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Management decisions impact wheat stem sawfly populations

Management decisions impact wheat stem sawfly populations
An adult wheat stem sawfly on a smooth brome leaf within a tree-row windbreak. Courtesy Photo

As we study steps farmers can take to decrease wheat stem sawfly pressure in winter wheat, two options seem to successfully provide a level of control for sawfly in the Nebraska Panhandle ― spring tillage of wheat fallow and taking steps to conserve grasslands adjacent to wheat fields.

By choosing management techniques that target specific life stages of the wheat stem sawfly (WSS), wheat farmers may be able to reduce sawfly populations by simply managing the soil and grasses within which they live.

The wheat stem sawfly is a native, stem-mining insect pest of economic importance in the Great Plains. During the post-World-War-II “Green Revolution” in the Northern Great Plains, the native WSS 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 had adapted an earlier emergence period (of approximately three weeks), allowing it to successfully infest earlier-maturing winter wheat crops and expand into the High Plains of Nebraska.

Over the last decade, these WSS 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. As the adult WSS’s emerge 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 approximately one 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.

WSS infestations impact wheat both through stem lodging and decreased wheat yields.

During the sawfly’s lifecycle, there are two windows of opportunity when management decisions may significantly reduce WSS pressure. The first management practice impacts the overwintering WSS pupal stage with a spring disk operation in wheat fallow.

In dryland wheat, no-till farming practices help conserve soil moisture and nutrients; however, they unfortunately can also help conserve sawfly pupae. Tilling dryland fallow increases the risk of depleting soil and soil moisture. This risk can be greater with fall tillage (depending on soil type).

One benefit to tilling for sawfly control in the spring (compared to fall), is that generally wetter, clumpier soils in the spring may allow for less aggressive tillage practices to have a greater reduction of sawfly survival. Our on-farm research with 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 WSS populations up to 80%.

The second management practice impacts the surrounding grassland landscapes, rather than the wheat field. Present-day sawfly populations in Nebraska still retain their ability to infest native and nonnative grasses, such as smooth brome, intermediate wheatgrass, and western wheatgrass, but with much lower survival than in wheat.

This is because due to successful breeding, wheat stands have little variation in stem height and width from one tiller to the next. However, non-cultivated grasses like those above 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 due to their narrower inner stem or thicker stem wall.

The WSS larva trapped within feral grasses may create an ideal condition for the WSS’s natural enemies: two species of braconid parasitoid wasps. Bracon cephi and Bracon lissogaster attack the WSS’s larval stage living within the stem. Female parasitoids locate WSS larva within a stem, stick their ovipositor inside, and paralyze the WSS larva. Then they insert their own egg(s) into the stem to feed upon the paralyzed WSS 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 several grasses are able to host this unique dynamic between the WSS and its natural enemies, including smooth brome, intermediate wheatgrass, and western wheatgrass. Grasslands, tree-row windbreaks, ditches, CRP ground, unfarmable areas surrounding power poles, etc. may all 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.

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