Troublesome bug | Producers consider response
A surprise contagion of aster leafhoppers in Manitoba, Saskatchewan and North Dakota has left entomologists and growers scratching their heads.
Producers have spotted high populations of aster leafhoppers in cereal fields and are wondering how to respond to the pests.
John Gavloski, an entomologist with Manitoba Agriculture, has been trying to answer their questions.
“People are just seeing higher levels and it’s just not something they’re accustomed to seeing a lot of,” he said.
Producers want to know if leafhoppers have reached high enough numbers that it is worthwhile to spray in cereal crops, but Gavloski said it’s not an easy question to answer because little research has been done on aster leafhoppers and field crops on the Prairies. Substantial populations of the insect may develop only once every 10, 15 or 20 years, he added.
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Aster leafhoppers are troublesome because they can transmit the disease aster yellows to oilseed and cereal crops.
They aren’t a significant pest in the northern Plains in most years because the climate is too cold for them to over-winter. However, this year’s mild winter likely allowed more aster leafhoppers to survive.
In canola, aster yellows causes leaves to turn purple and can deform pods or produce pods without seeds.
In wheat, aster yellows is often confused with barley yellow dwarf.
In general, the disease is more of a threat to canola, Gavloski said.
In an update on aster yellows in May, the Canola Council of Canada said five to 20 percent of plants in some prairie fields were infected with the disease in 2007. A five percent infection rate correlates to a 3.5 percent loss. At a 20 percent infection the yield loss could be 20 percent.
Fortunately for canola growers, aster leafhoppers prefer to feed on cereal crops, said entomologist Janet Knodel of North Dakota State University.
“They prefer the wheat and the small grains. I don’t think they will move into the canola until we start harvesting our spring wheat.”
Knodel thinks spraying for leafhoppers is a waste of money. The insecticide may kill the leafhoppers in one field but a new generation of pests would soon move in from a neighbouring field or blow in from another region.
“It’s not economically feasible for (growers) to be spraying every seven to 10 days,” she said.
Carrot growers in Manitoba do need to spray frequently for aster leafhoppers because aster yellows can affect the vegetable’s taste and appearance.
Gavloski also doesn’t support spraying field crops.
“(But) I can’t go on record saying it (the impact) is not going to be economical because we just don’t have the research,” he said.
Applying an insecticide would also kill beneficial insects, which may control other pests during the growing season.
There isn’t much growers can do except wait to see if aster yellows develops in their crops, Knodel said.
“There’s nothing they can do (to treat aster yellows). The only thing they can do is prevent the vectoring of the disease and that’s not economically feasible.”
The level of aster yellows infection in aster leafhoppers can vary. The insects are more likely to transmit the disease to plants if the infection level is high.
Knodel and Gavloski have sent insect samples to Agriculture Canada’s laboratory in Saskatoon to determine the level of infectivity. They expect to have results in a couple of weeks.
Aster leafhoppers may soon over-winter on the northern Plains because of climate change, Knodel said. As a result, regional entomologists might have more opportunity in the future to study the insect.
Aster Leafhoppers 101
Aster leafhoppers are small (1/8 of an inch), wedge-shaped and green to yellow with three pairs of spots on their faces.
Leafhoppers feed on plant sap and are a vector for aster yellows, a phytoplasma disease. Disease symptoms are similar to barley yellow dwarf virus, appearing at first as yellowing or a chlorotic appearance. Leaves then turn more reddish-purple with brown edges.
Aster leafhoppers do not transmit barley yellow dwarf virus, which is transmitted by cereal aphids.
Local populations of leafhoppers typically have low infection rates of aster yellows and need to feed on diseased plants to obtain the aster yellows phytoplasma.
Migratory aster leafhoppers have a higher infectivity rate.
Source: North Dakota State University
