Hot dry conditions favour lygus and diamondback moths, so farmers must make sure to keep monitoring their crops
TURIN, Alta. — When there are hoards of insect pests eating the canola, the decision to use insecticide is obvious. When numbers are barely at the economic threshold, the choice is more difficult.
In the case of cabbage seedpod weevil, early seeded fields in southern Alberta had numbers close to the threshold, but later seeded ones did not.
“This was a really good example of a year where you probably could have left most of your canola unsprayed,” said Alberta Agriculture insect specialist Scott Meers.
He said there are few major insect issues in Alberta’s south. His sweeps in the region show few cabbage seedpod weevils, lygus bugs or diamondback moths and quite a few beneficial insects that attack some of those pests.
However, cabbage seedpod weevil has few natural enemies, so low levels of that pest are fortunate. Meers speculated that low weevil pressure might be due to dry fall conditions last year that killed them before they could pupate. That and a cold winter may have worked in tandem to reduce weevil numbers.
As for other canola pests, “lygus loves dry hot weather,” Meers reminded those at a July 11 canola crop walk at Serfas Farms near Turin. The region has received only 75 millimetres of rain since April so many dryland crops are struggling.
Lygus numbers in the south are low, but when little spraying is done for cabbage seedpod weevil, it often means higher numbers of lygus later in the season.
“Typically if we’re seeing one to two lygus per sweep at early flower, then we’re into spraying later in the season,” said Meers.
The usual spraying threshold for lygus is 2.5 per sweep, but Meers said he doesn’t get excited about lygus pressure until at least three per sweep.
Diamondback moths also like hot and dry conditions, so Meers said farmers should monitor their fields. He has found parasitoids in his sweeps that may help control diamondback, so it is important to make careful spraying decisions based on adequate sweeps of the field.
Ten sweeps in four field locations is the bare minimum required to gauge insect pressure, he said.
“If you do one set of 10 sweeps … you have about a 75 percent chance of making the wrong decision,” he said.
“One set of 10 sweeps is not enough. It’s lazy and you are making a big decision.”
Some farmers in the Barons, Alta., region have sprayed for flea beetles in canola this year, and Meers said those insects are a bigger issue in the south than has been seen in recent years.
He suggested that lower seeding rates, prompted by the high cost of canola seed, results in thinner stands and thus a greater concentration of flea beetles. A good plant stand is a big part of flea beetle control, said Meers.
Seed treatments help control the problem but “we are pushing the capability of our seed treatments,” he said.
Meers also said the striped flea beetle, which is more aggressive than the black type, is moving steadily southward.
Striped flea beetles have been found from Crossfield to as far south as Calgary.
“It’s a completely different beast,” said Meers about crop damage and control.
“The game is changing and if it goes to stripes, it’s a whole different story.”
Bertha armyworm is not an issue in southern Alberta this year but is likely to reach spraying threshold levels in the Camrose and Beaver County regions, according to recent monitoring data.