An Update on Status of Wheat Stem Sawfly in Nebraska

An Update on Status of Wheat Stem Sawfly in Nebraska

Wheat Fields

June 24, 2015

By now, many wheat growers in Nebraska are familiar with the wheat stem sawfly. Within the past couple weeks, we saw peak emergence of wheat stem sawfly adults throughout much of the wheat production areas of Nebraska.

Wheat stem sawfly damage and adult sawfly
Figure 2. Wheat stem sawfly damage (L) and adult sawfly.

The Panhandle Research and Extension Center (PREC) Entomology Lab has conducted a survey of wheat stem sawfly infestation in Nebraska since 2011 (Table 1). Our results indicate an infestation that continues to grow in both intensity and geographic range. Understanding how this population moves through the landscape over time is an important component in helping biologists understand how any invasion establishes and then to develop appropriate management strategies. To date, stem sawfly populations have been very sparse in southwestern Nebraska and eastward. In 2015 the Panhandle REC will be joining forces with the West Central Research and Extension Center (WCREC) Agroecosystems Entomology Lab in North Platte to ensure better survey coverage throughout all of the major wheat producing areas of Nebraska.

A few wheat producers contacted UNL entomologists this spring to inquire about how this cool wet spring was going to impact sawfly populations. The short answer was "we don't know." However, because of survey efforts this year, we will be able to compare this year with previous years to determine that impact, if any. Based on work by a graduate student developing an adult stem sawfly sampling plan, our current sawfly populations were not affected by this year's cool and wet conditions. A recent sample revealed as many as 300 stem sawfly adults in 20 sweeps. 

Along with these surveys, we have also collected samples of other grass species that were adjacent to wheat fields to evaluate them for the presence of stem sawfly larvae (Table 2). We could find stem sawfly larvae present in downy brome, intermediate wheatgrass, Japanese brome, smooth brome, and rye. Since the wheat stem sawfly is a native to North America, it isn't too surprising to find it infesting other hollow-stemmed grasses. Obviously, grasses such as downy brome are problematic in their own right in our wheat production systems.

Researching Management Options

Of course, we aren't idly watching this insect spread across the landscape, but are researching solutions. In collaboration with the UNL Wheat Breeding Program, we continue to evaluate wheat varieties for resistance to wheat stem sawfly (Table 3). In last year's State Variety Trials, we saw some solid-stemmed wheats (e.g., Warhorse) produce decent yield along with a high level of sawfly resistance (based on larval infestation counts). We are also recording higher levels of one stem sawfly parasitoid, Bracon cephi, with as many as 400 collected last year at one field near Hemingford. Unfortunately, based on the stem sawfly emergence this year, parasitoid populations have yet to catch up with stem sawfly populations or are not yet seasonally adapted to this area. As we learn more about these parasitoids and what management options may best improve their habitat, we will be sure to keep you informed. The WCREC Agroecosystems Entomology Lab has been studying how conservation practices, such as cover crops and wildflower strips, can impact beneficial insects, such as parasitoids and predators. These practices could play an important role in suppression of pests like the wheat stem sawfly.

Jeff Bradshaw
Extension Entomologist, Panhandle REC
Julie Peterson
Extension Entomologist, West Central REC

Table 1. Mean percent of stem sawfly-infested tillers out of a random subsample of 100 headed tillers of wheat. Numbers in parentheses equal the number of wheat fields associated with a given mean for four years (2011-2014) of the Nebraska Wheat Stem Sawfly Survey.
State County 2011 2012 2013 2014
Colorado Logan 0 (1) 1 (1) 3 (1)
  Sedgewick 0 (1) 0 (1)
Nebraska Banner 34.9 (7) 53 (6) 52.3 (3) 87 (1)
  Box Butte 14.2 (6) 36.8 (4) 72.3 (4) 95 (1)
  Chase 0 (1)
  Cheyenne 11 (4) 49 (1) 62 (1) 77 (1)
  Dawes 30 (1) 30 (1) 55 (1)
  Deuel 0 (1)
  Franklin 0 (2) 0 (2) 0 (1)
  Garden 1 (1) 1 (1) 0 (1) 6 (1)
  Gosper 0 (2) 0 (2) 0 (2) 0 (2)
  Harlan 0 (1)
  Kearney 0 (1)
  Kimball 7 (1)
  Morrill 20.5 (2) 27 (2) 88.5 (2) 73 (1)
  Perkins 0 (1)
  Scotts Bluff 58 (3) 55.8 (4) 83 (1)
  Sheridan 0 (2) 0.7 (3) 10.5 (2) 5 (1)
  Sioux 2 (1) 0 (1)
Wyoming Laramie 32.5 (2) 47.5 (2) 84 (2)

 

Table 2. Percent wheat stem sawfly-infested stems of various grass species from select western Nebraska locations.
Grass Variety County Site Description Number of Stems Collected % Infested
Downy brome Banner Edge, infested wheat 100 43
Downy brome Cheyenne Edge, infested wheat 50 38
Downy brome Dawes Adjacent, wheat 30 0
Downy brome Scotts Bluff Edge, infested wheat 100 2
Intermediate wheatgrass Scotts Bluff CRP, near infested wheat 100 57
Japanese brome Cheyenne Edge, infested wheat 17 23.5
Smooth brome Banner Road intersection corner 15 60
Smooth brome Banner Edge, infested wheat 75 70.7
Smooth brome Dawes Edge, wheat 7 0
Smooth brome Morrill CRP, distant from wheat 35 14.3
Smooth brome Scotts Bluff CRP, near infested wheat 125 23.2
Rye Scotts Bluff Infested wheat field 50 4

 

Table 3.  State Variety Trial wheat stem sawfly infestation rates and yield of select winter wheat varieties.
Variety Infested Tillers (%)
(Sample of 150)
Live Larvae
(%)

 
Grain Yield
(bu/ac)

Bushel Weight
(lb/bu)

Freeman 63.3 29.3 74 59
Robidoux 67.3 36.7 72 58
Warhorse 28.7 9.3 72 59
NEO9521 65.3 38.0 72 61
Overland 86.7 60.7 71 62
Bearpaw* 38.7 18.0 69 62
Hatcher 78.7 52.7 66 59
Goodstreak 58.7 36.0 66 59
Judee* 62.7 26.0 63 60
Pronghorn 55.3 36.7 57 61
Turkey 60.7 33.3 53 62
* These varieties are commonly referred to as "solid stemmed" varieties.