Aphids arrived in Darke County last week
By Justin Petrosino, ANR Extension Educator
OSU Extension, Darke County 
August  22, 2011 

I hope everyone is enjoying the Great Darke County Fair. I have had my share of fun grilling at the Pork Producers grill and eating enough fried foods elsewhere to make any heart skip a beat! Today let’s take a fair break to talk about economic thresholds. 

Economic thresholds are a level of a pest infestation that a farmer, consultant or crop production specialist should recommend an application of a pesticide. Thresholds are typically established for insects and diseases and are rarely used for weeds. With weeds there is too much variability in competition, species composition, and not enough data that agrees to develop good thresholds. Therefore, we typically rely on scouting and past experience in planning herbicide programs. 

For insects and diseases research has been conducted for many years, often decades on what level of pests cause economic injury. Economic injury occurs when the pest reduces yields enough that applying a pesticide will preserve sufficient yield to pay for the application. In simple terms it is a breakeven point. However, an economic injury level and an economic threshold are two different concepts! Economic thresholds are typically at a lower pest infestation than an economic injury level. This is for several reasons. The first is timing; by the time a farmer realizes they are close to an economic injury level, purchases a product, and applies it, he may have surpassed the injury level and lost yield. Also the products applied are preventative rather than curative. Applying a fungicide to a corn plant with grey leaf spot will prevent the spread of the disease. Any leaf area already lost will not be regained. Therefore we recommend an application before the pest reaches critical parts of the plant, like the ear leaf on corn or flag leaf on wheat. 

When it comes to insects there are a few more curveballs thrown into the works. Let’s talk about soybean aphids. They have finally made it to Darke County for this year. While at a program Tuesday night I found the first few by pure luck. Thursday before the start of the fair I took a trip from the south eastern edge of the county in Monroe Township north to Allen Township. The results of the survey: aphids are thin on the ground! 

The economic threshold for aphids is 250 aphids per plant with an increasing population. I was lucky to find one per stop (20 plants) and had to resort to a sweep net to find any. The economic injury level for aphids is actually 600 aphids per plant. OSU recommends spraying at 250 aphids per plant because aphid population growth can progress rapidly and 250 can quickly increase to 600. The increasing population caveat has to do with natural predators. We have plenty of insects that feed on aphids and limit population growth. Spraying too early or if the population is holding steady at 250 will kill the beneficial insects. Aphids will rebound or migrate in much faster than the beneficials can. This will often require a second insecticide application for control. The farmer will lose twice. The first application was made when aphids were not causing economic injury, and he will have to pay for a second application to prevent yield loss if the population rebounds. 

So the current recommendation for aphids in Darke County is to scout fields for an increasing population at or near 250 aphids per plant. If the threshold is reached there are many effective insecticides. A list in available in OSU Bulletin 545. If the question of a fungicide tank mix is raised, ask why the fungicide is needed. It is too late to control white mold, and foliar fungicide applications will not treat sudden death syndrome, Phytophthora, or brown stem rot. The only disease that may be present is frogeye leaf spot. However, most varieties of soybeans planted in Ohio have good resistance to frogeye. So if you need a break from the fair take a stroll through your soybeans!