online, she said.

The milkweeds that monarchs rely upon also are a good source for lacewings, she said.

“If I look at every milkweed plant in my neighborhood it will have at least some lacewings on it,” she said. “If you can afford to wait, your beneficial species will come along.”

And a dead lawn isn’t always the end of the world, she said.

“If part of your yard gets taken out, maybe come back in there with a wildflower mix,” she said. “Maybe the worms are nature’s way of asking you to shrink your lawn to make more room for nature and to let nature take care of itself.”

Kelly Bostian is an independent journalist writing for The Conservation Coalition of Oklahoma Foundation, a 501c3 non-profit dedicated to education and outreach on conservation issues facing Oklahomans. To learn more about what we do and to support Kelly’s work, see the About the CCOF page.

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