[UPDATED: Jan. 31, 2018] Economic thresholds are developed and publicized so farmers will know when it makes sense to spray for a particular pest. For example, the economic threshold might be that if you have two bugs in your sweep net, the yield you gain will make it worth your costs and time to spray for that bug. But economic threshold numbers are only useful when they’re correct and up to date.
In recent years there has been much discussion on lygus bug control and whether or not current economic thresholds are indeed correct. While recent research validated thresholds, some producers and agronomists feel the numbers are too low, especially when applied to current canola production systems.
- Read more: Lygus bugs: Profile of a crop pest
Keith Gabert, Canola Council of Canada agronomist, and Héctor Cárcamo, research scientist, insect pest management, Agriculture and Agri-Food Canada, explain their differing opinions.
Ian Wise and Bob Lamb developed the first economic thresholds for lygus bug in Manitoba sometime in the 1990s. According to Cárcamo, they suggested a threshold of 1.5 lygus per sweep at the early pod stage of conventional canola (not including hybrid cultivars), using the prices for canola at that time. Ten or so years later, Cárcamo conducted further research that confirmed those numbers.
“In fact, several years ago we actually lowered the threshold to take into consideration the higher price of canola,” he said.
The new threshold was set at one lygus per sweep, which some think is too low. Carcamo openly admits there is a problem with the concept of economic thresholds. Some, he said, will look at the tables and forget to consider that plants can compensate for insect feeding.
“When you develop thresholds you kind of force a linear relationship,” he said. “You have a graph where you have, say, yield on the y-axis and insect numbers on the x-axis, and usually you get a very messy relationship.”
“To be able to develop the threshold you have to have an equation where you have a regression coefficient, and then you use that and relate it to the price of canola and the amount of grain that can be saved by spraying,” he continued. “It becomes a little artificial, in my opinion, and I think we should use a bit more common sense sometimes and not get too carried away with stretching that threshold to the very high end just because the prices are very high.”
Cárcamo has conducted three years of cage studies in different regions. His findings support the listed thresholds. “The data, to the best that I can analyze it, suggests those thresholds,” he said.
Gabert thinks current thresholds are dated, though, and don’t match what he’s seeing in the field. “They’re quite specific in terms of what’s the value of your canola and what’s your application cost per acre,” he said. “If you actually follow the chart and canola is $6 and it’s going to cost you $8 to spray, well, then you might consider spraying with 11 lygus bugs per sweep.”
“On top of that, there are two charts, based on the stage of the crop (see below), so it makes it look like we’re very exact on how much damage this insect can cause and how dedicated or effective we should be at controlling the insect,” he continued. “I have to say that that hasn’t necessarily been my personal experience or the feedback that I’ve got from growers.”
“While it seems that there is some yield potential to be lost to lygus bugs, it has never been described to me as a linear or a growing loss from more and more lygus bugs,” he concluded.
According to Gabert, advisors in the Black soil zones tend to increase the threshold numbers before advising growers to spray. “The charts end somewhere in the mid-30s and I have advisors using numbers in the 60 to 80 (lygus per 10 sweeps*) range — or maybe even higher,” he said. “They’re sort of padding the threshold numbers, which is likely a good thing. If they’re not seeing a lot of damage, they want to make sure that not only do they break even, but actually make money from spraying this insect.”
The future of the threshold
Gabert is currently part of a team that’s working with Cárcamo to investigate what’s going on at the field level. Their field-scale studies began in the 2016* growing season when, unfortunately, insect populations were historically low. They plan to continue their research in the hopes of getting some more concrete field data.
In the meantime, Gabert suggests that growers scout early in order to establish whether or not the insect is present in the field. “Plan to scout again and make a relatively quick decision near the end of flowering or prior to pod ripening,” he said. “And get the benefit of controlling this insect for a number of weeks before swath timing would become a concern.”
“If you’re going to control an insect and try to prevent its damage, it makes sense to remove it after flower, near the end of flowering when the crop is really trying to fill those pods,” he continued. “Rather than maybe waiting a few more weeks when it’s still perhaps advisable to spray, but you won’t get as much time or benefit out of controlling them.”
*Editor’s note: Thanks to an online reader, we have caught and fixed some errors. We originally indicated that these numbers were for ONE sweep — they are actually numbers for TEN sweeps. Thank you also to Keith Gabert, who wrote, “10 sweeps removes some of the averaging issues that occur when growers use a sweep net, so you’ll find entomologists tend to avoid even giving #/single sweep advice.”
Keith also passed on this information: “There’s a really clear indication that damage is less of a concern with adequate moisture… which also partially explains why growers in the Black soil zone — typically with good moisture that allows plant compensation — would be able to tolerate higher pest #’s.”
We noted that scouting “early” is good. Keith Gabert wrote, “Growers generally think that scouting early would mean in terms of early in crop development or season — instead of relative to the staging listed on the lygus bug chart which was my intent here.”