Grower-based selection of varieties and systems for wheat stem sawfly control

Grower-based selection of varieties and systems for wheat stem sawfly control

Summary

This project exploits two characteristics of certain wheat varieties to develop a trap crop system to manage the wheat stem sawfly. First, solid-stemmed varieties cause a high level of insect mortality. Second, some varieties are not preferred by the sawfly for egg-laying. We established two field-scale trials whereby an attractive solid-stemmed wheat variety was used as a trap crop alongside varieties that varied in their ability to attract the female sawfly. The pairing of the attractive solid-stemmed variety with an unattractive hollow-stemmed variety provided excellent sawfly control, as female sawfly laid eggs primarily in the solid-stemmed variety.

Objectives/Performance Targets

1) Assess the potential of solid-stemmed Choteau spring wheat as a trap crop for control of wheat stem sawfly.
2) Select solid-stem lines from a Reeder/Choteau cross that have high end-use quality and vary in volatile production and stem solidness.
3) Determine the relative ability of the solid-stemmed Choteau/Reeder lines to trap wheat stem sawfly.
4) Provide meaningful outreach to growers regarding sawfly control in general and use of trap varieties specifically.

Accomplishments/Milestones

1) Assess the potential of solid-stemmed Choteau spring wheat as a trap crop for control of wheat stem sawfly.
2) Select solid-stem lines from a Reeder/Choteau cross that have high end-use quality and vary in volatile production and stem solidness.
3) Determine the relative ability of the solid-stemmed Choteau/Reeder lines to trap wheat stem sawfly.
4) Provide meaningful outreach to growers regarding sawfly control in general and use of trap varieties specifically.

Accomplishments/Milestones

Objective 1 is the primary focus of this proposal. In cooperation with producers in two sawfly-infested areas of state, we established two trials as indicated in the following diagram. Key varieties were Choteau (solid-stemmed), Reeder (hollow-stemmed and very attractive to female sawfly for egg-laying), and Conan (hollow-stemmed and less attractive to the female sawfly for egg-laying). Data were obtained throughout the season on sawfly infestation by collecting stem samples from four 1 meter sections in each plot in each replication.

Table 1. Experimental layout for one replication of the trap crop experiment. Each experiment had three replications. The producers planted the experiments using their own equipment. Size of each experiment ranged from twenty to forty acres.
Choteau Choteau
Reeder Conan
Reeder Conan
Reeder Conan
Reeder Conan
Choteau Choteau

The trial at Conrad proceeded as intended. A mistake was made in the planting of the trial in Havre, as the grower’s planter malfunctioned resulting in a mix of Conan and Reeder in each of the replications. While not what we intended, this trial provided interesting data. The two trials are summarized separately below.

Conrad Trial

Four 1 m sections were sampled from the trap crop border and the hollow-stemmed crop in each replication. We are in the process of splitting stems to determine the amount of sawfly infestation in each stem. Table 1 shows results from stem sampling on July 1. On this date, 82% of Choteau stems were infested when paired with Reeder, while approximately 60% of Reeder stems were infested. The average number of immature larvae was greater than one per stem for both Choteau and Reeder. Thus, Choteau was not an effective trap for sawfly when paired with Reeder. Conversely, over 80% of Choteau stems were infested when it was paired with Conan, as opposed to 11% of Conan stems. There was only 0.1 larva per stem in the Conan plots, while Choteau had 1.4 larvae per stem. In summary, our data indicate that the solid-stemmed variety Choteau was an excellent trap crop when planted alongside a variety not preferred by the sawfly for laying eggs. We are currently processing stems collected at later dates to determine the degree of sawfly mortality in the Choteau trap crop.

Table 2. Sawfly infestation in Reeder, Conan and Choteau based on JULY 1 sampling date in a trap crop experiment in Conrad Montana.

This table shows that within the variety pair Reeder/Choteau, there were 1.1 and 1.5 larvae per stem in Reeder and Choteau, respectively. Within the variety pair Conan/Choteau, there were 0.1 and 1.5 larvae per stem in Conan and Choteau, respectively.

Havre Trial

This trial was designed to be the same layout as the Conrad trial. However, the grain drill used by the grower malfunctioned, and instead of planting separate blocks of Conan and Reeder, the two varieties were mixed. This provided an opportunity to test a question posed by a grower – that is, in a mixture of attractive and unattractive varieties, would the sawfly have the ability to distinguish individual stems? If so, this may suggest that mixing an attractive solid-stemmed line and an unattractive hollow-stemmed line would provide the same advantage as the trap strips. Fortuitously, Reeder and Conan stems could be distinguished in the field, as Reeder is dark green while Conan is blue-green. We sampled 1 m sections of each replication to determine sawfly infestation (Table 2). The basic finding from this data is that the female sawfly were able to distinguish among the varieties in this inter-seeded scenario, suggesting that a mixture of a solid-stemmed attractive variety and a hollow-stemmed unattractive one may be a viable management option for growers.

Table 3. Percent wheat stem sawfly infestation of Conan and Reeder stems grown in inter-seeded blocks during the 2008 field experiment.

This table shows that 41.7 and 65 percent of stems were infested in Reeder at Zadoks 32 and 49, respectively. A total of 9.2 and 8.6 percent of stems were infested in Conan at these stages.

Objectives 2 and 3 relate to the need to optimize the system by providing 1) a maximally attractive solid-stemmed variety, and 2) a hollow-stemmed variety that matches the developmental profile of solid-stemmed trap variety. We developed lines from a cross between Reeder (highly attractive, hollow-stemmed) and Choteau (solid-stemmed) to identify the required genotypes. The lines were planted in a replicated experiment in a sawfly-infested site near Havre MT, along with Reeder, Choteau and Conan. The most important finding from this study was that Choteau is as attractive as Reeder to the sawfly, with over 45% infestation in both varieties. Infestation in Conan was around 15%, and none of the lines from the Reeder/Choteau cross were as unattractive to the sawfly as was Conan. Thus, this work showed us that Conan is unique among wheat varieties, and that Choteau should function as a very acceptable trap crop.

To obtain unattractive solid-stemmed lines, Conan will need to be used as a parent. We will plant several hundred Conan/Choteau lines in sawfly-infested site in 2009 to select lines with combinations of solid stems and sawfly attractiveness.

Objective 4 related to the need for outreach and education, which is critical to the eventual success of our research. The PIs, especially Talbert and Weaver, have given a dozen talks to growers around the state regarding the research. Additionally, we have widely distributed a flier made to detail the Western SARE experiments (attached). We have engaged in discussions with private breeding firms to better exploit the potential for the trap crop method. Outreach is vital to this project as, in the end, growers will need to shift from their current practice of planting a single variety. Data from this project will be crucial for testing the benefits of altering this approach by planting solid-stemmed trap varieties alongside their high-yielding hollow-stemmed favorites.

Plans for 2009

We have made arrangements with the same two producers to repeat the trap crop experiments in 2009. Design will be the same as that of 2008. In addition, we are planting trials containing progeny lines from a Conan/Choteau cross to identify lines that vary in attractiveness and solidness to optimize varieties available for the trap crop system. Finally, we are scheduled to give several talks to growers impacted by the sawfly. A key part of the talks will relate to the Western SARE project to induce growers to test the trap crop idea on their own farms.

Plans for 2010

The 2010 planting season will be an important one. We will have obtained three site/years of data regarding the trap crop approach to managing sawfly. Other grower-initiated questions have arisen, and more will no doubt be raised. Thus, in 2010, we plan to work with the same growers to address the potential of inter-seeded mixtures as an alternative to trap strips. Additionally, there are questions as to which hollow-stemmed varieties will work in this system. We will plant a trial with the leading candidate for varieties to provide guidance to growers.

Impacts and Contributions/Outcomes

We have completed the first year of a three-year project. Year-one results are very encouraging regarding the potential use of solid-stemmed trap crops as a companion to hollow-stemmed, unattractive varieties. The wheat stem sawfly was very destructive in 2008, and growers are searching for better ways to manage the problem. A few conclusions can be drawn from work to date.

1) The approach of planting a solid-stemmed, attractive variety as a trap crop beside a hollow-stemmed less attractive appears to be a viable approach for controlling the sawfly. This needs to be confirmed in 2009 plantings.

2) Growers will occasionally have trouble adhering to statistically sound experimental methods. All care needs to be taken to have a very straightforward design. Repetition over years and sites is critical.

3) Choteau is a very attractive variety. Coupled with its solid stems, it has the characteristics needed for a good trap crop.

4) Conan has unique characteristics that give it the characteristic of being unattractive to the wheat stem sawfly. A second variety, named Corbin, has been developed by the same company and may show a similar lack of attraction for the sawfly. This will be explored further.

5) Several talks have been given regarding the Western SARE project. Assuming similar results in 2009, the PIs will be confident to suggest the trap crop approach to our growers. Choteau is currently grown on over 500,000 acres in Montana, with Conan and Corbin occupying approximately 300,000. There is potential to replace much of this acreage using either seed mixtures or with strips of Choteau planted to protect Corbin or Conan.

6) Based on our first year’s data, we are optimistic that these experiments will result in economic gain for the Montana wheat industry.

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