Wheat stem sawfly (Cephus cinctus) has been a pest of concern in winter wheat in Montana, Colorado, North Dakota, and Nebraska for a few years now, and it is causing severe losses to wheat producers in these states. We are beginning to measure their impact on wheat production in South Dakota as well. In a research project funded by the South Dakota Wheat Commission, we are testing the efficacy of several winter and spring wheat varieties with wheat stem sawfly resistance traits. Next year we will also examine how effective oats and barley are as a trap crop for wheat stem sawfly. Oats are particularly good as a trap crop because females of the wheat stem sawfly are preferentially attracted to oats as an egg-laying host but larvae can’t complete development in oats. Thus a perimeter of oats planted around wheat can serve as an extremely effective trap for the wasps and protect the wheat crop.
Wheat stem sawflies are wasps that belong to a primitive wasp family Cephidae. They are native to the Great Plains and often use wild grasses as hosts but lately these insects have exploited wheat as a host plant to a greater extent than ever before. Historically, these insects were primarily pests of spring wheat, but they are increasingly found to attack winter wheat as well.
Wheat stem sawflies are not very good fliers and the female choses the closest suitable host to lay her eggs. For this reason, the edges of wheat fields tend to suffer greater damage from this pest. Eggs are inserted below the uppermost node, and larger stems are preferred to the more slender stems. The female may lay more than one egg per stem but due to cannibalism only one larva will complete development inside each stem.
Larvae are light in color without easily discernable markings and have chewing mouthparts that sclerotize shortly after hatching. They assume a characteristic ‘S’ shape when disturbed or removed from the stems. Immature sawflies develop for about 30 days, feeding up and down the stem, and reach up to half an inch when they finish their development. Larvae then move down toward the soil line, cut a V-shaped notch around the stem, and continue moving down the stem toward the crown. The stem is then plugged up with their frass below the notch. Stems often break at the weak points created by larval notching, thus creating a stub that the larvae overwinter in before pupating the following spring.
The majority of damage to wheat comes from cutting of the stems and lodging of wheat just before harvest. Lodged wheat cannot be harvested, although it can be swathed to recover some of the crop. Wheat stem sawflies do not always cause lodging of wheat but larvae also affect the crop directly through their feeding inside wheat stems, primarily by reducing seed weight. Larval feeding alone is estimated to reduce the yield by about 10-20% in infested wheat that is harvested.
Once wheat stem sawfly infests wheat, there are no rescue treatments available. Pre-planting practices and using resistant varieties of wheat are the only available management options. It is generally advised to implement one of the preventative strategies if infestation levels were around 15% the previous year. Cutting the stem and inspecting it for larvae is the most accurate method of assessing infestation levels. Rotating to another crop or planting a trap crop (e.g., barley, rye or oats) along the edges of wheat fields will reduce their populations. Wheat stem sawflies will lay their eggs in these trap crops but larvae will not survive to maturity. Using a trap crop may be especially effective in low to moderate infestations; high wheat stem sawfly populations will likely cause significant damage to wheat despite the trap crop.
We will continue monitoring populations of this pest in South Dakota and establish the effectiveness of resistant varieties of wheat. If you suspect wheat stem sawfly infestations or have wheat lodging shortly before harvest, please contact Ada Szczepaniec at 605.688.6854.
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