Canada
June 2, 2014
Early weeds, insects above thresholds, and sclerotinia stem rot in a moist summer are three situations where canola growers are likely to get a good return from a pest management investment. Not all situations provide such economic clarity. This article describes three situations where spray applications may not provide a return on investment.
Second in-crop herbicide applications, fungicide applications to prevent blackleg, and insecticides tank-mixed with herbicide are three pest management situations where economic returns may not add up.
“These applications may slightly reduce pest severity, but as for making a positive contribution to profit, we just don’t see it very often,” says Gregory Sekulic, agronomy specialist for the Canola Council of Canada. “And they could do more harm than good by putting needless selection pressure on existing populations for resistance and by damaging beneficial insects.”
—Second in-crop herbicide applications. A preseed burnoff and one early in-crop herbicide application are often enough to remove the economic threat from weeds. As long as canola plant populations are on target, the crop should have enough ground cover to out-compete any subsequent weeds.
“Second in-crop applications may make the crop look “cleaner,” but they often do not provide an economic return, and weeds left to escape do not contribute significantly to weed seed banks,” Sekulic says.
If made after the recommended leaf staging of the crop, second applications can also injure the crop enough to reduce yields.
—Fungicide for blackleg. Blackleg that infects early is most likely to cause economic yield loss later in the season. However, unpublished AAFC study results from the past few years do not show an economic benefit to early-season fungicides for blackleg control — except when susceptible crops are grown under high blackleg pressure.
“Until there is third party data that clearly demonstrates improvement in yield from a fungicide use in absence of disease, we suggest that blackleg fungicides only be used in situations with high blackleg pressure and when varieties are susceptible,” Sekulic says. An important management step with blackleg is to scout at harvest for obvious signs of damage, and look to crop rotation and different varieties if improved management is required.
—Insecticides tank mixed with herbicide. The ideal economic herbicide timing often occurs before flea beetle control is warranted, and holding off herbicide until flea beetle feeding gets serious could be costly. In the case of cutworms, target sprays as opposed to full field coverage are often enough. Most other insects rarely cause economic damage this early in the season.
“It may only cost a few dollars an acre to add an insecticide to the herbicide tank mix, but that cost comes right off your profit margin if there are not enough insects to cause economic harm,” Sekulic says. “This practice also causes unnecessary damage to beneficial insects.”
Natural insect predators can stop pest insect threats from escalating, and may prevent the need for sprays later in the season. One good example is the Diadegma insulare parastitic wasp that attacks diamondback moth. During large influx years like 2003 and 2005, canola growers in Western Canada were spared millions of dollars on millions of acres as diadegma took out over 90% of diamondback moth larvae.
When making an insect management decision, scout for damage, identify the cause, and check the specific thresholds for each insect to make sure the threat is sufficient enough to provide an economic return for a pesticide application.
Insect economic thresholds represent the break-even point for an insecticide spray. If insect counts are at the threshold, yield benefit from an insecticide spray will be enough to cover the product and application cost of the spray. When insect numbers rise above the threshold, there will be a return on investment.
“Pest management decisions that consider the whole economic spectrum, including potential hidden losses from late herbicide applications and unnecessary insecticide applications, will reduce costs per bushel produced and improve economic and environmental sustainability at the same time,” Sekulic says.
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