By Brian Hefty
At what point do you have enough insects in your field to justify spraying? No one has the exact answer to this, but by using an economic threshold, you can at least make an educated guess.
Let’s use the outdated 250 soybean aphid threshold many people use as an example. When the 250 aphid threshold came out in the early 2000s, soybeans were worth maybe $6 a bushel at best. Insecticide was about $8 an acre, and the average yield back then was just shy of 40 bushels per acre. Back then, to spray your own fields, we figured a cost of about $2 per acre.
Today we figure $3 per acre to run over the ground on our farm, so that cost is up, but all the other factors go the other way. Soybeans are worth double what they were. A full rate of the same insecticide we used 10 years ago is now only $2 per acre, and I would sure hope your yields are up vs. 10-12 years ago. On our farm we have been averaging just shy of 60 bushels per acre the last few years.
What I’m trying to say here is THE ECONOMICS HAVE CHANGED. When the economics change, the threshold changes IF you are using a true economic threshold for spraying insects. Numerically, here’s how I look at it.
If cost in the early 2000s was $8 for insecticide and $2 for spraying, that’s a total investment of $10 per acre. Our data shows the 250 aphid threshold even back then was way high, but let’s just assume for a second that 250 was the correct number. If 250 aphids were costing $10 worth of economic damage, that means 25 aphids would cause $1 worth of damage if the feeding remained the same when the numbers were higher or lower. If today’s cost is $2 for insecticide plus $3 for spraying for a total of $5, that means that the 250 aphid threshold should now be 125 (25 X 5 = 125 aphids per plant).
Let’s take it a step further. Since back then the crop was only worth $240 per acre versus what we can now get today, I can make a good case for saying that based on my economics, I can spray at an even lower threshold. For me, I now figure 60 bushel soybeans times $12, which is $720 per acre. I don’t know if I would take it this far, but I’m now generating 3 times the gross income per acre, so shouldn’t my threshold be 1/3 what it used to be? If I already lowered my threshold to 125, could I then afford to lower it even more and spray at 42 aphids per plant (125 / 3 = 42)…I probably would.
Now let me add one additional piece to this. According to what I understand about the 250 aphid threshold, beneficial insects come into play somewhat in determining the threshold. In other words, you shouldn’t spray too early because you want to leave the aphid predators like lady beetles alone. Today, Transform insecticide can be sprayed. Transform DOES NOT KILL LADY BEETLES, yet is lethal to soybean aphids. How does that affect your thinking and your economic threshold?
Here’s my whole point. Unfortunately I have found many entomologists and agronomists throwing out the “economic threshold” term without actually putting the economics to it. On the farm, we all obviously want more yield, but we have to have profit to go along with that added yield. When your crop is valuable and treatment costs are cheap, an economic threshold by its very definition should have you spraying at a lower level of insects than when your crop is not worth much and treatment costs are high.
If your agronomist or entomologist is giving you an “economic threshold” that doesn’t vary when the economics change, it’s time to start looking for better advice.