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Higher crop prices mean lower insect thresholds

With higher grain prices it takes fewer insects and lower levels of feeding to trigger an economic pay-off for spraying. Generally these economic thresholds are lower today than they were just a couple years ago, says Scott Hartley, insect management specialist with Saskatchewan Agriculture in Regina.

He points to lygus bug as an example. Existing tables show that at the end of flowering, it would be economical to spray if you found 12 lygus bugs during 10 sweeps. That is based on costs of about $10.50 per acre to apply an insecticide and canola prices at $6.80 per bushel.

Today, if canola is in the $12 to $14 per bushel range, the threshold is likely about half that many — about six bugs found in 10 sweeps, says Hartley.

Determining more precise thresholds at higher crop prices takes research time and dollars, he says. “There really isn’t a point were we wrap up calculations and say “here are the figures for 2008 crop production conditions”,” says Hartley. “Establishing disease thresholds is an ongoing process that is reviewed as conditions change.”

Hartley says too there are many variables that come into play as far as determining a treatment threshold for an individual farm. Cost of application, whether a producer has his own spraying equipment, number of acres to be treated, cost of chemical, and the growing season are among the key variables.

Each provincial department of agriculture has posted economic thresholds for specific pests on specific crops on their respective websites. These thresholds are a good reference point, says Hartley, although they should be used as a guide, rather than an absolute.

For flea beetles in canola, for example, the threshold refers to when 25 per cent of cotyledon surface is destroyed. Hartley describes this as an action threshold. If a producer is seeing 25 per cent damage on cotyledon surface there is still time to act, he says. “We have seen situations where there has been as much as 40 per cent damage, and then with spraying the crop has survived and yielded quite well,” he says. “So that 25 per cent damage is a point at which a producer wants to start looking at spraying.”

With the wheat midge, the existing threshold is one midge for every four to five wheat heads. That’s based on $3 per bushel wheat. Again with higher wheat prices that threshold should be adjusted down, says Hartley. He adds that the threshold of one midge to every four to five wheat heads refers to a yield threshold, whereas with Canadian Grain Commission standards the economic threshold for grade is considerably lower. To optimize grade, the existing threshold is closer to one midge for every eight to 10 wheat heads.

“With current wheat prices, targeting one midge for every eight, nine or 10 wheat heads is probably a good yield threshold today,” he says. Pest numbers for a grade threshold haven’t been determined but will be one midge for every eight to 10 wheat heads or lower.

With bertha armyworm thresholds in canola, Hartley notes there is a scale based on different market prices for canola, but the scale needs to be adjusted to reflect today’s $12, $13 and $14 per bushel canola prices. The threshold scale also needs to be updated to reflect higher yielding hybrids.

“Other factors that come into play is the timing of the crop and the pest,” he says. “ With bertha armyworms if the pest is feeding primarily on the leaves of a hybrid variety, and not the pods, then the plants and crop can withstand a lot more insect pressure without an economic loss in yield.”

Diamondback moths present a similar situation. If the pest infests a leafy crop early before bolting, the impact is less than if the pest infests the crop later and damages the growing point of the plants.

Hartley would like to see research dollars invested in determining better thresholds for aphids that affect both wheat and pea crops.

Lee Hart is field editor of Grainews, based in Calgary. Contact him at 403-592-1964 or by e-mail.

More threshold details

For more specific information on the economic threshold of insect pests, check the websites of your provincial department of agriculture. The following Internet addresses go directly to reports on insect thresholds, however as some of the addresses are quite long, you can also go to the main agriculture department websites, click on crops, and follow the links.

Manitoba Agriculture, Food and Rural Initiatives

Saskatchewan Ministry of Agriculture

Alberta Agriculture and Rural Development

Your nearest AgKnowledge, AgInfo Centre or GO Team office will also have specific advice on insect thresholds. In Manitoba, contact the provincial ag department by calling the main government number toll free 1-866-MANITOBA (1-866-626-4862). In Saskatchewan, call the Agriculture Knowledge Centre toll free at 1-866-457-2377. And in Alberta call the provincial Ag Info Centre toll-free at 310-FARM (310-3276).

For information specific to canola pests, contact a Canola Council of Canada specialist: Derwyn Hammond, Manitoba, 204-729-9011; Jim Bessel, North Saskatchewan, 306-373-6771; David Vanthuyne, Eastern Saskatchewan, 306-782-7799; Doug Moisey, Parkland East, 780-645-3624; Matthew Stanford, Chinook, 403-758-6660; John Mayko, Parkland West, 780-764-2593; or Anne Vos, Peace Region, 780-835-4632.

About the author

Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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