Similar to fall of 2015, Hessian flies adults are emerging and laying eggs in December. Larvae will hatch from these eggs and will feed throughout the cold months, killing tillers.
Here are some bits of information to help you decide whether or not to spray:
1) Spray only susceptible varieties. A list of high yielding varieties with Hessian fly resistance ratings (if known) can be found here.
2) Do not spray wheat that was planted with an insecticidal seed treatment. A NCSU master’s student sprayed wheat treated with an insecticidal seed treatment twice a month during the fall in multiple locations during 2014 and 2015. The seed treatment alone was enough to hold the flies back through the fall.
3) Check your earliest planted wheat for Hessian fly larvae and pupae. A good way to do this is do take a shovel and carefully dig up plants. Keep all the soil on the shovel and dissect the plants. Focus in the soil on dead tillers that may have rotted; pupae and larvae may be harboring in these dead tillers that are not apparent on the surface.
4) See if you have eggs. A photo of an egg is provided in this earlier article. There is no threshold for spraying eggs, although we are working on developing one. I get worried when there is at least one egg per tiller, but that doesn’t necessarily mean this will turn into anything or mandate a spray.
5) Spray as soon as you can if there are eggs, but not right before a rain. We think that the insecticide works best if you can get some residual to kill adults. It is possible that the insecticide can kill larvae when they hatch from the eggs and move down the leaf to feed below the soil surface. A spray targeted when eggs first show up should work the best if these assumptions are true. See this article for an efficacy trial.
6) Consider other pests and beneficials that might be in your field. If you have aphids in the field, you might get more bang for your buck on a spray by eliminating this insect. Fall infestations of aphids can be associated with barley yellow dwarf virus in the spring. Also remember that if you end up spraying, you will knock out any beneficials that might help you in the winter hold back things like aphids, winter grain mite, or even Hessian fly.