Grasshoppers are on the move looking for green vegetation to eat.
The good news is they aren’t much interested in ripe cereal and canola crops, soybeans aren’t their preferred food and corn has so much leaf area it can withstand high populations, says Manitoba Agriculture entomologist John Gavloski.
“It’s not an outbreak,” Gavloski said in an interview Aug. 14. “There’s localized spraying going on. A lot of it is around field edges, but there have been some whole fields (sprayed). It’s localized where people are doing a bit of spraying. It’s not widespread.”
A lot of cereal and canola crops are being harvested, as well as ditches cut for hay, and that has grasshoppers looking for new food sources — preferably vegetation that’s lush and green.
Often grasshoppers will be more plentiful on field edges after moving in from ditches, Gavloski said.
“Don’t make the assumption that what you see in the first 10 or 15 feet into the field is what the whole field is like,” he said. “It may be quite different. In a lot of cases if people do have a heavier population they can get away with just doing an edge spray.
“If they seem to be focusing on the vegetation outside the field if it stays lush enough they may not even move in (to the field). If they do move, is it an edge effect or more widespread?
“Soybeans aren’t their favourite food. They’ve got a lot of other things they’d rather feed on.”
There are reports of grasshoppers in alfalfa. If they overwintered there as eggs, populations could be higher than if they have moved in from ditches, Gavloski said.
“You should keep an eye on it,” he said.
One option is to cut the alfalfa if it’s at the right stage for making hay.
“They shouldn’t hang around too long after a cut because the food quality is all of a sudden going down hill,” Gavloski said. “Especially this time of year they are going to be adults and mobile. They should move out after the field has been cut.”
The economic threshold for spraying grasshoppers in field crops, including alfalfa, is a bit vague at around eight to 12 per square metre, Gavloski said. The tougher part is estimating their numbers.
“It is a hard thing to assess because you’re walking and things are jumping and you’re supposed to be estimating numbers,” he said.
If farmers do spray crops to control grasshoppers they need to adhere to the insecticide label’s pre-harvest interval. That can range from as little as one day to several weeks.
“When you get to this time of year there becomes very few (insecticide) options in some crops,” Gavloski said.
Grasshoppers, which are in the adult stage are harder to kill now too.
Because they can fly they are more spread out.
The best time to scout and control grasshoppers is between June 20 and July 20 after most eggs have hatched, but the grasshoppers are still juveniles and concentrated in ditches and field edges, he said.
Dry, warm weather is ideal for grasshopper egg laying. That could mean lots of grasshoppers again in 2019, but not necessarily, Gavloski said.
“If we get some very, very heavy rains in late May or early June that can really knock the grasshopper population back,” he said.
Gavloski conducts an annual survey of grasshopper egg laying in the fall and publishes the results early in the new year.
“There are no guarantees, but we try to provide some sort of forecast based on these late-summer populations, more to just make sure people are out there scouting in June the following year,” Gavloski said.