Progress 11/01/00 to 10/31/04
Outputs
The goal of this research project was to determine how ecological factors, specifically local density and proximity of the exotic weed musk thistle (Carduus nutans), affect non-target damage to native species such as wavyleaf thistle (Cirsium undulatum), by Rhinocyllus conicus, a European weevil used as a biocontrol agent for musk thistle. Theory predicts that co-occurrence with musk thistle could increase or decrease R. conicus damage to native thistles. Damage may be less where wavyleaf and musk thistles co-occur (associational defense), if weevils are drawn from the native by their preferred host, musk thistle. Alternately, co-occurrence may increase damage to native thistles (associational susceptibility), if weevils on musk thistle spillover onto natives. Our results should help reduce R. conicus non-target effects by providing data needed to evaluate two management strategies: 1) reduce musk thistle abundance to minimize weevil spillover onto natives vs. 2) use
musk thistles as a `trap crop' to draw weevils from natives. We quantified R. conicus use of the wavyleaf thistle in three ways: regional surveys (2001-2003), experimental manipulations of R. conicus (2002-2003), and quantification of R. conicus oviposition in relation wavyleaf density on loess soils (2004) in southwest Nebraska. Earlier data from the Sand Hills, where musk thistle is very rare, showed more R. conicus damage to wavyleaf thistles in dense patches (>5 stems in a 3 m radius) than to isolated plants (>20 m from any bolting thistle). Our results provide strong evidence that proximity to musk thistles and greater local musk abundance lead to greater R. conicus damage to wavyleaf thistles. Regional surveys of damage to wavyleaf in relation to proximity to musk patches showed that wavyleaf >30 m from musk patches had fewer R. conicus eggs than wavyleaf in musk patches, supporting the spillover hypothesis. The experimental results, while smaller scale and less definitive
because of the mortality imposed on our transplants by severe drought, are consistent with spillover. Our data on effects of local wavyleaf thistle density on R. conicus damage to wavyleaf thistles in prairie on loess soils showed that wavyleaf at sites with high musk density had more R. conicus eggs than wavyleaf at sites with low musk density. Also, wavyleaf in dense thistle patches had more R. conicus eggs than did isolated thistles. The significant variation among sites in the difference in R. conicus damage between wavyleaf in dense and sparse patches could not be explained by musk abundance, total R. conicus damage, or size of wavyleaf plants at the site. Overall, damage to wavyleaf increased with thistle abundance, especially musk abundance, and all of the work together eliminates the `trap crop' hypothesis and supports the spillover hypothesis for explaining effects of R. conicus on native species. In sum, our results suggest that wavyleaf thistle experiences associational
susceptibility when it co-occurs with musk thistle. The management implications are that R. conicus non-target damage to native Cirsium species will be most effectively limited by reducing musk thistle abundance.
Impacts
By addressing the relationship between R. conicus damage to native thistles and musk thistle abundance/proximity, our results allow land managers to evaluate two opposing strategies for reducing damage by R. conicus to native Cirsium species. These strategies to limit non-target R. conicus damage are either: 1) reduce local musk thistle density and spatial extent, or 2) judiciously use musk thistle, R. conicus preferred host, as a `trap crop' to draw weevils away from fragile native species. Since we consistently found greater damage to native thistles near musk thistle and at sites with more musk thistles, the results strongly suggest that non-target damage by R. conicus will be best controlled by reducing musk thistle abundance. Management based on these data could be important in minimizing R. conicus damage if it invades habitats of rare native Cirsium species, such as the federally listed, threatened C. pitcheri in the U.S. National Lakeshore Parks around Lake
Michigan. More generally, our results provide insight into causes of variation in herbivore-mediated indirect effects among host plant species in basic and applied contexts. First, they support hypotheses that the magnitude of non-target damage by biocontrol insects to adopted native host plants will be strongly affected by the local abundance of the exotic, target weed. Second, they provide data consistent with suggestions in the community ecology literature that insect herbivore abundance is crucial to determining where and when herbivore-mediated indirect effects among alternate host species will occur.
Publications
- Russell, F. L. and S. M. Louda. 2004. Phenological synchrony affects interaction strength of an exotic weevil with Platte thistle, a native host plant. Oecologia 139:525-534
- Rand, T. A., F. L. Russell and S. M. Louda. 2004. Local vs. landscape scale indirect effects of an invasive weed on native plants. Weed Technology 18:1250-1254.
- Louda, S. M., T. A. Rand, F. L. Russell and A. E. Arnett. 2005. Assessment of Ecological Risks in Biocontrol: Input from Retrospective ological Analyses. Biological Control: in press.
- Russell, F. L. and S. M. Louda. 2005. Insect abundance, phenology and associational defense influence floral herbivory by an invasive insect. Oecologia. In review.
- Russell, F. L., S. M. Louda and T. A. Rand. 2005. Variation in herbivore-mediated indirect effects of an invasive plant on a native plant. In draft.
- Russell, F. L. and S. M. Louda. 2006. Spatial variation in Rhinocyllus conicus response to density of an adopted native host plant, Cirsium undulatum. In prep.
- Russell, F. L. and S. M. Louda. 2006. Does weed density explain variation in Rhinocyllus conicus damage to a target host plant, Carduus nutans (musk thistle)? In prep.
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Progress 10/01/02 to 09/30/03
Outputs
The goal of our research is to determine how ecological factors, specifically local density and proximity of the exotic weed musk thistle (Carduus nutans), affect the amount of damage to a native thistle species, wavyleaf thistle (Cirsium undulatum), by Rhinocyllus conicus, a European weevil introduced to North America for biological control of musk thistle. This project will contribute to the development of management strategies that manipulate musk thistle populations to reduce negative, non-target effects of R. conicus on native plants. In 2003 we quantified R. conicus damage to naturally-occurring C. undulatum in relation to distance to musk thistle patches and musk patch density at 14 sites in southwestern Nebraska. Wavyleaf thistles more than 30 m from musk patches had significantly fewer R. conicus egg cases per flower head than wavyleaf thistles in musk patches. Musk patch density did not affect the amount of R. conicus damage of wavyleaf thistles. However,
abundance of R. conicus adults in nearby musk patches explained a significant amount of variation among sites in damage to wavyleaf thistles by R. conicus. Damage to wavyleaf thistles was greater where R. conicus adults were more abundant in nearby musk patches. These results are consistent with results from identical surveys conducted in 2001 and 2002, providing strong evidence that musk thistle indirectly, negatively affects wavyleaf thistle by increasing R. conicus floral and seed herbivory. Follow-up data will be collected in summer 2004. We also conducted two transplanting experiments in 2003. The first experiment examined interaction effects of distance from musk patches and local densities of wavyleaf thistles on R. conicus use of wavyleaf. The second experiment examined effects of R. conicus abundance in musk patches on use of neighboring wavyleaf. In the first experiment, we found a significant interaction effect of distance and local wavyleaf density on the number of R.
conicus adults observed on wavyleaf. More adults were observed per plant in dense wavyleaf stands near musk thistles, but wavyleaf density had no effect away from musk thistles. Use of wavyleaf by R. conicus was not affected by distance to musk patches in this experiment. In the second experiment the ratio of R. conicus adults to musk thistle flower heads in the musk patch did not affect use of neighboring wavyleaf. In both experiments, our ability to detect treatment effects was limited by the small number of wavyleaf transplants that bolted. We suggest that the best management strategy for reducing negative, non-target effects of R. conicus on native thistles is to reduce sizes and spatial extents of musk thistle populations. This recommendation is based on results from three years of surveys of R. conicus use of naturally-occurring wavyleaf thistles. These results consistently showed decreased use of wavyleaf thistles with increasing distance from musk patches for distances up to
220 meters.
Impacts
Unintended, negative effects on native species are the primary concern with the safety of introducing exotic, herbivorous insects to limit the spread of weed populations. Our research will contribute to the development of management strategies to reduce detrimental, non-target effects of such an insect, Rhinocyllus conicus, on native thistle species by manipulating populations of musk thistle (Carduus nutans), the exotic, intended target of R. conicus. Specifically, we are evaluating the hypothesis that proximity to musk thistle and local density of musk thistle will affect the amount of damage to native thistles by R. conicus. Management strategies based on our findings could prove critically important in limiting R. conicus impact if it invades habitats of endangered native Cirsium species, such as C. pitcheri in Michigan lakeshores and sand dunes.
Publications
- Rand, T.A., F. L. Russell and S. M. Louda. 2004. Local vs. landscape scale indirect effects of an invasive weed on native plants. Weed Technology. submitted.
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Progress 10/01/01 to 09/30/02
Outputs
The goal of our research is to determine how ecological factors, specifically local density and proximity of the exotic weed musk thistle (Carduus nutans), affect the amount of damage to a native thistle species, wavyleaf thistle (Cirsium undulatum), by Rhinocyllus conicus, a European weevil introduced to North America for biological control of musk thistle. This project will contribute to the development of management strategies that use manipulation of musk thistle populations to reduce negative, non-target effects of R. conicus on native plants. In 2002 we conducted two experiments to determine whether proximity to musk thistle patches causes high levels of damage to wavyleaf thistles by R. conicus. In the first experiment, we transplanted wavyleaf thistles to create small, sparse and dense wavyleaf patches at 5 m and 50 m from musk patches. In the second experiment, we transplanted wavyleaf thistles 5 m and 15 m from small musk patches and we manipulated the
abundance of R. conicus adults in the musk patches to examine whether high ratios of weevils to their preferred resource, musk thistle, causes spillover onto native plants. In the first experiment, we found adult R. conicus on wavyleaf thistles 5 m from musk patches, but not on thistles transplanted 50 m from musk patches. The number of R. conicus egg cases on wavyleaf flower heads, however, did not differ significantly between transplants 5m and transplants 50 m from musk. The second experiment showed a trend toward decreased R. conicus use with increased distance from musk thistles, but the trend did not reach statistical significance. An extreme drought in 2002 significantly shortened the field season by reducing transplant survival rates in both experiments. If drought is less severe in 2003, we expect stronger patterns from both experiments. Also, given the drought in 2002, we should be able to extend the funded research (using the savings to the grant from this summer) to
follow-up on these experiments an additional summer (2004) if we are allowed to extend the final date for the proposed work. The opportunity to do this will be especially important if 2003 is also a drought year, as is forecast. In a second component of this project, we quantified number of R. conicus egg cases on naturally-occurring wavyleaf thistles in relation to distance to musk patches and local musk thistle density at 12 sites in southwestern Nebraska. We found significantly less damage to native thistles 80 m from musk thistle patches than within musk patches. Local density of musk thistle did not significantly affect use of wavyleaf thistle by R. conicus. These results are consistent with results from an identical survey we conducted in 2001. Our results to date from the first two years of field work support the hypothesis that musk thistle indirectly, negatively affects wavyleaf thistles by increasing rates of flower and seed herbivory by R. conicus at local spatial scales.
Further, our results from 2001 and 2002 suggest that the best management strategy for reducing negative, non-target effects of R. conicus on native thistles is to reduce the area covered by musk thistle.
Impacts
Unintended, negative effects on native species are the primary concern with the safety of introducing exotic, herbivorous insects to limit the spread of weed populations. Our research will contribute to the development of management strategies to reduce detrimental, non-target effects of such an insect, Rhinocyllus conicus, on native thistle species by manipulating populations of musk thistle (Carduus nutans), the exotic, intended target of R. conicus. Specifically, we are evaluating the hypothesis that proximity to musk thistle and local density of musk thistle will affect the amount of damage to native thistles by R. conicus. Management strategies based on our findings could prove critically important in limiting R. conicus impact if R. conicus invades habitats of endangered native Cirsium species, such as C. pitcheri in Michigan lakeshore sand dunes.
Publications
- Initial results from the first year of this study were included in a manuscript now in press. Also, the survey data for the first two years are now analyzed and a draft manuscript is waiting for the 2003 data prior to submission. Results also have been presented in two oral presentations, including an invited presentation at a meeting of the National Association of County Officials in North Platte NE, and another invited presentation to the Nebraska Department of Agriculture Weeds Supervisory Board.
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Progress 10/01/00 to 09/30/01
Outputs
In summer 2001 we described variation in R. conicus use of wavyleaf thistles (Cirsium undulatum) with musk thistle density and distance from musk thistle patches at 21 sites in central Nebraska mixed grass prairie. In addition, we experimentally quantified the relationship between number of R. conicus ovipositions on wavyleaf thistle flower-heads and seed loss. R. conicus use of wavyleaf thistles decreased with distance from musk thistle patches at sites that had high numbers of R. conicus eggs per musk thistle plant. Distance did not significantly affect use of wavyleaf thistles at sites with low numbers of eggs per musk thistle plant. The difference between these sites in effect of distance on use of native thistles resulted from significantly greater use of native thistles in or near (30 - 50 m) musk patches at sites with high egg loads on musk thistles than at sites with low egg loads on musk thistles. Use of native thistles far from musk patches (200m) did not
differ between sites with high and low egg loads on musk thistles. These data indicate that where weevils are abundant relative to musk thistle they will spillover onto non-target host plants near musk thistles. These preliminary results suggest that musk thistle cannot be used as a trap crop to draw R. conicus from native thistles. Instead, they suggest that musk thistles should be eradicated near native thistles to help prevent use of natives R. conicus. Data from our experiment have not yet been analyzed. Casual observations during summer 2001 suggested that local density of wavyleaf thistles might affect the amount of use of these thistles by R. conicus. Therefore, we constructed a transplant experiment to examine effects of wavyleaf thistle density and distance from a musk thistle patch on R. conicus use. We transplanted wavyleaf thistles to create dense (12 bolting wavyleaf / 20 sq. m) or sparse (3 bolting wavyleaf / 20 sq. m) patches at two distances (5 m and 50 m) from a musk
patch. Data from this experiment will be collected during summer 2002.
Impacts
Rhinocyllus conicus, an exotic biocontrol agent introduced in the United States to control musk thistle (Carduus nutans), previously has been shown to use and reduce seed production by native North American thistles. Data that we collect on ecological factors affecting use of native thistles by R. conicus will help establish management practices to reduce unintended, negative effects of this weevil on native species. Specifically, we are quantifying effects of musk thistle proximity and local density on use of native thistles by R. conicus. These data will show whether musk thistle can be used as a trap crop to draw R. conicus from native thistles or whether musk thistles should be aggressively eradicated near native thistles to prevent spillover of R. conicus from its intended host. This information will help preserve the endangered thistle, Cirsium pitcheri, if R. conicus disperses or is distributed to sand dunes habitat around the Great Lakes.
Publications
- No publications reported this period
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