In addition to the use of cultivars with solid stems that better tolerate the stem boring activity of the sawfly larva, seeding and cultivation strategies used in wheat production can impact insect pest populations (Beres et al., 2011b; Morrill et al., 1993; Weaver et al., 2004). Tillage was one of the first control methods advocated to manage WSS populations. Criddle (1922b) recommended that infested stubble be turned over and sealed using a mold-board plow, preferably in fall so that pathogens attacking the rotting stubble would also invade the overwintering larvae. Although considered effective, plowing did not kill all sawflies (Ainslie, 1920), and it destroyed beneficial insects that attack WSS (Runyon et al., 2002). The plow was eventually replaced with low disturbance implements such as the Noble blade (Mathews, 1945), and concomitant with large blocks of fallow, the change in farming practice likely enhanced WSS populations (McGinnis, 1950; Weaver et al., 2004). Other studies that investigate tillage as a management tool for WSS report that burial of stubs is not necessary, but removal of soil from the crown is necessary so that overwintering stubs are exposed to lethal temperatures (Holmes and Farstad, 1956). Similar numbers of larvae emerge from tillage operations that do not remove soil from the crown compared to undisturbed stubble (Goosey, 1999). Morrill et al. (1993) reported that WSS mortality in shallow-tilled fields was 90 and 10% in fields that were left undisturbed.
Biological tilling can also reduce WSS overwintering success. In a Montana study (Hatfield et al., 2007), sheep (Ovis aries) were grazed in fields of winter wheat stubble that had been infested with WSS. Compared to the control or mechanical tillage, mortality of WSS was greatest in plots that had been grazed by sheep. The effects of trampling caused by the foraging activity of sheep also increased WSS mortality. This method may have promise in areas with a crop–livestock interface, but further study is required to determine impacts to soil health from extensive grazing and trampling.
The timing of tillage operations may also be an important factor in reducing WSS populations. Holmes and Farstad (1956) recommended late spring operations in place of fall or early spring as larvae entering the pupal stage of development in late spring cannot return to diapause. However, there is disagreement over the efficacy of tillage as a management tool (Weiss et al., 1987) and concern that tillage negatively impacts soil health, which creates an incongruity between conventional tillage and modern conservation farming systems.
Traditionally, producers adopted systems of frequent summer fallow, monoculture cereal production, and intensive mechanical tillage (Zentner et al., 2002). The utility of conventional tillage diminished with the innovation of nonresidual herbicides such as glyphosate for weed control. The benefits of minimum or no-till systems have been reported extensively (Janosky et al., 2002; Lindwall and Anderson, 1981); and Derksen et al. (2002) suggested that the conventional wheat–fallow system in the northern Great Plains could be modified with diversified cropping systems. Benefits of minimum tillage, no-till and direct seeding practices include increased soil moisture retention, improved residue preservation and reduced soil erosion (Larney et al., 1994; Peterson et al., 1996; Roget et al., 1987). Lower prices for cereal grains, improvements in equipment and machinery design, influence of government policy, and concern for environmental sustainability of intensive tillage also contributed to the shift in land use (Zentner et al., 2002). The result was a significant reduction in the area left fallow each year across the Canadian prairies (Fig. 1a) (Anonymous, 2009) and in Montana and North Dakota (Fig. 1b) (Anonymous, 2007).
Despite the adoption of conservation farming there is still a sizeable area fallowed in semiarid regions of the northern Great Plains. Of the approximate 5 million ha of land fallowed each year in Canada and Montana (Fig. 1), a portion is due to weather-related factors that prevent timely sowing of spring annual crops; however, the remaining portion is by design and part of cropping systems that integrate summer fallow phases, which tend to be concentrated in the semiarid regions prone to WSS attack. Furthermore, the replacement of narrow strips alternating wheat and fallow with large blocks of wheat-fallow may enhance the ability of the WSS to persist from year to year (Weaver et al., 2004). A better understanding of the impact that modern seeding systems have on WSS populations is required. As an alternative to wheat–fallow systems and conventional tillage to manage WSS, our objective was to determine if residue management and recropping infested wheat stubble would inhibit WSS emergence and to test the hypothesis that the implements used in modern direct-seeded, continuously cropped systems would affect WSS populations.
MATERIALS AND METHODS
A location in the traditional distribution area of the WSS was selected at Coalhurst, near Lethbridge, AB, Canada (49°44′ N, 112°57′ W). The site is an Orthic Dark Brown Chernozem clay loam soil (Typic Boroll) with 27% sand, 32% silt, 41% clay with 3.9% organic matter content and a pH of 7.5. The site was established in the early 1960s as a nursery to conduct field evaluations requiring WSS infestation (Beres et al., 2005; Peterson et al., 1968). The site is managed with alternating 9.3 m wide by 325 m long strips of spring planted wheat and summer fallow (chemical). Soil nutrient status was determined from soil samples collected in fall and submitted to a commercial soil testing laboratory. Nitrogen and P2O5 fertilizer was applied mid-row or side-banded at seeding according to recommended rates for dryland wheat production (Beres et al., 2008; Selles et al., 2006). Separate experiments were established on the spring wheat stubble each fall using the winter wheat cultivars ‘AC Bellatrix’ (2003, 2004) or ‘AC Radiant’ (2005), and in spring using the hard red spring wheat cultivar ‘AC Barrie’ (McCaig et al., 1996). Susceptible cultivars (hollow stem) were used to maintain a robust population of sawflies in the nursery and to avoid any confounding negative effects that a solid-stemmed wheat crop might have on the sawfly population. A new strip of spring wheat stubble that was infested the previous growing season with WSS was selected for each year of the study for a period of 3 yr (2003–2004; 2004–2005; 2005–2006). The year in which crop insects were collected and crops were harvested is used to designate the study year; for example, winter wheat sown in the fall of 2003 is assigned the experimental year 2004.
A split-plot, 5 × 5 factorial, arranged in a randomized complete block experimental design was used each year. To study effects of residue management, five pre-seed harrow treatments were assigned to the main plot, which consisted of a (i) heavy tine harrow (Rite Way Manufacturing, Regina, SK, Canada) with light spring tension, 20° tine angle (Fig. 2a); (ii) heavy tine harrow with high spring tension, 5° tine angle (Fig. 2a); (iii) a rotary drum harrow (Phoenix rotary harrow, Excel Industries LLC, Waseca, MN) with low angle (25°) setting (Fig. 2b); (iv) rotary drum harrow with high angle (45°) setting (Fig. 2b); and (v) a control– ‘no pre-seed harrowing’). The five levels of direct seeding system treatments to recrop the infested spring wheat stubble were assigned to the subplot and consisted of a commercial zero tillage air drill configured with knife-type openers spaced (i) 23 cm or (ii) 30 cm apart; (iii) a commercial zero tillage air drill configured with high disturbance shovel-type sweep openers spaced 23 cm apart; (iv) a low disturbance plot seeder equipped zero tillage disc-type openers spaced 20 cm apart; and (v) a control of ‘no seeding–chemical fallow’. The commercial zero tillage air drills were manufactured by Vale Farms (Conserva Pak Models CP 129A and CP1212A, Indian Head, SK, Canada) and equipped with a Valmar air delivery system (Valmar Airflo Inc., Elie, MB, Canada). The low disturbance plot seeder was manufactured by Fabro Manufacturing (Swift Current, SK, Canada) using John Deere MaxEmerge (Moline, IL) disc openers and a Flexi-Coil air manifold product delivery system (CNH, Saskatoon, SK, Canada). The pre-seed harrowing and direct seeding treatments were performed at a perpendicular angle to the direction of the wheat stubble rows infested with wheat stem sawfly. Treatment combinations were replicated three times with subplot experimental unit dimensions 3.3 m wide by 10 m long. Each study area was treated with glyphosate (RoundUp, Monsanto, St. Louis, MO) a few days before seeding applied at a rate of 900 g a.i. ha−1 using a motorized sprayer calibrated to deliver a carrier volume of 45 L water ha−1 at 275 kPa pressure.
Shortly after the spring wheat experiment was sown, a 1 by 1 m triangular emergence cage (Dosdall et al., 1996) was placed near the center of each subplot for both winter and spring wheat experiments. The primary collection device consisted of a 700 mL glass jar with a funnel-shaped, amber lumite 530 μm mesh screen (BioQuip Products, Rancho Dominquez, CA) inserted into the jar and secured by a ring-lid. The jar was inverted and placed through a circular opening on top of the cage cut wide enough to accommodate the outside diameter of the ring-lid. Sticky cards (Contech Inc, Delta, B. C. Product No. 611– bright yellow) measuring 7.6 by 12.7 cm were mounted on wooden stakes and placed inside the emergence cage to act as a secondary trap. To encourage movement into the collection devices, plant growth inside the cage was terminated with an application of glyphosate. Three times weekly, from 21 June to 26 July, newly enclosed adults emerging from the previous crop stubble were collected from the cage jars and sticky cards. Specimens of WSS were grouped by gender and separated from the WSS parasitoid, Bracon cephi (Gahan) (Hymenoptera: Braconidae). Before crop harvest, the emergence cages were removed and at crop maturity, a 1.5 m wide strip running the length of the plot was harvested from each subplot with a Wintersteiger Expert (Wintersteiger AG, Salt Lake City, UT) plot combine equipped with a straight cut header, pickup reel, and crop lifters.
Data were analyzed with the PROC MIXED procedure of SAS (Littell et al., 2006). Count data for insects were subjected to log transformations [log10(x+1)] to stabilize variances among treatments (Steel et al., 1997). Homogeneity of error variances was tested and any outlier observations were removed before a combined analysis over years was performed. For the combined analysis of data over the 3 yr of the study, the model was:where μ is the overall expected response, harrow refers to the main plot pre-seed harrowing treatments, seeding system refers to the seed drill treatments used to recrop the infested wheat stubble, and ε refers to the residual error variance not accounted for in the model. For analyses of both raw and transformed data, the effects of replicate, year, and interactions of replicate, year and the main plot “harrow” were considered random; treatment effects were considered fixed and significant if P ≤ 0.05. Year was considered a random effect to remove the variability among years from the residual error and to get the average treatment effects over the 3 yr where WSS pressure was rated moderate to high (Fig. 3). Mean separation tests were performed using a Fisher's protected LSD (Steel et al., 1997). Pearson correlation coefficients were calculated between WSS emergence and grain yield in both spring wheat and the winter wheat systems using the PROC CORR procedure of SAS (Littell et al., 2006).
A grouping methodology, as described by Francis and Kannenberg (1978), and later adapted to agronomy studies (Beres et al., 2010; Gan et al., 2009; May et al., 2010), was used to further explore treatment responses. The mean and coefficient of variation (CV) were estimated for each level of the treatment. Means were plotted against CV for each level of the treatment. The overall mean of the treatment means and CVs was included in the plot to categorize the biplot ordination area into four quadrats/categories: Group I: High mean, low variability (optimal); Group II: High mean, high variability; Group III: Low mean, high variability (poor); and Group IV: Low mean, low variability.
RESULTS AND DISCUSSION
Populations of WSS during the study period were moderate to severe. The range of individuals emerging was 22 to 45 adults m−2 in the spring wheat and 36 to 65 adults m−2 in the winter wheat experiments; both were planted on spring wheat stubble. This translates to a range of 220,000 to 640,000 individuals ha−1. The emergence of adults was greatest in 2005, which peaked in the first week of July (Fig. 3a). Emergence peaked 1 wk later in 2004 and a week earlier in 2006 despite a later sowing date. However, using growing degree day (GDD) accumulation from 1 May of each year collected at the Agriculture and Agri-Food Canada meteorlogical site in Lethbridge, Alberta (Tbase = 5°C), the peak emergence for all 3 yr appears similar and within a range of 17 GDD (578–595 GDD) (Fig. 3b). The selection of the 5°C base temperature matches well with a 4.6°C GDD base for spring wheat development in the semiarid prairie (Davidson and Campbell, 1983) and with an overwintering WSS threshold of approximately 4°C derived from data in Perez-Mendoza and Weaver (2006) (M. Buteler and D. Weaver, personal communication, 2011).The first 2 yr also had similar populations, but the flight period in 2006 was shorter, peaked earlier (28 June), and ended abruptly with lower overall adults. This may be due to increasing numbers of parasitoids and insect-host asynchrony effects from below average rainfall during 1 June to 31 July, which may have caused earlier initiation of stem elongation leading to an earlier harvest date compared to the other years of the study (Table 1; Fig. 3).
|Location||Coalhurst, AB, Canada
|Latitude and longitude||49°44′N, 112°57′W
|Residue management||27 Sept.||10 Sept.||7 Oct.|
|Winter wheat seeding||27 Sept.||10 Sept.||7 Oct.|
|Spring wheat seeding||24 Apr.||13 Apr.||12 May|
|Winter wheat harvest date||3 Sept.||8 Sept.||28 Aug.|
|Spring wheat harvest date||3 Sept.||8 Sept.||28 Aug.|
|1 Sept. to 30 Mar. (LT average = 145)||113.4||144.4||247.5|
|1 Apr. to 31 May (LT average = 83)||126.2||38.6||71.0|
|1 June to 31 July (LT average = 107)||120.1||281.8||97.9|
|Total (long-term average = 335)||359.7||464.8||416.4|
The sex ratio of this species is usually equal (McGinnis, 1950), but our collections comprised approximately three times more WSS females than males (Table 2). Sawflies are haplodiploid, with the sex of the progeny determined by selective egg fertilization at the time of oviposition (Cook, 1993; Flanders, 1946). Luginbill and McNeal (1958) reported that females prefer plants with large stems for oviposition, and Morrill et al. (2000) found that plants with large stems confer several fitness advantages on WSS progeny. Sex ratios of WSS are male-biased in small stems and female-biased in large stems (Cárcamo et al., 2005; Morrill et al., 2000). The wheat crops planted in our study sites in the year preceding WSS emergence collections were hollow stem plants seeded at the recommended rate and grown in soil treated with optimal fertilizer application. It is probable that these conditions favored development of vigorous plants, with large stems, and this resulted in a female-biased sex ratio of offspring. Although we cannot confirm its presence, van Wilgenburg et al. (2006) noted that the presence of Wolbachia bacteria could affect sex determination in the Hymenoptera.
|No. adults m−2
||No. adults m−2
|Factor||Treatment||Males||Females||Total WSS||Males||Females||Total WSS|
|Harrow (main plot)||Control-no harrow||14a† (1.2)||38a (3.2)||52a (4.1)||7 (1.0)||27a (3.1)||34a (3.9)|
|Heavy tine 20°||13ab (1.5)||31ab (2.5)||44ab (3.7)||6 (1.1)||19b (3.3)||24b (4.3)|
|Heavy tine 5°||9b (1.1)||25b (2.5)||35b (3.4)||6 (0.8)||17b (2.4)||23b (3.0)|
|Rotary harrow 25°||13a (1.4)||33a (3.0)||46a (3.9)||7 (1.0)||24a (2.6)||32a (3.4)|
|Rotary harrow 45°||14a (1.5)||36a (3.4)||51a (4.6)||5 (0.7)||19b (2.3)||25ab (2.9)|
|Seed drill (subplot)||Chemical fallow-no seeding||17a (1.6)||48a (3.7)||65a (5.0)||11a (1.4)||32a (4.2)||45a (5.4)|
|Disc opener||14a (1.3)||33b (2.6)||48b (3.4)||6b (0.9)||18bc (2.1)||24bc (2.8)|
|Knife opener30 cm row spacing||10b (1.3)||30b (2.4)||42bc (3.3)||4bc (0.4)||18c (2.0)||22c (2.3)|
|Knife opener23 cm row spacing||11b (1.3)||26b (2.4)||38c (3.3)||5b (0.5)||20b (2.2)||25b (2.5)|
|Sweep opener23 cm row spacing||10b (1.0)||26b (2.6)||36c (3.4)||5c (0.7)||18bc (2.4)||23c (3.0)|
|Pr > F||Harrow (H)||0.047||0.042||0.039||0.211||0.008||0.026|
|H × D||0.714||0.697||0.321||0.036||0.294||0.049|
In the spring wheat study, heavy tine harrows and the high angle setting on the rotary harrow reduced female and total WSS but had no effect on male WSS emergence (Table 2). A reduction in adult emergence from harrowing in the winter wheat study occurred only with the 5° (high tension) heavy harrow. Using a tine harrow with an angle setting of 5° (high tension) before seeding reduced WSS adult emergence by approximately 35% in both the winter wheat and spring wheat systems. Morrill et al. also reported negative impacts to WSS populations (1993) if the operation sufficiently exposed the stubble. The authors do not report the percentage of stubs uprooted onto the soil surface from harrowing or what type of harrow implement was used. However, the operation seemed to create high disturbance of the stubble as the authors recommend only harrowing field border to limit the risk of soil erosion (Morrill et al., 1993).
The seed drill factor also affected emergence patterns of WSS (Table 2). Irrespective of seed drill type, recropping spring wheat stubble infested with WSS reduced the adult population compared to leaving the stubble undisturbed in both the fall and spring systems. In the winter wheat system, greater reductions in sawfly emergence were observed with drills equipped with knife and sweep opener configurations spaced 23 cm apart, whereas in the spring system, the sweep and wider row spacing of the knife opener (23 cm) was more effective than the disc drill and narrow spacing of the knife opener (Table 2). This is plausible because the disc drill provides the least disturbance to stubble at seeding, which is highly desirable when sowing winter wheat (Fowler, 1983), and less disturbance would have the least impact on diapausing larvae of WSS.
The significant interaction between harrowing and seed drill in the spring wheat system (P < 0.049) was explored and summarized in Table 3. When compared to the “chemical fallow” or control of “no harrowing”, combinations of the other harrow and recropping treatments should decrease WSS emergence to be considered effective. After the harrowing, the heavy tine harrow set at 5° tension and the 45° rotary harrow had the lowest total WSS emergence, which was further reduced by 50% or more in the tine harrow main plots when using any drill configuration except the knife opener spaced 23 cm apart (Table 3). Reducing the tension of the tine harrow to 20° diminished efficacy before recropping, but if followed by the drill configured with 30 cm knife or sweep openers, the emerging population was reduced by approximately 70%. A similar result was observed if a 25° rotary harrow was used with the drill equipped with 30 cm knife openers, but no additional reductions were observed when using the 45° rotary harrow. The results suggest that in a spring wheat system the most effective system for reducing WSS emergence would be to combine pre-seed heavy tine harrows with a drill configured with knife openers spaced 30 cm apart, and that higher spring tension may improve efficacy. This recommendation could be easily implemented as most air drills equipped with knife openers similar to ones used in this study are generally sold in the 30 cm row spacing configuration, and heavy tine harrows are commonly used to manage crop residue.
|Harrow (main plot)
|Effect||Treatments||Control-no harrowing||Heavy Tine20° Setting||Heavy tine5° Setting||Rotary harrow25° Setting||Rotary harrow45° Setting||Mean over all harrow treatments|
|Seed drill (Subplot)||Chemical fallowNo recropping||54a† (13.8)||56a (18.3)||34a (9.3)||45a (9.2)||37a (8.5)||45a (5.4)|
|Disc opener18 cm row spacing||29b (5.8)||23b (5.0)||15b (4.5)||31ab (9.0)||25a (6.3)||25bc (2.8)|
|Knife opener30 cm row spacing||29b (6.3)||15 d (4.5)||20b (5.4)||23b (4.6)||22a (4.8)||22c (2.3)|
|Knife opener23 cm row spacing||35ab (7.0)||18bc (3.4)||27a (6.0)||25b (5.2)||22a (5.0)||25b (2.5)|
|Sweep opener23 cm row spacing||24b (6.0)||17cd (5.8)||18b (6.2)||34ab (7.7)||24a (7.9)||23c (3.0)|
|Mean over all seed drill treatments||34 a (3.9)||24 b (4.3)||22 b (3.0)||32 a (3.4)||25 ab (2.9)||–|
Previous studies reported that shallow tillage or operations involving harrows following tillage were only effective if the crown was uprooted and all soil was removed (Goosey, 1999; Holmes and Farstad, 1956; Morrill et al., 1993), which causes WSS mortality from exposure and desiccation. Modern farm systems incorporate harrowing before seeding to spread out trash cover to facilitate even crop stand establishment. Furthermore, because the low disturbance disc drill was sometimes as effective at reducing WSS emergence as the other drills, our results cannot be explained by crown upheaval and removal of all soil from the crown. We acknowledge that changes in emergence probably reflect altered mortality in the sample area. However, it was beyond the scope of this paper to directly establish this cause-and-effect relationship. It is possible that the treatments most effective at reducing WSS emergence damaged the anchored stubble sufficiently to kill the sawflies through exposure, or inflicted lethal effects on the pupae in spring.
Populations of the parasitoid Bracon cephi were low but increased in the final 2 yr of the study (data not shown). This is not surprising as there is generally a lag phase before the population of a natural enemy increases in response to host populations. Trap captures of this species were low and usually in the range of one to four specimens per m2. In general, the practice of pre-seed residue management using the harrows selected in this study did cause reduced parasitoid emergence. The magnitude of reduction was greater in the winter wheat system as parasitoids were more abundant, which suggests operations performed in the fall are not as detrimental to B. cephi. However, the overall population density of B. cephi was likely too low in this study to draw definitive conclusions (data not shown). Other studies report that aggressive tillage such as plowing under wheat stubble significantly increases the mortality of the C. cinctus parasitoids Bracon lissogaster (Muesebeck) and B. cephi (Runyon et al., 2002). The methods employed in our study are not as destructive as plowing or other aggressive forms of tillage; for example, the rotary harrow is commonly used for in-crop weed control in organic farming systems (Frick and Johnson, 2002). Thus, it is not clear if the reductions of B. cephi observed when harrowing or direct seeding into stubble infested with WSS interfered with parasitism patterns.
The merit of any cultural practice used to control an insect must include grain yield as a proxy for sustainability. Grain yield was affected by the seed drill factor but pre-seed harrowing did not influence grain yield in the spring or winter wheat systems (Table 4). For spring wheat, grain yield was optimized with the drill configured with knife openers. In fall, plots seeded with the low disturbance disc drill produced more grain than those seeded with the other drills. In another study in southern Alberta, a drill equipped with disc openers improved plant population stands in winter wheat compared to a hoe or knife opener but grain yield was unaffected (McKenzie et al., 2007). The grain yield from plots seeded with the drill equipped with sweep openers was inferior to that from plots seeded with the other drills in both fall and spring (Table 4). These observations make agronomic sense as low stubble disturbance is a key to successful overwintering of winter wheat. The sweep opener is an outdated configuration but was selected to create an environment of high stubble disturbance, which likely inhibited seed to soil contact in the spring system and led to greater winterkill in the fall system. The interaction of harrowing and seed drill was significant (P < 0.041) and explored further in the spring wheat system (Table 5). The combinations that consistently reduced WSS populations also optimized grain yield potential. It is not possible to determine to what degree reduced emergence may have impacted the reported yield. Although there must be a relationship within the overall effect of the treatment, what is detrimental to WSS and what is simply beneficial to the non-insect part of the system cannot be discriminated
|Factor||Treatment||Spring wheat||Winter wheat|
|kg ha −1|
|Harrow (main plot)||Control-no harrow||2438 (122)||2568 (100)|
|Heavy tine 20°||2482 (123)||2344 (110)|
|Heavy tine 5°||2573 (115)||2342 (109)|
|Rotary harrow 25°||2530 (129)||2366 (118)|
|Rotary harrow 45°||2620 (123)||2468 (121)|
|Seed drill (subplot)||Disc opener||2483b† (117)||2706a (112)|
|Knife opener30 cm row spacing||2605ab (99)||2411b (87)|
|Knife opener23 cm row spacing||2715a (124)||2342bc (94)|
|Sweep opener23 cm row spacing||2294c (87)||2206c (94)|
|Pr > F||Harrow (H)||0.487||0.804|
|H × D||0.041||0.677|
|Harrow (main plot)
|Effect||Treatments||Control-No harrowing||Heavy tine20° Setting||Heavy tine5° Setting||Rotary harrow25° Setting||Rotary harrow45° Setting||Mean over all harrow treatments|
|Seed drill (subplot)||Chemical FallowNo recropping||–||–||–||–||–||–|
|Disc Opener18 cm row spacing||2532a† (332)||2334bc (247)||2330b (98)||2778a (334)||2412b (205)||2483b (117)|
|Knife Opener30 cm row spacing||2471ab (232)||2758a (225)||2656ab (227)||2471bc (186)||2678ab (279)||2605ab (99)|
|Knife Opener23 cm row spacing||2553a (286)||2611ab (284)||2915a (252)||2673ab (299)||2849a (306)||2715a (124)|
|Sweep Opener23 cm row spacing||2207b (127)||2223c (230)||2318b (256)||2187c (189)||2527b (195)||2294c (87)|
|Mean over all seed drill treatments||2438a (122)||2482a (123)||2573a (115)||2530a (130)||2620a (123)||–|
A biplot of the yield responses confirmed that pre-seed harrowing in combination with a drill equipped with knife openers produced high grain yield and achieved greater overall stability when planting spring wheat. However, in the winter wheat system, treatments that caused lower stubble disturbance enhanced grain yield. For example, the disc drill with or without harrowing, or the knife opener drills without any pre-seed harrowing produced high grain yield and achieved high overall stability (Fig. 4).
Correlation analyses of the variables grain yield and total WSS emergence showed a marginal (P = 0.06) inverse relationship between grain yield and the total WSS that emerge from the spring wheat system (Table 6). However, in the winter wheat system, grain yield and WSS emergence were positively correlated (P = 0.003) and suggest the producer must suffer a yield penalty to lower populations of WSS in winter wheat (Table 6). Therefore, the practice of recropping to manage WSS is primarily recommended in spring, but, if performed in the fall, caution should be exercised to ensure that adequate snow trap potential (STP) is maintained (>20) after fall operations if the goal is direct-seeded winter wheat, that is, STP = [stubble height (cm) × stubble stems m−2]/100 (Fowler, 2002). An alternative may be to perform residue management or harrow operations in the fall and direct-seed a spring annual crop into the stubble in spring.
|Pearson correlation coefficients
|Wheat system||Males||Females||WSS total|
|Spring wheat||−0.05 ns‡||−0.16*||−0.15†|
Although we chose continuous wheat, any spring annual crop could be used when recropping infested wheat stubble. Several studies report benefits when broad leaf crops such as pulses are integrated into a semiarid cropping system (Brandt, 1996; Miller and Holmes, 2005; Miller et al., 2002; Miller et al., 2003; Zentner et al., 2002). Other studies report a decline in winter wheat yield when fallow phases are removed, although overall system profitability is maintained or even improved (Lyon et al., 2004).
Aggressive forms of tillage for the control of an insect or weed population are no longer considered sustainable in the northern Great Plains (Larney et al., 1994). The results of our study indicate that continuous cropping may be a sustainable form of stubble disturbance as part of integrated pest management strategy for WSS. Many producers opt for a wheat–fallow system based on the assumptions of water conservation during the fallow phase. The challenge, therefore, is to assess the effect of continuous cropping on the economic stability of the overall system. Even though chemical fallow is less destructive to soil health than conventional tillage, the practice is not efficient at conserving water. Only 15 to 20% of soil water remains in the upper 2 m of the soil profile for use by the successive crop (Stoskopf, 1985). In the context of economic sustainability, continuous wheat is more profitable than a wheat–fallow system in a semiarid environment in the brown soil zone of Saskatchewan (Zentner et al., 2006). Therefore, our results support other published works that suggest fallow could be eliminated, and that the benefits derived from continuous cropping include reduced pest pressure and an improvement to overall system profitability.
The WSS remains a key pest of hard red spring wheat, durum wheat (T. turgidum L.) and winter wheat throughout the northern Great Plains. We conducted this study to determine if the implements used in modern direct-seeded, continuously cropped systems would affect WSS populations. Pre-seed heavy tine harrowing treatments (residue management) reduced adult sawfly emergence but usually required a high tension setting. No-till planting into infested spring wheat stubble also lowered WSS emergence compared to leaving the field fallow. Grain yield was optimized in spring wheat with air drills equipped with narrow knife openers, and optimized in winter wheat with the low disturbance disc drill configurations. It is unclear if the stubble disturbance would negatively affect parasitism patterns of the WSS parasitoid population. The results indicate that there would be an incremental benefit of continuous cropping rather than fallowing infested wheat fields, however, the practice may only be sustainable in spring annual cropping systems as yield reductions were observed in fall treatments most effective at reducing WSS emergence. Moreover, these strategies should not be used in isolation as the reductions did not eliminate the WSS population. A systems approach is required that integrates these practices with diversified crop phases and resistant cultivars (Beres et al., 2011a). Future directions in cultural management practices should integrate multiple management strategies and include plant density and nutrient and harvest management. Continued research in the conservation of the beneficial parasitoid B. cephi should focus on the impacts of both preharvest and harvest management practices.