Recommendations for Treating Hessian Fly in the Spring
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Scattered reports are coming in about Hessian fly infestations in North Carolina wheat. Usually this occurs after a period of cold weather when growth has slowed, followed by a warm period when wheat should be growing and folks are making decisions about topdressing with nitrogen. Wheat fields that are heavily infested with Hessian fly are often described as going backwards. Fields that should be growing are not because tillers feed on by Hessian fly larvae have died. If you want to skip to the conclusion of this article, I do not recommend spring foliar insecticide sprays for Hessian fly because they are so difficult to time. For the rationale behind this, you can read on.
The best management methods for Hessian fly infestations are pre-plant decisions. The #1 Hessian fly management method is to plant a resistant variety. The #2 Hessian fly management method is to use a full rate of an insecticidal seed treatment. After this, things like conventional tillage, avoiding planting in or near last year’s soybeans double cropped with wheat, and planting in the recommended window can also help.
If you’ve got Hessian fly larvae or pupae now, realize that most of the damage is already done. These larvae have destroyed tillers that would have greatly contributed to yield. This begs the question, can you treat with a foliar insecticide in the spring as a rescue treatment to combat infestations initiated in the fall?
When the weather is warm and the pupae have sufficiently developed, Hessian fly adults will become active, mate, and begin laying eggs. This can happen anytime we have an extended warm spell during the winter or spring. Spring infestations can result in larval infestations that can cause heads to lodge. Furthermore, larvae and pupae can be carried up the stem that bears the head during elongation (see photo). This can cause the stems with the wheat heads to snap in addition to the loss caused by larval feeding.
Foliar spray target adult Hessian flies to prevent further egg laying. In order to properly time a spring spray, you will have to time it when adults are active. This can be very difficult to do for a couple of reasons. First, fields often have immature Hessian flies with developmental variability. I often dig up tillers and find larvae of different sizes alongside pupae, especially in the winter. In this situation, sprays might kill some adults and knock back egg laying, but other adults could emerge after the spray residual wore off (expect around a week residual in the absence of rain) and continue laying eggs. Indeed, this was the case in a 2013. We successfully decreased egg laying at seven days after treatment (see figure below), but Hessian fly adults moved in behind the spray, laid eggs, and erased any effect we had knocking back adults the previous week.
Secondly, it is really difficult to determine when pupae are going to emerge into adults. Folks have noted that when you squish young pupae they are white. When you squish older pupae they come out pink, since the developing fly begins to take on some color prior to emergence. We tried to take advantage of this in a trial, initiating a foliar spray when 50% of the pupae squished pink. Despite this, we saw no significant effect of a spray.
For these two reasons, I do not recommend spring foliar insecticide sprays in wheat for Hessian fly. If you want more extensive coverage of all our recommendations and fly biology, they can be found in Biology and Management of Hessian Fly in the Southeast, the Small Grain Production Guide, or this video.