Admittedly, there is considerable confusion in determining the threshold levels of insects prior to spraying a crop.

Is it 3 or 5 Japanese beetles per ear, or was that corn rootworm beetles, or was that something else? And by the way, does that threshold still apply if the crop value has increased threefold? Entomologists are doing as much number crunching as bug counting, and the answers are not always easily determined as some farmers may think. In the meantime, let’s listen in on their conversation about pest control thresholds.

The bug gurus at Iowa State University, Jon Tollefson, Matt O’Neal, and Marlin Edwards say soybean aphids may be the next pest that draws your attention, and their numbers on soybean plants can be Biblical in proportion. Since counting tens of thousands is not feasible, even for folks with a lot of time on their hands, their latest newsletter provides some guidelines to help with the task of deciding whether to spray. They want you to learn and use three levels of consideration:

• Damage Boundary is the earliest stage, in which insects can be measured to determine whether there is yield loss. While five aphids on a soybean will not have an impact, five Japanese beetles on a silking ear of corn will have a major impact. The entomologists say there is no reason to spend any money on pest control if numbers are not present to begin any level of yield loss. The fewer the bugs the less chance of any economic return for your efforts.
• Economic Injury Level is a significant threshold, and is the number of insects required for the value of the lost yield to equal the cost of control. In other words, if the chemical and custom application cost $30 per acre, then the bugs have to cause a two bushel per acre loss in beans, if beans are worth $15.
• Economic Threshold is the decision point according to the Iowa State entomologists, and that is the number of insects required for pest control to be applied to keep pest populations from reaching the Economic Injury Level.

Prior to the rise in market values for soybeans, the soybean aphid Economic Injury Level threshold had been 654 aphids per plant, plus or minus 95 per plant, cost of control ranging from $8.65 to $21 per acre with a yield between 30 and 60 bushels per acre, and soybeans valued between $5.50 and $6.50 per bushel.

With $15 soybeans, that soybean aphid Economic Injury Level threshold drops to 452 aphids per plant, which are 200 fewer to count. Since the initial discovery of soybean aphids in 2000, entomologists have recommended a spray threshold of 250 per plant. But 250 aphids are also below the damage boundary in a new experimental protocol for the pest, meaning their presence at that level should not cause economic injury.

Research during the 2004 and 2006 aphid outbreaks indicated that populations rarely exceeded 250 per plant, and yield damage could not be quantified when fields were sprayed. Currently, aphids exist in soybeans at relatively low populations, but there is no guarantee they will multiply to treatable thresholds. That is why the Iowa State researchers are recommending use of the 250 Economic Threshold.

Along with the consideration of a new yardstick for making pest control decisions is the question about preventative spraying. Specialists say that only leads to the opportunity for resistance to be developed and for a resurgence of the aphids, should the preventative insecticide kill all of the natural predators.

Summary:

Soybean aphid populations should be on the radar of all soybean producers, since small numbers of aphids have been found in numerous locations. Spraying decisions should be based on existence of a number of aphids that could multiply into levels that will have an economic impact on the yield. There may always be enough aphids to count, but not always enough aphids to warrant a spray application.

Stu Ellis

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