Parasitism of the Wheat Stem Sawfly

Home , Braconidae, Bracon (wasp), Cephidae, Cephus, Cephus cinctus, Hymenoptera

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 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 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. Two Þelds contained throughout the entire study area. Tillage type was the current yearÕs stubble (sawßies that will emerge categorized by the amount of crop residue on the soil the following spring), and two Þelds contained the surface. Tilled summer fallow with Ͼ75% of the stub- previous yearÕs stubble (sawßies that emerged 5 to 6 ble remaining on the soil surface were designated as mo before measurement). 1132 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 95, no. 6

Table 2. Comparison of percent sawfly-infested stems (mean ؎ SEM), sawfly-cut stems, and sawfly parasitism in wheat fields bordering tilled and untilled summer fallow in Montana

Bordering Mean Ϯ SEM no. % Sawßy-infested t % Sawßy-cut t % Sawßy t Site P P P fallow Þeld stems/sample stems value stems value Parasitism value 1 Minimally tilled 76 Ϯ 787Ϯ 4 9 Ϯ 2 55 Ϯ 12 Ϫ0.49 0.67 Ϫ0.77 0.51 2.140.09 Untilled 71 Ϯ 992Ϯ 315Ϯ 341Ϯ 8 2 Minimally tilled 125 Ϯ 15 77 Ϯ 3 14 Ϯ 2 48 Ϯ 9 Ϫ1.640.20 0.03 0.97 Ϫ1.22 0.16 Untilled 136 Ϯ 984Ϯ 814Ϯ 358Ϯ 6 3a Minimally tilled 88 Ϯ 11 44 Ϯ 4 6 Ϯ 2 27 Ϯ 9 0.23 0.83 Ϫ2.01 0.09 0.79 0.44 Untilled 96 Ϯ 643Ϯ 812Ϯ 720Ϯ 7 4Minimally tilled 84 Ϯ 16 39 Ϯ 5** 5 Ϯ 1 45 Ϯ 12 Ϫ5.37 0.01 Ϫ0.740.60 Ϫ2.37 0.07 Untilled 96 Ϯ 12 53 Ϯ 78Ϯ 369Ϯ 16 3b Minimally tilled 96 Ϯ 834Ϯ 6 10 Ϯ 2 18 Ϯ 7 2.39 0.11 0.88 0.44 0.44 0.29 Untilled 101 Ϯ 10 25 Ϯ 89Ϯ 115Ϯ 3 5 Intensely tilled 148 Ϯ 21 38 Ϯ 4** 15 Ϯ 2** 7 Ϯ 7** 24.65 Ͻ0.01 21.98 Ͻ0.01 Ϫ4.47 Ͻ0.01 Untilled 159 Ϯ 16 9 Ϯ 20Ϯ 039Ϯ 12 6 Intensely tilled 196 Ϯ 17 11 Ϯ 2 3 Ϯ 2** 5 Ϯ 3** 0.02 0.98 4.96 0.02 Ϫ16.62 Ͻ0.01 Untilled 211 Ϯ 15 11 Ϯ 40Ϯ 081Ϯ 17 7 Intensely tilled 144 Ϯ 13 38 Ϯ 12 11 Ϯ 3** 20 Ϯ 11 1.33 0.17 8.12 0.01 Ϫ1.18 0.17 Untilled 130 Ϯ 12 35 Ϯ 94Ϯ 332Ϯ 16 8 Intensely tilled 111 Ϯ 752Ϯ 14 9 Ϯ 2** 56 Ϯ 9** 1.02 0.28 7.73 Ͻ0.01 Ϫ4.54 0.03 Untilled 109 Ϯ 12 42 Ϯ 43Ϯ 174Ϯ 8 9 Intensely tilled 166 Ϯ 21 72 Ϯ 8 19 Ϯ 4 31 Ϯ 4** Ϫ0.10 0.93 0.87 0.54 Ϫ5.56 0.01 Untilled 186 Ϯ 13 84 Ϯ 718Ϯ 459 Ϯ 3 10 Intensely tilled 121 Ϯ 96Ϯ 2** 4 Ϯ 2 0 Ϯ 0** Ϫ8.56 Ͻ0.01 Ϫ2.36 0.07 Ϫ7.57 0.01 Untilled 115 Ϯ 318Ϯ 48Ϯ 322Ϯ 3 11 Intensely tilled 241 Ϯ 941Ϯ 6 6 Ϯ 3 15 Ϯ 8** 0.25 0.81 Ϫ0.81 0.49 Ϫ3.28 0.02 Untilled 211 Ϯ 16 40 Ϯ 98Ϯ 531Ϯ 6 12 Intensely tilled 98 Ϯ 11 28 Ϯ 7 8 Ϯ 2 29 Ϯ 9 1.140.39 0.06 0.97 0.29 0.82 Untilled 114 Ϯ 2425 Ϯ 58Ϯ 427 Ϯ 13

For each comparison, df ϭ 3; t and P values apply to comparison in preceding column. ** indicates signiÞcant difference at ␣ ϭ 0.05. a 1998. b 1999.

Statistical Analysis. The experimental design al- respectively). There was no difference in mean per- lowed for careful comparison between tilled and un- centage of sawßy infestation between the Þeld bor- tilled summer fallow Þelds at each site. However, the dering intensely tilled fallow and untilled fallow at six great variation in agricultural practices and environ- of the eight sites (Table 2). However, intensive tillage mental conditions across sites precluded an analysis of that incorporates sawßy-infested stubble into the soil variance (ANOVA) procedure by violating the un- was recommended as a control tactic (Criddle 1922, derlying assumptions for this approach. Therefore, a Farstad et al. 1945). Recently, Goosey (1999) reported paired t-test was conducted for each site, and general no difference in sawßy mortality between intensely conclusions are drawn based on these repeated inde- tilled and untilled Þelds, which is consistent with our pendent analyses. The t-test procedure of the SAS Þndings. Sawßies overwinter below ground in stubs Statistical Package (PROC t-test, SAS Institute 1990) that are often completely buried and following emer- was used in a paired comparison by site, evaluating gence must dig to reach the soil surface (Table 3). The mean percentage of sawßy infestation, cut stems, and ability of sawßies to escape from their soil-covered sawßy parasitism between Þelds that bordered tilled stubs upon emergence presumably renders intensive and untilled summer fallow. Percent data were trans- tillage ineffective as a method of sawßy control. Four formed using the arcsine transformation. of the eight Þelds bordering untilled fallow had fewer sawßy-cut stems than the neighboring intensely tilled Þelds. This decrease in sawßy-cut stems can be par- Results and Discussion tially explained by the increase in sawßy parasitism Sawßy parasitism was statistically greater in the Þeld seen in Þelds bordering untilled fallow. that bordered untilled summer fallow than the Þeld that bordered intensely tilled summer fallow at six of the eight sites, suggesting that intense tillage in which Table 3. Depth of wheat stem sawﬂy stubs in four untilled ﬁelds in Broadwater Co., Montana in 2001 all stubble is buried could be detrimental to sawßy parasitoids (Table 2). Sawßy parasitoids overwinter Total stubs Total stubs Avg. depth Ϯ SEM Stub age Field with apex not with apex of above ground in stems and might be unable to dig to (months) the soil surface if buried. Samples from Þelds border- soil-covered soil-covered buried stubs (mm) ing intensely tilled summer fallow contained notably A 5Ð6 27 48 5.2 Ϯ 0.9 fewer total parasitoids than Þelds bordering untilled B 5Ð6 28 47 4.8 Ϯ 1.4 summer fallow (177 and 415 total parasitoids in Þelds C 17Ð18 18 57 7.2 Ϯ 2.1 D 17Ð18 21 547.7 Ϯ 1.3 bordering intensely tilled and untilled summer fallow, December 2002 RUNYON ET AL.: TILLAGE IMPACTS WHEAT STEM SAWFLY PARASITISM 1133

There was no difference in sawßy parasitism be- CORE 4 Conservation. 2001. Conservation for agricultureÕs tween Þelds that bordered minimally tilled and un- future: Crop residue management. http://www.ctic. tilled summer fallow (Table 2). Minimally tilled sum- purdue.edu. mer fallow, in which Ͼ75% of the stubble remains on Criddle, N. 1922. The western wheat-stem sawßy and its the soil surface, is similar to untilled summer fallow in control. Canadian Department of Agriculture Pamphlet that few parasitoids are buried. Minimum tillage that 6, Ottawa, Canada. Farstad, C. W., K. M. King, R. Glen, and L. A. Jacobson. 1945. exposes sawßy stubs on the soil surface showed po- Control of the wheat stem sawßy in the Prairie Provinces. tential as a control practice (Holmes and Farstad Canadian War-Time Production Series, Agricultural Sup- 1956). Freezing and desiccation are important mor- plies Board Special Pamphlet 59, Ottawa, Canada. tality factors for sawßy larvae in exposed stubs (Salt Goosey, H. B. 1999. In Þeld distributions of the wheat stem 1946, 1961). Although there is high mortality in ex- sawßy (Hymenoptera: Cephidae), and evaluation of se- posed stubs (Morrill et al. 1993), current tillage meth- lected tactics for an integrated pest management pro- ods do not expose enough soil-free stubble to affect gram. M.S. thesis. Montana State University, Bozeman. sawßy populations the following year (Goosey 1999). Hitchcock, O. B. 1942. Wheat stem sawßy. Montana Exten- In the semiarid northern Great Plains, water avail- sion Series, Montana State College Series A-37, Bozeman, ability is a major factor limiting plant growth (Willis MT. Holmes, N. D. 1954. Food relations of the wheat stem saw- et al. 1983). Managing fallow land with intensive till- ßy, Cephus cinctus Nort. (Hymenoptera: Cephidae). Can. age leaves the soil surface free of residue and increases Entomol. 86: 159Ð67. water loss and soil erosion (Young et al. 1983). Min- Holmes, N. D. 1977. The effect of the wheat stem sawßy, imum tillage and chemical fallow increase soil surface Cephus cinctus Nort. (Hymenoptera: Cephidae), on the residue, thus decreasing evaporation and run-off yield and quality of wheat. Can. Entomol. 109: 1591Ð98. (Wiese et al. 1967), decreasing soil erosion and in- Holmes, N. D., and C. W. Farstad. 1956. Effects of Þeld creasing moisture by trapping snow (Larney et al. exposure on immature stages of the wheat stem sawßy, 1994). The developments of a wide variety of new and Cephus cinctus Nort. (Hymenoptera: Cephidae). Can. J. improved herbicides (Wiese 1983) and of new seeding Agric. Sci. 36: 196Ð202. equipment capable of penetrating heavy crop residue Holmes, N. D., W. A. Nelson, L. K. Peterson, and C. W. Farstad. 1963. Causes of variations in effectiveness of (Larney et al. 1994) have reduced the need for tillage. Bracon cephi (Gahan) (Hymenoptera: Braconidae) as a No-till management of fallow land in Montana has parasite of the wheat stem sawßy. Can. Entomol. 95: increased from an estimated 228,000 acres in 1991 to 113Ð26. 456,000 acres in 1998 (CORE 4 Conservation, 2001). Larney, F. J., C. W. Lindwall, R. C. Izaurralde, and A. P. Producers substituting minimum tillage operations Moulin. 1994. Tillage systems for soil and water conser- in favor of chemical fallow should see no increase in vation on the Canadian Prairie, pp. 305Ð328. In M. R. sawßy-cut stems, and no reduction in sawßy parasit- Carter [ed.], Conservation tillage in temperate agroeco- ism. Intensive tillage is not an effective sawßy control systems. CRC Press, Boca Raton, FL. practice and was detrimental to sawßy parasitoids. Montana State University Extension Service. 1997. Mon- Replacing intensive tillage with minimum tillage or tana crop health report. Special edition: Wheat stem sawßy. Crop Health Report 10: 1Ð7. chemical fallow should result in a cumulative increase Morrill, W. L. 1997. The wheat stem sawßy, Cephus cinctus in sawßy parasitism over time. Therefore, it seems Norton (Hymenoptera: Cephidae), and associated para- probable that an overall decrease in sawßy-cut stems, sitoids in the northern Great Plains of North America. used by producers to determine the extent of their Trends Entomol. 1: 171Ð74. sawßy problems, should result. Morrill, W. L. 1998. Parasitism of the wheat stem sawßy (Hymenoptera: Cephidae) in Montana. Biocontrol 12: 159Ð63. Acknowledgments Morrill, W. L., J. W. Gabor, and D. Wichman. 1993. Mor- tality of the wheat stem sawßy (Hymenoptera: Cephidae) B. G. FitzGerald is thanked for his many hours spent at low temperatures. Environ. Entomol. 22: 1358Ð61. dissecting wheat stems. We thank Greg Johnson, Xinzhi Ni, Morrill, W. L., J. W. Gabor, D. K. Weaver, G. D. Kushnak, and Kevin OÕNeill for helpful comments on the manuscript. and N. J. Irish. 2000. Effect of host plant quality on the This project was supported by the USDA, CSREES, Western sex ratio and Þtness of female wheat stem sawßies (Hy- Regional integrated pest management (IPM); Montana Ag- menoptera: Cephidae). Environ. Entomol. 29: 195Ð99. ricultural Experiment Station; and the Montana Wheat and Morrill, W. L., D. K. Weaver, and G. D. Johnson. 2001. Trap Barley Committee. This is contribution No. J-2001Ð59 of the strip and Þeld border modiÞcation for management of the Montana Agricultural Experiment Station. wheat stem sawßy (Hymenoptera: Cephidae). J. Ento- mol. Sci. 36: 34Ð45. References Cited Nelson, W. A. and C. W. Farstad. 1953. Biology of Bracon cephi (Gahan) (Hymenoptera: Braconidae), an impor- Ainslie, C. N. 1920. The western grass-stem sawßy. U.S. De- tant parasite of the wheat stem sawßy, Cephus cinctus partment of Agriculture Bulletin 841, Washington, DC. Nort. (Hymenoptera: Cephidae), in western Canada. Blevins, R. L., and W. W. Frye. 1993. Conservation tillage: Can. Entomol. 85: 103Ð107. An ecological approach to soil conservation. Adv. Agron. Platt, A. W., and C. W. Farstad. 1946. The reaction of wheat 51: 33Ð78. cultivars to wheat stem sawßy attack. Sci. Agric. 26: 231Ð47. Callenbach, J. A., and M. P. Hansmeier. 1944. Wheat stem Salt, R. W. 1946. Moisture relationships of the wheat stem sawßy control in severely infested areas. Montana Ex- sawßy (Cephus cinctus Nort.) I. Some effects of dessica- tension Service Circular 56, Bozeman, MT. tion. Sci. Agric. 26: 622Ð30. 1134J OURNAL OF ECONOMIC ENTOMOLOGY Vol. 95, no. 6

Salt, R. W. 1961. A comparison of injury and survival of Wiese, A. F. 1983. Weed control, pp. 463Ð488. In H. E. larvae of Cephus cinctus Nort. after intracellular and ex- Dregne and W. O. Willis [eds.], Dryland Agriculture. tracellular freezing. Can. J. Zool. 39: 349Ð50. ASA, CSSA, SSA, Madison, WI. SAS Institute. 1990. SAS/STAT userÕs guide. SAS Institute, Wiese, A. F., E. Burnett, and J. E. Box. 1967. Chemical Cary, NC. fallow in dryland cropping sequences. Agron. J. 59: 175Ð7. Somsen, H. W., and P. Luginbill, Jr. 1956. Bracon lissogaster Willis, W. O., A. Bauer, and A. L. Black. 1983. Water con- Mues., a parasite of the wheat stem sawßy. U.S. Department servation: Northern Great Plains, pp. 73Ð88. In H. E. of Agriculture Technical Bulletin 1153, Washington, DC. Dregne and W. O. Willis [eds.], Dryland Agriculture. Troeh, R. F., J. A. Hobbs, and R. L. Donahue. 1999. Soil and ASA, CSSA, SSA, Madison, WI. water conservation: Productivity and environmental pro- Young, R. A., M. J. Lindstrom, and R. F. Holt. 1983. Soil tection. Prentice Hall, Upper Saddle River, NJ. conservation: Central and northern Great Plains, pp. 219Ð Weiss, M. J., and W. L. Morrill. 1992. Wheat stem sawßy 229. In H. E. Dregne, and W. O. Willis [eds.], Dryland (Hymenoptera: Cephidae) revisited. Am. Entomol. 38: Agriculture. ASA, CSSA, SSA, Madison, WI. 241Ð45. Weiss, M. J., W. L. Morrill, and L. L. Reitz. 1987. The in- ßuence of planting date and spring tillage on wheat stem Received for publication 3 January 2002; accepted 1 June sawßy. Montana Agric. Res. 4: 2Ð5. 2002.