Hessian flies were found recently on a field of wheat in southeast Kansas. This was on a field of Everest, a variety with a rating of “3” for Hessian fly on a scale of 1-9, with 1 being most resistant and 9 being most susceptible. The long open fall in 2014 may have led to an extended period of adult Hessian fly activity and possible multiple broods. Even varieties with resistance to Hessian fly can occasionally become infested if the insect pressure is high enough, and we are now finding that high temperatures my negatively affect some of the resistance genes.
The Hessian flies seen now would have infested the wheat in the fall. Hessian flies may emerge on warm fall days, usually in September or October. The longer these conditions persist in the fall, the longer Hessian fly activity will continue – even into November at times in Kansas.
Adults lay their eggs in the fall, usually on the upper surface of wheat leaves. After the eggs hatch, about three to 10 days later, the larvae (maggots) will crawl down the plant and begin feeding just above the crown between leaf sheaths and stem. The larvae will feed for up to 30 days, and will finish before cold weather. After feeding, the larvae will form hard, mahogany-colored capsules called “flaxseeds,” in which they overwinter. These are the puparia. There may be more than one such cycle in the fall, depending on how warm the weather is and how early the first cycle gets started.
A similar cycle occurs in the spring. Adults will emerge after the weather warms up, anytime from early March through April. The eggs will be deposited on wheat in the same or nearby fields, and flaxseed can later be found on oversummering wheat stubble. More than one such generation can also occur in the spring.
Fall infestations in wheat can be difficult to detect. Shoots will be stunted if infested, and plants may even die. There will be undeveloped central shoots with a broad, thickened and darker green leaf.
Spring infestations can also be hard to detect until harvest. Tissue at the base on an infested plant will stop growing while the remaining, uninfested plant tissue will continue growing. This weakens the stem, leading to partially filled heads or stem breakage at the point of feeding.