Integration of Biological Control and Native Cover Crops for Canada Thistle Control

Weevil Pressure.
The foliage of all Canada thistle plants in weevil addition treatments sustained similar amounts of heavy feeding damage by adult weevils (i.e., windowpaning damage to upper and lower leaf surfaces; Zwolfer and Harris 1966; Prischmann-Voldseth et al. 2012). The majority of Canada thistle plants in weevil addition treatments sustained some larval damage (main stem damaged = 72%, not damaged = 28%), and the incidence of larval weevil presence was statistically similar among treatments (PC x SN, P = 0.858; PC, P = 0.739; SN, P = 0.096). On average, experimental plants sustained relatively light levels of larval damage. Plants in a field setting experience variable levels of larval pressure (Rees 1990; Liu et al. 2000; Burns 2012); in ND a late-summer survey of 192 weevil infested Canada thistle stems at 46 sites where weevils were released revealed that 43% were lightly damaged (rating of 2) while 58% had moderate or severe damage ratings (unpublished data). Larval damage ratings from Canada thistle plants grown with low levels of soil nutrients (mean ± SE = 1.75 ± 0.21) were lower than plants grown with high levels of soil nutrients (2.44 ± 0.24; Year x PC x SN, P = 0.569; Year x SN, P = 0.191; PC x SN, P = 0.346; Year, P = 0.191; SN, P = 0.045), although larval damage was similar when common sunflower was present (2.00 ± 0.24) or not (2.19 ± 0.25; Year x PC, P = 0.569; PC, P = 0.569). The development and performance of herbivorous insects is often linked to plant quality, and endophagous insects (e.g. stem-miners, gall makers) often do better on vigorous plants growing in resource rich environments (Price 1991, 1997; De Bruyn et al. 2002). Additionally, second generation adult weevils were not observed at the end of the experiment, so late season adult feeding did not impact our results.
Canada Thistle Size.
Canada Thistle Height. Canada thistle main stem height was affected by weevil herbivory, common sunflower presence, and soil nutrients, but interactions among treatments were absent (Table 1). Canada thistle plants exposed to weevils produced main stems that were 15% shorter than plants not exposed to weevils (Figure1A). This is a slight height difference that was not accompanied by a corresponding decline in shoot biomass (Figure 2A). A field study that quantified Canada thistle height as affected by several insect biological control agents including H. litura determined that plants were often, but not always, shorter at the control site than at release sites (Liu et al. 2000). The authors concluded that environmental differences among sampling times may have been responsible for these inconsistent results. They also speculated that insect damage may have stimulated an elongation response. Another study found that stem mining by H. litura did not affect Canada thistle stem height (Peschken and Wilkinson 1981). However, both of these studies were conducted in the field, where differences in soil nutrients or other herbivorous insects could have influenced the results.
Canada thistle plants grown alongside common sunflower produced main stem shoots that were 28% shorter than shoots produced by plants grown without common sunflower neighbors (Figure 1B). This reduction in Canada thistle plant height associated with competition is not mirrored by other studies that measured Canada thistle height as affected by plant competition and may perhaps be attributed to functional differences in the plant competitors used in the various studies. Bacher and Schwab (2000) used an herbaceous seed mix and Cripps et al. (2010) chose a mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.). Both studies reported no difference in height between Canada thistle plants grown with and without plant competitors (Bacher and Schwab 2000; Cripps et al. 2010).
Common sunflower has a fast growth rate, which gives the plant the ability to quickly shade plant neighbors, thus reducing the neighboring plant’s growth (Perry et al. 2009). Previous research has reported negative impacts of shading on Canada thistle growth (Evans 1984); these results may help to explain the above results. Although common sunflower presence had a negative impact on Canada thistle height in our study, competition can also lead to height increase in response to shading (Aarssen 1995; Jaremo et al. 1996). Common sunflower presence may have exerted competition for light above ground and competition for soil resources below ground. The reduction of soil nutrients imposed by common sunflower competition could be primarily responsible for the reduction in Canada thistle height.
Canada thistle plants grown in low nutrient soil produced main stem shoots that were 30% shorter than shoot produced by plants growing in high nutrient soil (Figure 1C). Most previous results regarding Canada thistle growth responses to nutrients are from studies that focused on only the effect of nitrogen. One previous study (McIntyre and Hunter 1975) found that Canada thistle shoot length was reduced by 53% when plants were grown with 31.5 kg ha-1 N compared to plants grown with 315 kg ha-1 N (the N levels in this study were 142 and 60 kg ha-1, for comparison).
Canada Thistle Main Stem Shoot Biomass. Our experiment did not detect an impact of H. litura weevil presence on Canada thistle main stem shoot dry biomass (Table 1, Figure 2A). These findings are similar to those reported by Collier et al. (2007) and Ferrero-Serrano et al. (2008), who both reported no effect of weevil attack on Canada thistle shoot biomass. Conversely, Sciegienka et al. (2011) reported that H. litura herbivory negatively impacted Canada thistle shoot biomass in greenhouse experiments. The differences in reported impacts of H. litura on Canada thistle shoot biomass could be attributed to genetic differences among the weevils, differences in attack severity, and also impacts of differential environmental conditions on biological control agent performance (Menalled et al. 2004). In our study, while adult feeding damage was heavy on all rosettes, larval damage was light overall, and this may be a potential reason why weevil presence did not affect main stem shoot biomass.
Canada thistle plants grown with common sunflower neighbors or in low soil nutrients produced less main stem shoot biomass (64 and 74 % less biomass, respectively) than Canada thistle plants grown in high soil nutrients or without common sunflowers (Table 1; Figure 3). Perry et al. (2009) found that common sunflower competition resulted in reduced Canada thistle shoot biomass under controlled greenhouse conditions. Friedli and Bacher (2001), who grew Canada thistle in competition with perennial ryegrass, Italian ryegrass (Lolium perenne L. ssp. multiflorum (Lam.) Husnot), and orchardgrass (Dactylis glomerata L.) also found that competition reduced shoot biomass. In contrast, Canada thistle biomass was not affected when it was grown with alkali sacaton (Sporobolus airoides (Torr.) Torr.) and needle and thread grass (Hesperostipa comata (Trin. & Rupr.) Barkworth) (Ferrero-Serrano et al. 2008). Differences among studies may perhaps be attributed to the physiological and morphological differences in the specific plant competitors used in experiments or to experimental differences such as pot size, water or nutrient levels, and planting density. Furthermore, our study was performed outdoors and subject to variable weather conditions whereas the previous studies were conducted in highly controlled greenhouse environments. The natural weather fluctuations in our study may have played a large role in the results. Additionally, Ferrero-Serrano et al. (2008) and Sciegienka et al. (2011) studies were conducted in 7.6 L plastic containers whereas in our study, microcosms were 19 L plastic containers. Experiments have demonstrated that plant growth can be negatively affected by pot size and that often when plants are grown in pots that are too small they may become root bound and a general reduction in growth occurs (Townend and Dickinson 1995) and therefore their treatments may have been more detrimental due to potentially stressed root bound Canada thistle plants.
Canada Thistle Reproductive Potential.
Canada Thistle Flower Number. Weevil presence, common sunflower presence, and soil nutrients each influenced Canada thistle flower number, but no interaction terms were significant (Table 1). Canada thistle plants that were exposed to H. litura weevils produced 40% fewer flowers than plant that were not exposed to weevils (Figure 3A). Furthermore, flower number: main stem shoot biomass ratios did not differ between weevil treatments (P=0.115, data not shown), demonstrating that this effect resulted from more than a simple reduction of biomass. Our study is the first (to our knowledge) to document a negative impact of H. litura presence on Canada thistle flower number, but we did not quantify seeds per flower. However, one previous study found that exposure to H. litura weevils and seed head parasites did not reduce the number of seeds per seed head (Larson et al. 2005). Also, Canada thistle is a dioecious plant and male Canada thistle shoots have been shown to produce fewer flowers than female shoots (Becker et al. 2008). We did not identify the gender of individual Canada thistle plants, so this could be a source of error in our study.
Canada thistle plants grown with common sunflower neighbors produced 65% fewer flowers than Canada thistle plants grown alone (Figure 3B). Plant competition also negatively impacted Canada thistle flower production in a previous study in which a mixture of three grass species (perennial ryegrass, Italian ryegrass, and orchardgrass) reduced Canada thistle flower production by 81% compared to Canada thistle plants grown without grass competition (Friedli and Bacher 2001). In our study, flower number:main stem shoot biomass ratios did not differ between common sunflower treatments (data not shown), indicating that the decrease in flower number associated with common sunflower presence scaled with biomass reduction. This was likely caused by a reduction in soil nutrients available to Canada thistle plants.
Canada thistle plants grown in low soil nutrients produced 68% fewer flowers than Canada thistle plants grown in high soil nutrients. A previous study (Loehle 1987), along with our study, demonstrated that increased soil nutrients are associated with production of more reproductive structures by perennial plants. A model constructed by Loehle (1987), which evaluated energy partitioning in clonal plants, predicted that increased soil nutrients would be associated with a decrease in the cost of producing sexual reproductive structures, which ultimately leads to increased seed production. Our results, which showed that Canada thistle plants grown in stressful low soil nutrient environments produced fewer flowers than Canada thistle plants grown in high soil nutrients, support this theoretical model. Also, the ratio of flowers: main stem biomass was 45% greater for Canada thistle plants grown in high soil nutrients (P=0.006, data not shown). This demonstrates that the increase in flower number in high soil nutrients was not a simple function of increasing plant biomass.
Canada Thistle Side Shoot Number. Side shoot production is important because it is indicative of vegetative reproductive capacity, which is the primary means by which Canada thistle spreads (Donald 1994). We failed to detect any impact of H. litura weevil presence on Canada thistle side shoot number (Table 1, Figure 4A). In a microcosm study performed under controlled greenhouse settings, Sciegienka et al. (2011) reported that Canada thistle plants exposed to H. litura produced on average 29% fewer side shoots than Canada thistle plants without weevil exposure. In our study not every Canada thistle plant that received the H. litura treatment sustained visible larval stem mining damage. Sciegienka et al. (2011) did not report the extent of H. litura larval mining; if the damage was more extensive than in our study, this may have been another source of variation between the two studies that could have played a large role in the discrepancy between results. Differences could also be related to the environmental conditions in which the two experiments were conducted (greenhouse versus outdoor).
Canada thistle side shoot number was affected by both plant competition and soil nutrient level (Table 1). Canada thistle plants grown with common sunflower plants produced 57% fewer side shoots than Canada thistle plants grown without common sunflower plants (Figure 4B). These results are not consistent with results reported by Ferrero-Serrano et al. (2008), who reported plant competition by alkali sacaton and needle and thread grass grown together and alone with Canada thistle in microcosm experiments had no impact on the number of side shoots produced by Canada thistle throughout the experiment. We hypothesize that differences in competitor functional type (i.e., perennial grass vs. annual forb) may be the basis for these differential results.
Canada thistle plants grown in low soil nutrients produced 39% fewer side shoot than plants grown in high soil nutrients (Figure 4C). The negative impact of low soil nutrients on Canada thistle side shoot production is similar to findings by McIntyre and Hunter (1975), who reported a 68% decrease in side shoot production by Canada thistle plants grown in soil treated with 21 ppm nitrogen compared to Canada thistle plants grown in soil treated with 210 ppm nitrogen.
Canada thistle Final Root Biomass. Canada thistle root biomass was not affected by either weevil presence or soil nutrients (Table 1, Figures 5A and 5C). The lack of weevil impact on root biomass is consistent with previous research showing that H. litura herbivory failed to reduce Canada thistle root biomass in two of three experimental runs (Collier et al. 2007). However, Ferrero-Serrano et al. (2008) and Sciegienka et al. (2011) both reported a negative impact of H. litura on Canada thistle root biomass. These two studies found that H. litura attack decreased Canada thistle root biomass by 81 and 18%, respectively. Variables that could have caused differential responses include the size of the pots or microcosms, the initial size of the Canada thistle plants, the intensity or severity of weevil attack, soil resource status, and climatic variables.
Root biomass is critically important for Canada thistle spread and persistence, but common sunflower plant presence was the only treatment that negatively affected Canada thistle root biomass (Table 1, Figure 5B). Common sunflower presence resulted in an 83% reduction in root dry biomass. Previous studies (Ferrero-Serrano 2008; Friedli and Bacher 2001) have reported reductions in Canada thistle root biomass resulting from intraspecific competition with grass competitors. Surprisingly, soil nutrients did not affect Canada thistle root biomass (Table 1, Figure 5C), in contrast to previous results that showed when Canada thistle infestations were treated with 100 kg ha-1 nitrogen fertilizer, root biomass excavated from a 1 and 2 yr old stand from the top 20 cm of soil was approximately three times greater than the root mass from unfertilized plots (Nadeau and Vanden Born 1990). These results are somewhat perplexing because the mechanism for negative impacts on root biomass resulting from competition would presumably be competition for soil nutrients. But since soil nutrients did not affect root biomass, perhaps the mechanism for changes in root biomass caused by common sunflower presence can be attributed to changes in root:shoot ratios caused by shading. Also, root:shoot ratios differed between Canada thistle plants grown with common sunflower and those grown without (P=0.001, data not shown).
Summary and Management Implications.
Though interactive effects among H. litura herbivory, soil nutrients, and common sunflower presence were absent, these treatments did have substantial individual and additive effects on many measures of Canada thistle growth and reproductive output. Our results indicate that both H. litura weevil attack and interspecific plant competition can reduce Canada thistle shoot height, which is a proxy for size and thus competitive ability and general robustness. Therefore, integrating interspecific competition with insect biological control could place additional stress or pressure on Canada thistle populations beyond that supplied by biocontrol alone. But our results also highlight the importance of selecting a plant competitor with specific functional characteristics, since other studies have demonstrated no impact of intraspecific competition on Canada thistle height. Common sunflower proved to be a good competitor against Canada thistle, but this species is often also considered undesirable by land managers. Often, plant species that compete strongly enough to inhibit invasive perennials will also inhibit desirable native vegetation (Perry et al. 2009). Seeding of a fast-growing competitive annual forb such as common sunflower could potentially be the first step in a restoration process which would be followed by restoration of more desirable perennial native vegetation.
New Canada thistle infestations occur via wind dispersed seed and Canada thistle seed also remains viable in the soil for long periods of time (Piper and Andres 1995; Tiley 2010); therefore, decreasing seed inputs into the soil seed bank would be advantageous for future restoration efforts. Our research demonstrated the ability of H. litura and common sunflower presence to negatively impact Canada thistle flower production. Although these results are encouraging, we did not quantify the number of viable achenes produced by each flower head nor did we discriminate among male vs. female shoots, so our results should be considered preliminary.
Difficulties controlling Canada thistle are often attributed to its deep, extensive, and regenerative root system, which facilitates invasiveness, persistence, and tolerance to control measures (Lukashyk et al. 2008; Tiley 2010). However, similar previous studies investigating the impacts of plant competition on Canada thistle did not quantify root biomass impacts (Edwards et al. 2000; Perry et al. 2009). Our research demonstrated the utility of using common sunflower as a plant competitor against Canada thistle, because competition by common sunflower negatively impacted multiple morphological characteristics of Canada thistle, most notably a reduction in root biomass. Chemical control measures often fail to translocate sufficiently (Armel et al. 2005; Petersen and Swisher 1985) to the root system of Canada thistle to cause detrimental damage to the entire network of clonal plants. Control measures that could target and negatively impact the prolific Canada thistle root system would therefore be beneficial.
Canada thistle patches spread and creep into new territories via adventitious shoot production throughout the growing season, making the patches larger and more difficult to manage (Tiley 2010). In our study, low soil nutrients and plant competition reduced side shoot numbers. Planting common sunflower could potentially slow down the rate of expansion of Canada thistle patches, especially in areas where soil nitrogen level are relatively low or in areas where soil nitrogen could be reduced via filter strips or other approaches that mitigate nitrogen pollution (Morghan and Seastedt 1999; Paschke et al. 2000; Vasquez et al. 2008).
The success of biocontrol agents depends not only on the control agent’s ability to damage individual plants, but also on the ecology of both the agent and the weed species in question (McFadyen 1999). Our results suggest that H. litura weevils alone (at least at the level of weevil pressure present in our study) will have minimal impacts on the spread and persistence of Canada thistle, because weevil herbivory does not appear to reduce root biomass, which is the primary means by which this weed spreads and persists. Most practitioners acknowledge that single biological control agents rarely are sufficient to effectively manage an invasive weed species (DiTomaso 2000). Clearly, no single biocontrol measure appears to be a silver bullet for managing Canada thistle. However, results of our research indicate that combining plant competition with biological control could be a potentially effective strategy for reducing population densities of this troublesome weed, especially in areas where low soil nutrients somewhat limit Canada thistle growth and proliferation.

• Tables and Figures