• Many growers wish to tank-mix an insecticide with a herbicide or fungicide, to avoid making additional trips across a field.
• Doing so when insects are not present, or when thresholds are not reached, does not make economic sense, and has the potential to flare secondary pests such as armyworms and spider mites.
Economic thresholds are based on the concept of economic injury.
Economic injury is the point at which the cost of a management action is equal to the injury that the insect is causing to the crop. It is essentially a break-even point.
Economic thresholds are set below the break-even point and are conservative so that growers will not experience any loss. A good example of an economic threshold is the dynamic threshold for stink bugs in cotton. Although growers can make a higher yield by spraying weekly for stink bugs, in the vast majority of cases, it is much more profitable to use the dynamic threshold, spraying only when needed.
In contrast, most thresholds used in soybeans are different. Below is an explanation for why thresholds in soybeans should remain static once soybeans are above $10 per bushel.
Kudzu bug: The kudzu bug threshold of one nymph per sweep (15 nymphs in a 15 sweep sample) is not a true economic threshold. Research has shown that treating at these levels should avoid ANY yield loss. Therefore, the threshold is unaffected by the price of beans.
Defoliating pests: These include pests like green cloverworm, armyworms, and bean leaf beetle, to name a few. Much like the kudzu bug threshold, the defoliation thresholds (click here for the thresholds) are set at a point at which growers will not incur any yield loss. Therefore, the threshold is unaffected by the price of beans.
Corn earworm: The threshold for corn earworm is a true economic threshold, which changes with row-spacing, price of the crop, and cost of control.
You can play with an online calculator by visiting this website. However, this calculator has one critical flaw. Research has shown that once soybean prices exceed $10 per bushel, that soybeans will compensate for any additional loss and that the threshold should remain static.
How can this be? The reason is that the calculator is based on an equation that drops the number of insects required before a treatment lower and lower as soybean prices go higher and higher.
At some point, the calculator will require that the number of insects be so low that you would have to constantly treat the soybeans to keep the number of earworms below the threshold. The flaw in this logic is that the calculator does not include the plant’s ability to compensate for the feeding done by very few caterpillars.
Therefore, our thresholds should remain constant for corn earworm as prices increase anywhere above $10 per bushel.
Stink bugs: Stink bug thresholds will likely be raised in the future, based on recent research in southeastern Virginia. Therefore, our thresholds for stink bugs are likely too low, even in the face of $16 soybeans.