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Crossing the threshold

Nobody likes to see hungry insects in their field — but spraying in the absence of an economic rationale is bad economics and agronomics

Man holding a weed in his hand

Farmers and insects have always had a complex relationship and it hasn’t become any simpler with modern chemistry. There’s no doubt that a lethal application of an insecticide can do wonders for crop production if it’s needed and timely. But it’s a waste of good money if it isn’t. Every year farmers have to make the call to spray or not depending on whether a complex array of factors add up to what we call the economic threshold.

“It’s a process for producers to determine whether an application of an insecticide is economic,” explains Agriculture and Agrifood Canada entomologist Owen Olfert. “They want to control a certain pest if the economic injury level has been reached or exceeded.”

It’s a fact of life that no field ever farmed has produced at 100 per cent efficiency. That’s due to the huge number of curve balls Nature throws. Insect damage is going to lower yields but this shouldn’t always send farmers out with their sprayer. They have to watch for the economic injury level. They must answer the question: Does the looming economic loss make it worthwhile to take chemical action against a biological process? If the answer is “no” then there’s no point wasting time and money on spraying. If the answer is “yes” then a farmer can fill up the tank.

To make the call, farmers need to have a handle on the potential value of this year’s crop along with the cost per acre of buying the spray and running their sprayers. They should also have a good idea of what insect pests will be threatening their crops this year and what will be required to control them.

Adding up the costs is a good starting point and an experienced farmer with a handle on input prices and a well-kept set of records should have no trouble doing this. The biological factors, however, are another story. They’re completely unpredictable and this goes to the heart of that complex relationship. One thing about insects that we’ve come to know in the 10,000 years we’ve been farming is how well and how quickly their populations can expand to take advantage of an abundant food source.

Ecologists call this strategy r-selection and it hinges on small body size, early maturity and lightning fast reproduction. These species are highly adaptable and thrive in unstable and unpredictable environments. Because of their fast reproduction, their populations boom when conditions line up, and they crash just as dramatically afterward. Their numbers, when graphed, show a rapid climb towards a peak with an equally rapid fall when predators and disease catch up.

Some plants act this way too and we easily recognize this same behaviour in all of our annual crop species. Their wild ancestors survived by using the same r-selective strategy, grabbing the opportunity when land was disturbed and bare soil became available. They’d germinate, grow quickly and produce huge numbers of seeds only to die off just as quickly, leaving those seeds on the ground to wait for their chance to germinate and flourish. This certainly explains why our pests are so at home among our crops. They’ve been doing this dance together for millions of years.

The first insecticides worked very well and DDT in particular brought about a revolution in pest control. Insect pests were completely decimated for a number of generations, until the selective pressures favoured the progeny of resistant parents. 

The tables turned back in the 1960s when DDT became virtually useless in the field, leading one old cotton farmer to observe that he was killing more weevils by running over them with the tractor than by spraying them with DDT.

A decade later, with new understanding of population genetics and new chemistries, we started to play a more complex game called Integrated Pest Management or IPM. Farmers would scout their fields and, if the pest numbers were high enough, then they’d go in and spray. Otherwise they’d just leave the fields alone and let the natural enemies do the job. It was during this period we began to establish economic thresholds.

“Essentially it started for environmental reasons, but from a farmer’s perspective the economic threshold was promoted as an economically desirable thing to do, just to make sure that if you’re putting a pesticide out there’s an economic reason for doing that,” Olfert says. “So there was the biological component with economic benefits such as preserving natural enemies.”

That’s where we’re at today. A judicious farmer will look at the projected costs of control balanced against the projected population levels of insects within the fields. To do that we look at the different factors influencing insect behaviour. The first is the weather.

“The weather can be a big factor and the complex of insects present this year can change,” explains John Gavloski, provincial entomologist in Manitoba. “In some regions this year we had a very cold winter with minimal snowfall until January. In areas where they didn’t have a lot of snow their risk of cutworms and Bertha armyworms is a bit lower, because they overwinter right in the field. Other insects, like grasshoppers or flea beetles, overwinter in areas that tend to get a lot of snow accumulation so they’re going to be affected less. Again, your whole insect complex can change because of differences in overwintering strategies.”

Once they get past the winter, the spring weather can be just as important. Grasshoppers love it when it’s hot and dry. If that’s what greets them when they hatch, then prolonged dry spells in the spring may lead to serious grasshopper problems. On the other hand, if it’s cool and moist, the grasshoppers may die off and not be a problem at all, but these are the conditions in which wheat midge thrive. Watch for winds as well.

“A lot of insects don’t overwinter here,” Gavloski says. “They blow in with the prevailing winds or migrate in. Your risk of having them two years in a row really depends on when they blow in, what numbers blow in and the fact that they are mobile.”

Needless to say scouting is important. Good population estimates are integral to insect management. Scouts may look for damage because rough estimates may be done that way but proper counts are more accurate and will give you a better idea of how many are out there. 

Accurate identification is also crucial. Because different pests have different habits, economic thresholds will vary with species. For example, in the case of Bertha armyworm the economic threshold is the same as the economic injury level. There’s one generation every year, so once they’ve hatched the population doesn’t tend to grow beyond that.

Aphids, on the other hand, reproduce very fast so there are several generations per year. In that case the economic injury level is set around 670 aphids per plant. The economic threshold, however, is set much lower.

“It’s around 250 per plant on average and the reason they made separate economic injury levels and economic thresholds is because you’ve got an insect that reproduces quickly,” Gavloski says. “So you need a number that’s going to keep them from going above that economic injury level by the time you get your spray on.”

A great deal of research into the biology and ecology of our insect pests goes into setting these thresholds and there’s still a lot of work to do. Not only is there a wide range of species that we know but there’s always new invasive species coming into the fields.

In some of the latest research we’re starting to factor in things like the population levels of natural enemies. In many cases it’s the work of parasitoid wasps, many too small to be easily seen, and with some pests these wasps are crucial in keeping pest numbers below economic injury levels. 

“One of the advances occurring right now is using apps as decision-making tools and there are one or two of them out right now,” Gavloski says. “There is a soybean aphid app where you count your aphids but you also estimate numbers of key natural enemies. It’s what they call a dynamic action threshold where it’s also incorporating information on natural enemies. With computers and apps the computing can be done simply enough but if you had to do it by hand, it would get complicated.”

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