By Bethany Thomas and Jeff Bradshaw
The wheat stem sawfly is a native stem-mining insect pest of economic importance within the Great Plains. During the agricultural revolution of the late 1900s in the northern Great Plains, the native wheat stem sawfly transitioned from infesting only large, hollow-stemmed wheatgrasses to infesting the introduced cultivated spring wheat fields.
By the end of that century, independent sawfly populations adapted an earlier emergence period (of about three weeks), allowing it to successfully infest earlier-maturing winter wheat crops and expand into the High Plains of Nebraska.
Over the past decade, these sawfly populations have increased dramatically in the Nebraska Panhandle, with the highest recorded populations in wheat fields in Box Butte and Cheyenne counties.
In Nebraska, the sawfly's lifecycle begins with adult emergence, peaking at the end of May in the southern Panhandle and the beginning of June in the northern Panhandle. During the adult sawfly's emergence from wheat stubble, female sawflies stick their ovipositor inside large hollow-stem wheat tillers and insert a single egg in each stem.
After a week, the larva hatches from the egg and feeds on the inside of the stem for about a month. Once the host plant begins to desiccate, the sawfly bores downward to the base of the stem above the crown, cuts a notch in the stem, and fills the top of the stubble with a frass plug.
This is where the pupal sawfly hibernates until emergence next spring. Wheat stem sawfly infestations affect wheat through both stem lodging and decreased wheat yields.
Windows of opportunity
During the sawfly's lifecycle, there are two windows of opportunity when management decisions may significantly reduce sawfly pressure. The first management practice affects the overwintering wheat stem sawfly pupal stage with a spring disk operation in wheat fallow.
Dryland wheat under no-till practices helps conserve soil moisture and nutrients. However, this practice, unfortunately, also can help conserve sawfly pupae. When tillage is applied to dryland fallow, one risk is the depletion of soil and soil moisture. This risk can be greater with fall tillage (depending on soil type).
One benefit to spring tillage, for sawfly control, would be the generally wetter, clumpier soils in the spring may allow for less aggressive tillage practices to have a greater reduction of sawfly survival. The on-farm research treatment using a tandem disk has shown significant reduction of sawfly survival on dryland fallow.
It is likely that simply turning the stubble "upside down" within the dirt clods is sufficient for preventing sawfly emergence. A single pass using a tandem disk in late April to early May could help reduce wheat stem sawfly populations up to 80%.
Home for natural predators
The second management practice affects the surrounding grassland landscapes, rather than the wheat field. Present-day sawfly populations in Nebraska still retain their ability to infest native and non-native grasses, such as smooth brome, intermediate wheatgrass and western wheatgrass, but with much lower survival.
Because of successful breeding, wheat stands have little variation between wheat tillers, regarding stem height and width. However, noncultivated grasses still retain much of their original genetic variability and are more susceptible to fluctuations in temperature, precipitation and soil type. Sawflies that infest these feral grasses have a lower survivability because of their narrower inner stem or thicker stem wall.
The sawfly larva trapped within feral grasses may create an ideal condition for the sawfly's natural enemies: two species of braconid parasitoids. Bracon cephi and Bracon lissogaster attack the sawfly's larval stage living within the stem. The parasitoids have two complete life cycles in the summer, emerging one to three weeks behind peak sawfly densities and about one month later.
Female parasitoids locate sawfly larva within a stem, stick their ovipositor inside, and paralyze the sawfly larva. Then they insert their own egg into the stem to feed upon the paralyzed sawfly larva. The parasitoid may complete development and form a silken cocoon within the stem, either emerging in the summer or overwintering inside the stem to emerge the following summer.
From our studies, grasses able to host this unique dynamic between the wheat stem sawfly and its natural enemies include smooth brome, intermediate wheatgrass and western wheatgrass.
Grasslands, tree-row windbreaks, ditches, Conservation Reserve Program acres, unfarmable areas surrounding power poles, etc., all may be great habitats for the conservation of these parasitoid populations.
In order to increase the likelihood of successful parasitoid conservation, growers may consider reducing mowing and not disturbing grassy areas with smooth brome and wheatgrasses, as their presence may be vital for the health of wheat.
As we continue to study management options that can decrease sawfly pressure in winter wheat, two options seem to successfully provide a level of control for wheat stem sawfly in the Nebraska Panhandle ― spring tillage of wheat fallow and the conservation of surrounding grasslands.
By choosing management techniques that target specific sawfly life stages, we may be able to affect sawfly populations by managing the soil and grasses within which they live.
Thomas is entomology graduate research assistant at the University of Nebraska-Lincoln. Bradshaw is a Nebraska Extension entomologist.