Parasitism of the Wheat Stem Sawfly (Hymenoptera: Cephidae) by <I>Bracon cephi</I> and <I>B. lissogaster</I> (Hymenoptera: Braconidae) in Wheat Fields Bordering Tilled and Untilled Fallow in Montana

Abstract

We evaluated wheat stem sawfly, Cephus cinctus Norton, parasitism, infestation, and sawfly-cut stems in wheat fields bordering intensely tilled (no visible stubble residue), minimally tilled (>75% stubble residue visible), and untilled (chemical fallow, herbicide fallow management) summer fallow fields in north-central and south-central Montana. No difference in sawfly parasitism or sawfly-cut stems was found between fields bordering minimally tilled and fields bordering untilled summer fallow. Sawfly parasitism in fields bordering untilled summer fallow was greater than in fields bordering intensely tilled summer fallow at six of the eight sites examined. Sawfly-cut stems were greater in the field bordering intensely tilled fallow at four sites, with no difference in sawfly-cut stems between the intensely tilled and untilled field at the other four sites. Although it has never been reported, we have observed that many sawfly stubs are completely buried. Therefore, we measured the depth of sawfly stubs in four untilled fields in Broadwater County, MT. Two-thirds of the stubs were completely buried (206 of 300) with an average depth of 6 mm. Intensive tillage, which results in soil-covered stubble, is not an effective sawfly control practice, because sawflies typically overwinter below ground and upon emergence must dig to reach the soil surface. However, Bracon cephi (Gahan) and Bracon lissogaster Muesebeck overwinter above ground in stems and might be unable to dig to the soil surface if buried. The elimination of intensive tillage in favor of chemical fallow should result in greater sawfly parasitism over time. Producers replacing minimal tillage with chemical fallow should see no effect on sawfly parasitism.

Cultivar Preferences of Ovipositing Wheat Stem Sawflies as Influenced by the Amount of Volatile Attractant

“…Wheat stem sawßy infestation was determined by the presence of eggs, larvae, or frass resulting from the feeding of larvae within the stem, according to Runyon et al (2002). The number of immatures and percentage of infested stems per sample were recorded and compared between cultivars.…”

“…In addition, insecticides are not effective because all immature stages are within the plant and adult ßight periods are often Ͼ1 mo. Recent trends in wheat cultivation on the northern Great Plains include increasing Þeld size and greater adoption of no-till practices, which considerably alter aspects of wheat stem sawßy management (Runyon et al 2002, Weaver et al 2004. For example, planting larger Þelds and reducing tillage promotes a marked concentration of the wheat stem sawßy to the periphery of the crop (Nansen et al 2005a, Weaver et al 2005 and further enhances the "edge effect" that has long been reported as a characteristic of infestations (Criddle 1917).…”

“…However, although lodging caused by stem cutting by cryptic wheat stem sawßy larvae does provide a measure of relative host plant susceptibility, it is not a direct measure of the attractiveness of the cultivars. For example, larval mortality within the stems, primarily caused by parasitism, prevents stem lodging (Runyon et al 2002.…”

Oviposition Behavior of the Wheat Stem Sawfly When Encountering Plants Infested With Cryptic Conspecifics

“…High population densities are common in agroecosystems, where infestations by wheat stem sawßy can reach 80% of the available hosts (Runyon et al 2002). Females able to recognize previously visited stems would have greater Þtness.…”

Increased area of a highly suitable host crop increases herbivore pressure in intensified agricultural landscapes

“…In wheat crops, these two species represent the only important insect natural enemies of C. cinctus, with B. cephi generally dominating (Runyon et al, 2002). These beneficial enemies can reach high densities and reduce economic damage by C. cinctus (Morrill et al, 1998;Buteler et al, 2008), but populations are highly variable across wheat fields and regions (Shanower and Waters, 2006) and causes of their population fluctuations are not well understood.…”