Parasitism of the Wheat Stem Sawfly
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BIOLOGICAL AND MICROBIAL CONTROL Parasitism of the Wheat Stem Sawfly (Hymenoptera: Cephidae) by Bracon cephi and B. lissogaster (Hymenoptera: Braconidae) in Wheat Fields Bordering Tilled and Untilled Fallow in Montana 1, 2 1 1 3 J. B. RUNYON, W. L. MORRILL, D. K. WEAVER, AND P. R. MILLER J. Econ. Entomol. 95(6): 1130Ð1134(2002) ABSTRACT We evaluated wheat stem sawßy, Cephus cinctus Norton, parasitism, infestation, and sawßy-cut stems in wheat Þelds bordering intensely tilled (no visible stubble residue), minimally tilled (Ͼ75% stubble residue visible), and untilled (chemical fallow, herbicide fallow management) summer fallow Þelds in north-central and south-central Montana. No difference in sawßy parasitism or sawßy-cut stems was found between Þelds bordering minimally tilled and Þelds bordering untilled summer fallow. Sawßy parasitism in Þelds bordering untilled summer fallow was greater than in Þelds bordering intensely tilled summer fallow at six of the eight sites examined. Sawßy-cut stems were greater in the Þeld bordering intensely tilled fallow at four sites, with no difference in sawßy-cut stems between the intensely tilled and untilled Þeld at the other four sites. Although it has never been reported, we have observed that many sawßy stubs are completely buried. Therefore, we measured the depth of sawßy stubs in four untilled Þelds 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 sawßy control practice, because sawßies typically over- winter 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 sawßy parasitism over time. Producers replacing minimal tillage with chemical fallow should see no effect on sawßy parasitism. KEY WORDS Wheat stem sawßy, Cephus cinctus, Bracon cephi, Bracon lissogaster, tillage, chemical fallow IN THE NORTHERN Great Plains, the wheat stem sawßy, difÞcult to control because eggs, larvae, and pupae are Cephus cinctus Norton, is an economically important enclosed within host plants. Current management insect pest of wheat, Triticum aestivum L. (Weiss and practices do not provide adequate levels of control Morrill 1992, Morrill et al. 2001). Sawßy-infested (Morrill et al. 2001). plants have reduced yields and usually lodge, reducing Bracon cephi (Gahan) and Bracon lissogaster Muese- the amount of grain that can be harvested (Ainslie beck are the only wheat stem sawßy parasitoids that 1920, Platt and Farstad 1946, Holmes 1977). In Mon- commonly occur in wheat. Levels of parasitism vary tana alone, annual losses caused by sawßy infestations greatly among Þelds (Morrill 1997, 1998). The life have been estimated to exceed $25 million (Montana histories of these idiobiont ectoparasitoids are similar. State University Extension Service 1997). B. cephi and B. lissogaster have two generations per Female sawßies deposit their eggs in the developing year. Once a host has been located, females insert their stems of wheat, and larvae feed and complete devel- ovipositor into the stem, paralyze the sawßy larva, and opment within the stems. Mature larvae chew a notch deposit one to four eggs (Nelson and Farstad 1953, around the inside perimeter of the stems near ground Holmes et al. 1963). Larvae feed on the surface of the level and overwinter in the underground region of the host and consume the sawßy larvae through minute stem below this notch. Stems usually break at this lacerations made with the mandibles (Somsen and notch (called “sawßy-cutting”), leaving a “stub” that Luginbill 1956). Mature larvae spin a cylindrical co- serves as an overwintering chamber (Ainslie 1920, coon that is attached lengthwise to the inside of the Holmes 1954, Weiss and Morrill 1992). Sawßies are stem at each end by a disc-like plate. Larvae pupate in the spring, and adults emerge by chewing a circular 1 Department of Entomology, Montana State University, Bozeman, hole in the stem wall (Nelson and Farstad 1953). MT 59717. 2 E-mail: [email protected]. Alternate-year summer fallow-wheat production is 3 Department of Land Resources & Environmental Sciences, Mon- a typifying feature of dryland farming in Montana tana State University, Bozeman, MT 59717. (Willis et al. 1983, Troeh et al. 1999). This cropÐfallow 0022-0493/02/1130Ð1134$02.00/0 ᭧ 2002 Entomological Society of America December 2002 RUNYON ET AL.: TILLAGE IMPACTS WHEAT STEM SAWFLY PARASITISM 1131 Table 1. Research site locations in Montana, year(s) sampled, tillage type, and wheat variety Site Year County Location Tillage type Variety 1 1998 Chouteau 47Њ 50Ј N, 111Њ 17Ј W Minimum ÔRampartÕ 2 1998 Chouteau 47Њ 58Ј N, 111Њ 22Ј W Minimum ÔRampartÕ 3 1998/1999 Cascade 47Њ 41Ј N, 111Њ 37Ј W Minimum ÔRockyÕ 41998 Teton 47 Њ 57Ј N, 111Њ 47Ј W Minimum ÔVanguardÕ 5 1998 Toole 48Њ 16Ј N, 111Њ 49Ј W Intensive ÔErnestÕ 6 1998 Stillwater 45Њ 45Ј N, 109Њ 07Ј W Intensive ÔVanguardÕ 7 1999 Chouteau 47Њ 50Ј N, 111Њ 19Ј W Intensive ÔRampartÕ 8 1999 Chouteau 48Њ 00Ј N, 111Њ 08Ј W Intensive ÔVanguardÕ 9 2000 Teton 47Њ 49Ј N, 111Њ 59Ј W Intensive ÔTiberÕ 10 2001 Cascade 47Њ 41Ј N, 111Њ 32Ј W Intensive ÔVanguardÕ 11 2001 Pondera 48Њ 02Ј N, 111Њ 29Ј W Intensive ÔRampartÕ 12 2001 Chouteau 48Њ 02Ј N, 111Њ 10Ј W Intensive ÔRampartÕ system consists of the current crop adjacent to idle “minimum tillage”, and summer fallow Þelds with no Þelds in which the previous yearÕs crop was located. visible stubble were labeled “intensive tillage.” Sites Tillage and/or herbicides are commonly used to man- were sampled with a sweep net (before the appear- age weeds on fallow land in Montana. Tillage, me- ance of parasitoids) to ensure the presence of sawßies. chanical disturbance that results in inversion of the Stem Sampling. Plant samples were collected at soil, uproots and buries seedlings and mature weeds each location to determine sawßy infestation and par- (Troeh et al. 1999). Chemical fallow is an untilled asitism levels. Four samples were taken along the long summer fallow system in which weeds are controlled axis of each Þeld edge that bordered the adjacent with herbicides, and the soil is left undisturbed fallow Þeld. Mature plant stems were collected im- (Blevins and Frye 1993). Sawßies and their parasitoids mediately before harvest by uprooting entire plants. overwinter in postharvest wheat stubble. The effects Samples consisted of four replicates of all stems in a of stubble management on sawßy populations have 30-cm length of a row located within 1Ð5 m of the Þeld been extensively investigated (Criddle 1922, Callen- border. Field edges were sampled because sawßy ovi- bach and Hansmeier 1944, Farstad et al. 1945, Holmes position occurs as soon as suitable hosts are encoun- and Farstad 1956, Weiss et al. 1987, Morrill et al. 1993, tered; therefore, infestations are concentrated in Þeld Goosey 1999); however, nothing has been reported borders (Hitchcock 1942, Morrill et al. 2000). Samples about the effects of stubble management on levels of were taken from the same side of each Þeld (i.e., from sawßy parasitism. The objectives of this study were to east edge) at each location because sawßies ßy up- examine the effects of fallow management on popu- wind to existing crops, often causing sawßy infestation lations of wheat stem sawßy and its associated para- and parasitism levels to vary between opposing Þeld sitoids. SpeciÞcally, we wished to determine if we edges. could discern differences at the Þeld level for both Stem Processing. Each stem was dissected length- trophic levels, and if we could detect any direct impact wise with an X-ACTO knife (Hunt Corp., Statesville, of weed management on the number of sawßy-cut NC). Sawßy infestation was determined by the pres- stems, the criterion used by producers to gauge the ence of characteristic frass resulting from the feeding severity of their sawßy infestations. of the sawßy larva within the stem. The cleanly cut, frass-plugged lower portions of stems were easily rec- ognized as sawßy-cut stems. Sawßy parasitism levels Materials and Methods were determined by the presence of parasitoid co- Field Selection. Two wheat Þelds, one bordering coons and the distinctive adult parasitoid emergence either intensely tilled or minimally tilled summer fal- holes. Plants were held in the lab for 1 mo to allow low and an adjacent Þeld bordering untilled summer completion of parasitoid development and cocoon fallow, were compared at each of 12 sites in Montana construction before each stem was dissected. during 1998Ð2001 (Table 1). Sites varied in the Sawfly Stub Depth. The distance of the apex (point amount of annual rainfall, wheat varieties grown, ap- from which sawßies emerge) of sawßy stubs from the plication of soil amendments, and soil type. Insecticide soil surface was measured in four untilled Þelds in use is uncommon in dryland wheat production in Broadwater County, in October 2001. “Stub” depth Montana. The sites selected covered a broad area of was obtained by placing a meter stick level on the soil the agricultural landscape in the wheat producing area surface along a row, digging under the meter stick of Montana, with a distance of 310 km separating the until a stub was located, and measuring the distance sites furthest apart. The tilled and untilled Þeld at each from the top of the stub to the bottom of the meter site was carefully chosen to match wheat variety and stick. The depth of 25 stubs in Þve randomly selected Þeld size, but it was not possible to select similar sites rows was obtained in each Þeld.