Wheat Infested With Hessian Fly in Spring- Disc It, Spray It, or Let It Ride?

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Article by Dominic Reisig, NCSU Entomologist and Scott Tilley- Regional Grains Agent

Two common questions have been popping up as folks are noticing Hessian fly. The first is can a rescue insecticidal spray help me reduce populations? The answer is almost always no, as timing the insecticide to reach the fly when it is susceptible (for an individual fly, about a two day period when they are an adult and a two day period once they hatch from the egg and move to the plant base). The second is what sort of yield loss spring infestations might cause.

We have some good information on this based on studies published by Dr. David Buntin from the University of Georgia. Note that this information pertains to spring infestations only, as we cannot tolerate as many Hessian flies in the fall. There are also two ways to look at this. Probably the most helpful is to calculate the number of infested tillers in the field. You probably won’t lose yield until 20% of the tillers are infested. If 40% of the tillers are infested, you might expect a 7 ½ bu yield loss. At 80% infested tillers, you can expect about a 15 bu yield loss. Keep in mind that yield potential of the wheat in Dr. Buntin’s studies ranged from 24 to 38 bushels, so you might lose more if you have potentially high yielding wheat.

There is also a relationship between percent infested stems and fly larvae in the stem. Don’t expect yield loss if you have one or fewer flies per stem. Two flies per stem is roughly equivalent to 40% infested tillers and four flies per stem is roughly equivalent to 80% infested tillers.