Weed & Pest
Schell presents strategies to protect cropland from grasshoppers
While 2010 grasshopper treatments were successful in many rangeland areas of Wyoming, some of the farmland areas were hit hard, and UW Extension Entomologist Scott Schell is working on how to prevent that in 2011.
Of problems encountered in Wyoming crop production areas, Schell says the major species that affect farmland are migratory, differential, twostriped and redlegged.
“They’re all related, with the same genus and habits,” he says. “Some hatch earlier, some have higher reproductive potential, but they’re all mixed feeders, preferring forage first, then eating grasses and small grains. They’ll even chew on pine trees and other things considered inedible, like clothing on a clothesline and window screens. Anything is food to a grasshopper, if it doesn’t kill them when they eat it.”
He says the crop varieties have much higher reproductive potential than rangeland species, at 45 to 194 eggs per pod found in one study.
“If you look at a grasshopper area with 40 per square yard, and 20 are females laying one egg pod, that’s 900 eggs per square yard for next year,” explains Schell. “The egg density on field borders can be very high – that little bit of area that’s not cultivated can cause a lot of problems.”
The nymph stage is the time period when producers should look for grasshoppers and think about control for the species they find. Schell says the crop species, as nymphs, have black between their head and thorax, and a white swoosh underneath that. Many also have a black bar on the femur of the hind leg.
Schell says their extended hatching period is most important – there can be up to 52 days between the first and last hatches, and none of the current products have enough residual to cover the entire period.
Crop grasshoppers will come from the edges of a field, around borrow ditches, fencerows and the corners of irrigated circles.
“Those can be source habitats. They often lay their eggs in those field borders, and we have seen some situations where grasshoppers adapt to farming units with no till or minimum tillage and lay eggs in the middle of the field. Some minimum tillage implements, like a sweep, might not be enough to kill them,” says Schell. “Plowing, disking and rotary hoes create more disturbance, so farming practices are something to think about if you’re having problems with grasshoppers.”
Schell says the major goal is to keep grasshoppers out of a crop. “Once the grasshoppers get in a crop it’s much more expensive to kill them, so we want to treat source habitats early and often,” he says.
A 40-acre corn field with a 15-foot border has approximately 38.2 tilled acres and 1.8 acres remaining around the edges for grasshopper habitat. Schell calculates that’s enough to produce 200 eggs per square yard in the edges, for an average of 9.5 grasshopers per square yard over the entire field.
“That’s a density at which they can defoliate a corn crop,” he says.
To protect that field, Schell recommends spraying that 1.8 acres between May 20 and May 31.
“I’d scout it and see if there’s a hatch again, and spray again between June 20 and June 30,” he says, saying he’d repeat again between July 20 and 30, if necessary. “I’d use Dimilin at the label maximum, which is two ounces per acre.”
Dimilin has become the preferred product for treating grasshoppers because of its safety, selectivity and economy.
“It doesn’t affect other species and the residual is pretty long,” says Schell. “In rangeland situations one treatment is more than enough, if applied during the ideal time period. It has reliable results, and can be applied through a wide temperature range.”
The one challenge with Dimlin is that it’s not labeled for alfalfa, and Schell says the company is working to get a full label on the forage crop.
“It has a low impact on non-target insects, and I think it would be an ideal fit for our crop situations,” he adds.
Apart from Dimilin, other options are pyrethroid insecticides, which have a different mode of action and affect the nervous system of insects.
“They have good safety with low mammalian toxicity, and they work very quickly. One thing about Dimilin is that the grasshoppers have to eat it, and it won’t kill them until next molt period, which is five to seven days down the road, so you’ll see a gradual decline. Pyrethroids have a very rapid knockdown, and you will see dead grasshoppers and other insects, because it will affect any insects that come into contact.”
Schell says the advantage of pyrethroids is that there are many crop labels.
Carbaryl is another control possibility for grasshoppers, says Schell.
“It’s also a neurotoxin, it’s stable in a lot of temperatures and it has decent residual, easy handling and reliable results,” he explains. “It has to be applied at a higher volume, there is potential harm to people and wheat is no longer on the label.”
He says carbaryl is also available in bran or pellet baits, which will kill adults, but multiple applications are necessary. “Within one week it’ll all be gone, and you might have to apply it for seven or eight weeks,” he says.
Schell adds that organophosphate insecticides are labeled for wheat and alfalfa and are broad spectrum, non-selective and are labeled for chemigation with irrigated crops.
Another option for wheat land is seed treatments in winter wheat.
“After the seed sprouts and germinates, the new seedling has insecticide inside it to protect it,” says Schell, saying that one product is Rancona Crest, which also includes fungicides. “It’s expensive, but you don’t have to plant entire field with it – just the outer two rows of treated seed will help protect your field.”
It does have a grazing restriction, and Schell says sometimes you might be overrun despite its use. He says it’s easy to use if the seed is purchased pre-treated, but other products have to be treated by the farmer.
“None of them are perfect, but you’ve got to educate yourself to the pros and cons of each one to see what fits your situation,” says Schell.
Schell cautions that a cost/benefit analysis is a critical component of any treatment program.
“You don’t want to do a treatment that will cost you more than the benefit,” he says.
He recommends a computer program for both range and cropland treatments called CARMA, which was developed at UW by John Hastings and can be found at carma.johnhastings.org.
Schell also recommends visiting Wiki.bugwood.org/HPIPM:Main_Page, or simply Googling “High Plains IPM” for the most current crop labeling. “It’s difficult to keep up with all of them – they change constantly,” he says.