- Additional Plants: native plants
- Crop Production: cover crops
- Pest Management: biological control, integrated pest management, weed ecology
- Production Systems: agroecosystems, organic agriculture
Due to economic and environmental constraints, alternatives to chemical management of Canada thistle (Cirsium arvense L.) are frequently sought, but adequate non-chemical suppression of this species remains elusive. Previous research has shown that Canada thistle biological control agents, such as the stem-mining weevil (Hadroplontus litura Fabricus), have limited efficacy, but integrating additional control tactics with biological control may enhance its effectiveness. Furthermore, soil resource levels can substantially influence impacts of and interactions between insect herbivores and plant competitors. We investigated effects of a biological control agent (H. litura, a stem-mining weevil) integrated with a native annual potential competitor (common sunflower, Helianthus annuus L.) on Canada thistle under two nitrogen, potassium and phosphorous regimes in outdoor microcosms during 2010 and 2011. Weevils caused modest reductions in Canada thistle height and flower number, but did not significantly affect two critical measures of Canada thistle reproductive capacity: final root biomass and number of side shoots. Exposure to common sunflower neighbors reduced Canada thistle height, final main shoot biomass, flower number, side shoot number, and final root biomass. Most measures of Canada thistle performance except root biomass declined with decreasing soil nutrient concentrations. Effects of weevil herbivory and common sunflower presence were additive rather than interactive. Our results suggest that integrating weevils with plant competition may place additional stress on Canada thistle compared to weevils alone, especially in situations where low soil nutrients inhibit Canada thistle performance.
Canada thistle (Cirsium arvense L.) is a problematic invasive weed in many croplands, rangelands, and recreational areas in cooler temperate regions of world, including the U.S. and Canada. This invasive weed thrives in disturbed or moist environments and can lower the quality of grazing lands, out-compete native plants, and negatively impact crop yield (McClay 2002; McLennan et al. 1991; O’Sullivan et al. 1982, 1985). Canada thistle is a clone forming perennial with a deep root system that can spread extensively (Donald 1994; McClay 2002) and give rise to adventitious shoots from root buds throughout the growing season (Tiley 2010). The widespread invasiveness of this weed is often attributed to these characteristics. Canada thistle’s persistence and vegetative spread has also been associated with the plants root carbohydrate reserves (Tworkoski 1992). In general, root carbohydrate levels are lowest in spring and early summer as a result of active shoot growth and begin to increase in late summer and fall in preparation for overwintering. Although environmental conditions principally influence Canada thistle root carbohydrate levels, they can also be impacted when insects feed upon foliar plant tissue (Hein and Wilson 2004).
Despite extensive research, managing Canada thistle remains challenging (Cripps et al. 2011; Tiley 2010). Numerous control tactics for suppression and management of Canada thistle have been investigated, including herbicides (reviewed by Donald 1990) and mechanical tactics such as frequent mowing (Lukashyk et al. 2008), hoeing (Graglia et al. 2006), and tillage (Pekrun and Wilhelm 2004); although these practices can be effective they are management intensive and often costly (Graglia et al. 2006). While individual management tactics such as herbicides and mechanical methods have been moderately successful (Liu et al. 2000), these tactics often don’t provide long-term results (Evans 1984; Travnicek et al. 2005), and therefore can be prohibitively expensive (Sciegienka et al. 2011; Tichich and Doll 2006). Overall, results from previous research suggest that integrated pest management (IPM) may provide longer-lasting Canada thistle suppression when compared to relying on a single chemical or mechanical control tactic (Ferrero-Serrano et al. 2008; Sciegienka et al. 2011).
Biological control is often an important component of IPM programs for invasive weed management. Biological control is an attractive option for many land managers because, if released agents successfully establish and persist, the expense and potential environmental concerns associated with repeated herbicide applications can be avoided (Liu et al. 2000). Although there are several other insects that are approved biological control agents (Winston et al. 2008), Hadroplontus (formerly Ceutorhynchus) litura Fabricius, a phytophagous stem-mining weevil (McClay 2002), is typically considered one of the most effective for Canada thistle in North America (Coombs et al. 2004). Adult H. litura overwinter in the soil and emerge in early spring in synchrony with Canada thistle emergence (Zwolfer and Harris 1966); in eastern North Dakota this typically occurs in late April or early May (E. Burns, unpublished data). Females lay eggs singly or in small groups (2-5) in round feeding cavities on the leaves of Canada thistle rosettes (Zwolfer and Harris 1966; E. Burns, unpublished data). Larvae emerge after 5 to 9 d and mine the mid-vein of the leaf, eventually tunneling into the stem (Zwolfer and Harris 1966). A single stem is often mined by several larvae (average of 3-6) and becomes discolored due to larval feeding and frass (Zwolfer and Harris 1966; Rees 1990; Burns 2012). Mature third instar larvae exit the plant and pupate in the soil (Zwolfer and Harris 1966), which is generally in late June to mid-July in eastern North Dakota when plants are at the pre-bud to bud stage (Burns 2012).
Stem-mining by H. litura larvae during the midsummer is thought to cause more damage to Canada thistle than the foliar chewing damage done by adult weevils in the spring and fall (Liu et al. 2000; Zwolfer and Harris 1966). Though larval mining stresses the plant, it is able to continue growth during and after attack because vascular bundles are not damaged by weevil feeding (Peschken and Wilkinson 1981). While H. litura herbivory does not kill shoots, larval feeding may lead to reduced overwinter survival (Rees 1990), reduction in early season root sugar (Peschken and Derby 1992) and starch content (Hein and Wilson 2004), and increased susceptibility to pathogens and/or adverse environmental conditions (Rees 1990). Overall, previous research results concerning H. litura efficacy are mixed and suggest that H. litura alone does not effectively control Canada thistle (Peschken and Derby 1992; Reed at al. 2006), but that combining additional management tactics might improve Canada thistle suppression (Bacher and Schwab 2000; Ferrero-Serrano et al. 2008; Friedli and Bacher 2001).
One potential option is seeding highly competitive native vegetation along with releasing biocontrol agents. Ferrero-Serrano et al. (2008) found that combining H. litura and a native cool season grass greatly reduced Canada thistle root biomass and hypothesized that H. litura had a positive indirect effect on the grass by decreasing the competitive ability of Canada thistle. Other plants, such as common sunflower (Helianthus annuus L.) may have even greater competitive abilities. Common sunflower is an annual dicot native to North America and found throughout the United States, Canada, and Mexico (Burke et al. 2002). It is similar to Canada thistle in several ways (i.e., it is fast growing and often thrives in disturbed areas) (Burke et al. 2002; Perry et al. 2009), which likely enhances its ability to compete against Canada thistle for sunlight and nutrients (especially nitrogen). As one example, Perry et al. (1990) reported that competition from common sunflower reduced Canada thistle above-ground biomass in greenhouse experiments.
Assuming that biological control and plant competitor impacts will be similar at all locations and under all conditions is naive (Shea et al. 2005). For instance, in North Dakota, anecdotal evidence suggests that H. litura releases have resulted in Canada thistle suppression in some geographical areas, but not in others (Gramig, personal observation). Environmental variability and differences among various Canada thistle biotypes in their response to H. litura are possible explanations for these observations. Successfully combining plant competition with biological control may require understanding how environmental variables, such as water or nutrient availability, mediate plant-plant and insect-plant interactions (Shea 2005). For example, a study that investigated interactive effects of soil nitrogen, plant competition, and various insect biological control agents found that a flower head weevil, Larinus minutus Gyllenhal, reduced spotted knapweed (Centaurea stobe L. subsp. micranthos [Gugler] Hayek) seed production most severely in low nitrogen soils and that reduced plant competition was associated with increased L. minutus numbers per flower (Knochel and Seastedt 2010). Conversely, soil nitrogen and plant competition did not significantly affect impacts of a root-feeding weevil, Cyphocleonus achates Fahr (Knochel and Seastedt 2010). These results demonstrate that soil resources regulate biological control impacts, but that generalizing about the impacts of soil resources on the impacts of insect herbivory or plant competition across species is problematic.
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The goal of this study was to investigate the combined impact of H. litura and common sunflower on Canada thistle under different nitrogen, phosphorus and potassium (hereafter N, P and K) concentrations using field soil in outdoor microcosms. We focused on assessing impacts on plant parameters associated with vigor and reproductive potential, including root biomass, which is seldom investigated due to the difficulty of separating root tissue by species. We hypothesized that weevil attack and plant competition would reduce Canada thistle root biomass, in addition to reducing shoot height, shoot biomass, flower number, and number of side shoots. We further hypothesized that negative impacts of weevil attack and plant competition would be evident with low soil nutrients but not with high soil nutrients.