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Relationships among Edaphic, Climatic, and Vegetation Conditions at Release Sites and Aphthona nigriscutis Population Density
Biological Control 22, 46 –50 (2001) doi:10.1006/bcon.2001.0955, available online at http://www.idealibrary.com on
Relationships among Edaphic, Climatic, and Vegetation Conditions at Release Sites and Aphthona nigriscutis Population Density James S. Jacobs, 1 Roger L. Sheley, 2 Neal R. Spencer, 3 and Gerry Anderson 4 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana 59717 Received July 18, 2000; accepted April 17, 2001
Key Words: Euphorbia esula; leafy spurge; noxious weed management; biological control.
The role of site conditions on the success of leafy spurge control by Aphthona nigriscutis is poorly understood. Our objective was to determine the relationships among the climatic and edaphic conditions of A. nigriscutis release sites, A. nigriscutis population density, leafy spurge cover and density, and grass cover. We sampled 13 field sites in eastern Montana where A. nigriscutis had been established for 6 to 8 years. Sampling was done in June and July of 1998 along a randomly determined transect from the center of each release point to densely infested leafy spurge. Data were analyzed by step-down regression procedures using A. nigriscutis density, leafy spurge cover and density, cover of grass, forbs, litter, and bare ground as the dependent variables. Independent variables included vegetative and insect data sampled as well as site conditions. Average annual precipitation was the only edaphic or climatic site characteristic that influenced A. nigriscutis density; the beetle population density increased as precipitation increased. There was a negative association between A. nigriscutis and number of leafy spurge flowering stems and leafy spurge cover. Grass, litter, and bare ground cover increased as A. nigriscutis numbers increased. Within the range of site conditions sampled in this study, successful establishment and population expansion of A. nigriscutis appears to be related to the cover of leafy spurge, grass, forbs other than leafy spurge, litter, and bare ground. This information may aid in site selection for release of this important biological control agent or in creating conditions for optimum establishment. ©
INTRODUCTION
Biological control is important for the management of some rangeland weed species such as leafy spurge (Euphorbia esula L., Euphorbiaceae) because management using herbicides is rarely cost-effective, and herbicide use is restricted in riparian areas where this invasive weed often dominates. This long-lived, deeprooted perennial weed is rapidly spreading in the United States and Canada (Lajeunesse et al., 1999) and has an economic impact of $130 million each year in Montana, Wyoming, and South Dakota (Bangsund et al., 1993; Leitch et al., 1994). Development of an effective biological control program for leafy spurge requires an understanding of the relationships among the weed, the control agent, and the site conditions for successful establishment and maintenance of biological control agents. Thirteen species of biological control insects for leafy spurge have been approved by the USDA for introduction into the United States (Rees et al., 1996). Six species of the root- and shoot-feeding flee beetles in the genus Aphthona are well established and believed to provide the greatest control of leafy spurge (Lym, 1998). Aphthona nigriscutis Fourdas (Coleoptera: Chrysomelidae), the first successful Aphthona spp. to establish, was introduced into Canada in 1983 and into the United states in 1989 (Rees et al., 1996). It is currently established in Colorado, Idaho, Montana, Nebraska, North Dakota, Oregon, and Washington. The success rate of A. nigriscutis establishment has been variable, and at many sites where the beetle has established, the impact on leafy spurge has been low (Lym, 1998). Rees (1994) reported that in 1991 only 14 of 34 A. nigriscutis releases established. Habitat type, soil conditions, and the density of spurge have been suggested to affect A. nigriscutis establishment. In Canada, A. nigriscutis established well on Stipa grassland habitat sites (Rees et al., 1996). A. nigriscutis is
2001 Academic Press
1 To whom correspondence should be addressed. Research Assistant Professor, Department of Land Resources and Environmental Sciences, Montana State University, Bozeman. Fax: (406) 994-3933. E-mail: [email protected]. 2 Associate Professor, Department of Land Resources and Environmental Sciences, Montana State University, Bozeman. E-mail: [email protected]. 3 Laboratory Director, Northern Plains Agricultural Research Laboratory, USDA-ARS, Sidney, MT. E-mail: nspencer@sidney. ars.usda.gov. 4 Co-Principal Investigator, TEAM Leafy Spurge, USDA-ARS, Sidney, MT. E-mail: [email protected].
1049-9644/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.
46
47
RELATIONSHIPS BETWEEN SITE CONDITIONS AND Aphthona nigriscutis
TABLE 1 Characteristics of 13 A. nigriscutis Release Sites Sampled Site
Date established
Number of insects released
1
7/6/90
1000
2
7/6/90
1000
3
7/6/90
500
4
7/2/91
1000
5
7/27/91
500
6
7/6/90
500
7
7/13/92
500
8
7/2/91
500
9
7/6/90
500
10
7/2/91
500
11
7/18/90
500
12
7/2/91
1000
13
7/2/91
500
Location 46°40⬘01.88 N 104°30⬘06.53W 46°38⬘07.36 N 104°32⬘42.87W 46°08⬘37.52 N 105°05⬘22.48W 45°28⬘47.17 N 105°20⬘18.97W 45°08⬘37.52 N 105°48⬘11.21W 48°35⬘52.89 N 109°25⬘27.00W 48°34⬘01.73 N 109°29⬘34.64W 48°33⬘19.06 N 108°32⬘34.67W 48°21⬘22.98 N 108°21⬘22.27W 48°21⬘14.18 N 108°21⬘21.74W 48°05⬘36.29 N 104°25⬘52.81W 48°13⬘21.26 N 104°12⬘26.56W 48°12⬘53.49 N 104°16⬘14.90W
Elevation (m)
Habitat type
Precipitation (mm)
% Sand
% Silt
% Clay
825
AGSP/AGSM
330
66.1
23.8
10.0
853
STGR/BOGR
380
67.2
26.4
16.3
853
STGR/BOGR
330
57.3
31.4
11.3
945
STGR/BOGR
380
6.6
40.9
52.5
1097
STGR/BOGR
280
73.7
17.6
8.8
792
STGR/BOGR
280
53.7
36.3
10.0
762
STGR/BOGR
280
51.2
36.3
12.5
792
STGR/BOGR
280
62.1
20.2
17.7
701
STGR/BOGR
280
45.8
32.8
21.4
701
STGR/BOGR
280
56.3
27.6
18.8
640
PODE/COST
330
31.8
45.5
22.7
701
STGR/BOGR
380
77.3
10.1
12.6
671
STGR/BOGR
380
58.5
25.2
16.4
reported to prefer sites with sandy, well-drained soils (Rees et al., 1996). However, Lym (1998) observed that this species did not establish on sites with soil texture greater than 80% sand. The probability of A. nigriscutis establishment is also believed to be reduced where leafy spurge density is greater than 320 stems/m 2 (Lym, 1998). An understanding of the relationships among edaphic and climatic conditions and vegetation at release sites and A. nigriscutis establishment may help determine where this biological control agent may establish and have the greatest impact on leafy spurge. Therefore, the objectives of this study were to determine the relationships among edaphic and climatic conditions at release sites, habitat type, and A. nigriscutis impacts on leafy spurge population density and cover, grass cover, forbs, litter, and bare ground. We hypothesize that A. nigriscutis populations and grass cover will be related to soil texture, precipitation, and leafy spurge densities. MATERIALS AND METHODS
Field sites. To quantify the relationships among site conditions, A. nigriscutis, leafy spurge, and grass, we intensively sampled 13 A. nigriscutis field release sites in eastern Montana, U.S.A. Only sites with successful establishment of A. nigriscutis were sampled. A. nigriscutis was released at these sites from 1990 to
1992. Sites were selected to represent a range in habitat types, elevation, 30-year average annual precipitation, and soil texture. Three habitat types that are commonly infested with leafy spurge were selected. They were Agropyron spicatum (Pursh) Scribn. & J.G. Sm./Agropyron smithii Rydb. (Mueggler and Stewart, 1980) cool-season grasslands, Stipa comata Trin. & Rupr./Bouteloua gracilis (Kunth) Lag. ex Griffiths (Mueggler and Stewart, 1980) mixed cool-season/ warm-season grasslands, and Populus deltoides Marsh. (cottonwood)/Cornus stolonifera Michx. (Hansen et al., 1996) river bottoms dominated by cottonwood. The elevation at the release sites ranged from 640 to 1097 m. Average annual precipitation ranged from 280 to 380 mm. The soil texture of the sites, determined by using particle-size analysis by a hydrometer (Gee and Bauder, 1986), ranged from 7 to 77% sand and 9 to 53% clay. Site conditions and the number of insects released for each site are listed in Table 1. Sampling. Sites were sampled from late-June to mid-July 1998 during peak adult A. nigriscutis emergence based on air degree-day accumulation (Hansen, 1996). Each site was divided into strata that were determined by distance from the original point of A. nigriscutis release. A transect was established from the point of release and extended in a randomly chosen
48
JACOBS ET AL.
TABLE 2 Regression Model Predicting A. nigriscutis Density a Regressor variable
Coefficient
P value
Intercept Precipitation Spurge cover ⴱ strata Grass cover Other forb cover Litter cover Bare ground cover
⫺57.0000 2.0433 ⫺0.5263 3.0867 ⫺2.1290 3.6130 2.4370
0.0013 0.0003 0.0076 0.0127 0.1099 0.0006 0.0179
a
P ⫽ 0.0001, R 2 ⫽ 0.49.
direction to an area of dense leafy spurge past the apparent point of A. nigriscutis expansion. The transect was divided into five strata of equal length. A. nigriscutis density was measured within each strata by sweeping 10 randomly located 1-m 2 plots of vegetation using 3 sweeps of a 381-mm sweep net. Insects captured in sweeps were placed into plastic bags, frozen at ⫺18°C, and subsequently counted in the laboratory. Leafy spurge density and cover, cover of grass, other forbs, litter, and bare ground were sampled in 10 randomly located 0.2- by 0.5-m frames within each strata. Leafy spurge flowering stems and stems without flowers were counted in each frame. Cover was visually estimated for leafy spurge, other forbs, grass, litter, and bare ground using six cover classes: 0 –5% (1), 6 –25% (2), 26 –50% (3), 51–75% (4), 76 –95% (5), and 96 –100% (6) of the ground covered (Daubenmire, 1970). Statistical analysis. The relationships among site characteristics, population density of leafy spurge, all cover measurements, and A. nigriscutis density were analyzed using the step-down linear regression procedure (Neter et al., 1985). Site characteristics used as predictor variables were precipitation, elevation, percentage sand, percentage silt, percentage clay, sand: silt ratio, sand:clay ratio, silt:clay ratio, and strata (distance from release point). Vegetation characteristics used as predictor variables when they were not the regressor variables were the population densities of leafy spurge flowering stems, leafy spurge vegetative stems, and total leafy spurge and leafy spurge cover, grass cover, cover of forbs other than leafy spurge, litter cover, and bare ground cover. Inverse and quadratic transformations were also tested. A combination of P value, model simplicity, and R 2 values was used to identify the best model for each step-down procedure. Scatter plots of the residuals versus the standardized predicted values were used to evaluate heterogeneity of variance for each model. Models presented are significant at the P ⱕ 0.05 level.
based on data from all 13 sites combined, used precipitation, the interaction of leafy spurge cover and distance from release, grass cover, litter cover, and bare ground cover as independent variables (Table 2). A. nigriscutis density was positively related to precipitation. Within the precipitation range of the sites in this study (280 –380 mm per year), for every 2.5-mm increase in precipitation, the model predicted an increase of 2 insects. Aphthona nigriscutis adult density was low farther from the release point where leafy spurge cover was high. There was a positive association between A. nigriscutis density and grass, litter, and bare ground cover. Leafy spurge density. The mean total leafy spurge density was 86 stems/m 2 over all sites and over all strata within the sites, and ranged from 0 to 790 stems/ m 2. The linear regression model predicting total leafy spurge density used habitat type, leafy spurge cover, grass cover, and cover of forbs other than leafy spurge as independent variables (Table 3). Of the habitat types sampled, a higher density of total leafy spurge stems was associated with flood plains dominated by cottonwood trees and cool-season grasslands compared to warm-season grasslands. The number of leafy spurge stems increased by about 82, 19, and 17 stems/m 2 for each increase in cover class of leafy spurge, other forbs, and grass, respectively. The mean vegetative leafy spurge stem density was 60 stems/m 2 over all sites and over all strata within the sites, and ranged from 0 to 730 stems/m 2. The model predicting leafy spurge vegetative stem density was very similar to the model predicting total stem density (Table 4). Leafy spurge vegetative stem density increased by 54, 22, and 19 stems/m 2 with each increase in the cover class of leafy spurge, other forbs, and grass, respectively. The mean number of leafy spurge flowering stems was 26 stems/m 2 over all sites and over all strata within the sites, and ranged from 0 to 230 stems/m 2. The number of flowering leafy spurge stems was best predicted by the ratio of clay to sand, the interaction of A. nigriscutis density and distance from release, and leafy spurge cover (Table 5). For each 1% increase of clay in the soil relative to sand, leafy spurge flowering
TABLE 3 Regression Model Predicting Total Leafy Spurge Density a
RESULTS
Aphthona nigriscutis density. The linear regression model for predicting A. nigriscutis population density,
a
Regressor variable
Coefficient
P value
Intercept Habitat type Spurge cover Grass cover Other forb cover
⫺158.0000 0.9724 81.6600 18.8700 22.2700
0.0001 0.0040 0.0001 0.0252 0.0115
P ⫽ 0.0001, R 2 ⫽ 0.78.
49
RELATIONSHIPS BETWEEN SITE CONDITIONS AND Aphthona nigriscutis
a
TABLE 4
TABLE 6
Regression Model Predicting Leafy Spurge Vegetative Stem Density a
Regression Model Predicting Leafy Spurge Cover a
Regressor variable
Coefficient
P value
Intercept Habitat type Spurge cover Grass cover Other forb cover
⫺33.0000 0.9598 53.9700 18.8700 22.2700
0.0001 0.0027 0.0001 0.0076 0.0023
P ⫽ 0.0001, R 2 ⫽ 0.65.
stem density decreased by 2 stems/m 2. The interaction between distance from release and A. nigriscutis density indicated that there were fewer leafy spurge flowering stems where there were more A. nigriscutis. As leafy spurge cover increased, the number of leafy spurge flowering stems increased. Leafy spurge cover. Leafy spurge cover was positively associated with precipitation, increases in clay relative to sand, and warm-season grass sites relative to cool-season grass sites and cottonwood flood plains (Table 6). Leafy spurge cover was positively associated with leafy spurge flowering stem density more than vegetative stem density. Leafy spurge cover was negatively associated with A. nigriscutis density as distance from release increased. Grass cover was best predicted by habitat type, A. nigriscutis density, leafy spurge flowering stem density, leafy spurge vegetative stem density, litter cover, and bare ground cover (Table 7). Higher grass cover was associated with cool-season grassland habitat types compared to warm-season grasslands or habitats dominated by cottonwood trees. Grass cover was positively associated with A. nigriscutis density. There was a negative relationship between grass cover and leafy spurge flowering stem density and positive relationship with leafy spurge vegetative stem density. Increases in litter and bare ground were associated with decreases in grass cover.
Regressor variable
Coefficient
P value
Intercept Precipitation Elevation Clay/sand ratio Habitat type Flowering spurge stem density Vegetative spurge stem density A. nigriscutis density ⴱ strata
0.0900 0.0681 0.0139 0.0334 ⫺0.0045 0.0257 0.0018 ⫺0.0048
0.8277 0.0130 0.1064 0.0847 0.0246 0.0001 0.0198 0.0094
a
P ⫽ 0.0001, R 2 ⫽ 0.93.
DISCUSSION
ern Montana. On the xeric sites sampled with average precipitation ranging from 280 to 380 mm annually, precipitation was the only climatic or edaphic factor that influenced A. nigriscutis density. Results showed that sites with high annual precipitation within this range were more likely to have greater populations of A. nigriscutis than low precipitation sites. Previous observations of A. nigriscutis establishment have not identified precipitation as an important influencing factor (Rees et al., 1996; Lym, 1998). In Canada, A. nigriscutis established well on Stipa grassland habitat sites (Rees et al., 1996). Our study was limited to three habitat types, one of which was a Stipa grassland habitat. While the beetles established well on the Stipa sites we sampled, there was neither an increase nor a decrease in A. nigriscutis density associated with the two other habitat types. Habitat type was more important in predicting leafy spurge density and cover, and grass cover. Soil drainage and texture have been reported to influence A. nigriscutis establishment (Rees et al., 1996; Lym, 1998). Our results indicated that, within the range of the sites sampled, there was no relationship between soil texture and the population density of A. nigriscutis. This supports the observations by Lym (1998) that soils with less than 80% sand will not reduce the probability of A. nigriscutis establishment.
We used adult beetle density as a measure of the success of A. nigriscutis populations at 13 sites in east-
TABLE 7
TABLE 5
Regression Model Predicting Grass Cover a
Regression Model Predicting Flowering Leafy Spurge Density a Regressor variable
Coefficient
P value
Intercept Clay/sand ratio A. nigriscutis density ⴱ strata Leafy spurge cover
⫺33.0000 ⫺1.9288 ⫺0.1521 29.5386
0.0001 0.0003 0.0056 0.0001
a
P ⫽ 0.0001, R 2 ⫽ 0.91.
Regressor variable
Coefficient
P value
Intercept Habitat type A. nigriscutis density Flowering spurge stem density Vegetative spurge stem density Litter cover Bare ground cover
5.9092 ⫺0.0188 0.0248 ⫺0.0126 0.0039 ⫺0.4769 ⫺0.6731
0.0001 0.0001 0.0116 0.0024 0.0283 0.0012 0.0012
a
P ⫽ 0.0001, R 2 ⫽ 0.44.
50
JACOBS ET AL.
The sand content of soils at our sites ranged from 6.6 to 77.3% (Table 1). Leafy spurge populations typically consist of large (ca 1 m) flowering stems and numerous smaller (⬍150 mm) vegetative stems in the understory. Our results showed no relationship between vegetative or total leafy spurge stem density and A. nigriscutis density. However, there was a positive relationship between flowering leafy spurge density and A. nigriscutis density that was dependent on the distance from the point of beetle release. While A. nigriscutis adults were present on the vegetative stems close to the release point, they tended to congregate on flowering stems at the edge of their apparent impact on leafy spurge. Lym (1998) reported that the probability of A. nigriscutis establishment was reduced by leafy spurge stem densities grater than 320 stems/m 2. The number of leafy spurge flowering stems on the sites in this study was 230/m 2 or less whereas the number of total stems was as high as 790 m ⫺2. We believe that flowering stem density may be important in determining optimum establishment sites for A. nigriscutis. In addition, fewer reproductive leafy spurge stems associated with A. nigriscutis confirm observations that this biological control agent may be important in reducing leafy spurge seed production and population spread. This is meaningful for sustainable management of this weed, particularly along streams and rivers where herbicide use is restricted and waterways help spread the seed. There are two possible explanations for the negative association between leafy spurge cover and A. nigriscutis density by distance from release point. The increase in A. nigriscutis density with distance from the release point resulted in a decrease in leafy spurge cover, implying A. nigriscutis as an important biological control agent. Alternatively, reduced leafy spurge cover with increasing distance from the point of release created conditions ideal for A. nigriscutis colonization. Creating conditions of low leafy spurge cover, or flowering stem density, through herbicide application, mowing, or sheep/goat grazing may create improved conditions for A. nigriscutis establishment and population growth (Lym et al., 1996). Previous research found little or no response in grass production to reductions in leafy spurge density at A. nigriscutis release sites (Kirby, 1996). We did not sample grass production; however, our results showed a positive relationship between grass cover and A. nigriscutis density and a negative relationship between grass cover and density of leafy spurge flowering stems. This implies a competitive shift favoring grasses over leafy spurge. We believe that the increases in litter and bare ground cover associated with increased population density of A. nigriscutis indicate
an opening of niches for potential occupation by desirable species. ACKNOWLEDGMENTS Support for this study was provided by USDA-ARS TEAM Leafy Spurge. Special thanks goes to Mark Gaffri, USDA-ARS, for his help in location and access to the study sites.
REFERENCES Bangsund, D. A., Baltezore, J. F., Leitch, J. A., and Leistritz, F. L. 1993. Economic impact of leafy spurge on wildland in Montana, South Dakota and Wyoming. Agricultural Economics Report No. 304. Dept. of Agricultural Economics, North Dakota State Univ., Fargo, ND. Daubenmire, R. 1970. Steppe vegetation of Washington. Washington Agric. Exp. Stn. Tech. Bull. No. 62. Gee, G. W., and Bauder, J. W. 1986. Methods of soil analysis, Part 1. Physical and mineralogical methods. Agronomy Monographs No. 9:383-411. Am. Soc. Agronomy. Hansen, P. L., Pfister, R. D., Boggs, K., Cook, B. J., Joy, J., and Hinckley, D. K. 1996. Classification and Management of Montana’s riparian and wetland sites. Montana Forest and Conservation Experiment Station School of Forestry, Univ. of MT, Missoula, MT. Miscellaneous Publication No. 54. Hansen, R. W. 1996. Development and application of phenological models for leafy spurge biological control agents. FY96 Annual Report, USDA-APHIS-PPQ Bozeman Biological Control Laboratory, Bozeman, MT. Kirby, D. 1996. Biological control of leafy spurge (Euphorbia esula) with flea beatles (Aphthona spp.). Abstr. Soc. Range Manage. 40, 40. Lajeunesse, S., Sheley, R. L., Duncan, C., and Lym, R. 1999. Leafy spurge. In “Biology and Management of Noxious Rangeland Weeds” (R. L. Sheley and J. K. Petroff, Eds.), pp. 249 –260. Oregon State Univ. Press, Corvallis. Leitch, J. A., Bangsund, D. A., and Leistritz, F. L. 1994. Economic effect of leafy spurge in the upper Great Plains: Methods, models, results. Agricultural Economics Report No. 316. Dept. Agricultural Economics, North Dakota State Univ., Fargo, ND. Lym, R. G. 1998. The biology and integrated management of leafy spurge (Euphorbia esula) on North Dakota rangeland. Weed Technol. 12, 367–373. Lym, R. G., Carson, R. B., Messersmith, C. G., and Mundal, D. A. 1996. Integration of herbicides with flee beetles, Aphthona nigriscutis, for leafy spurge control. In Proceedings of the IX International Symposium on Biological Control of Weeds (V. C. Moran and J. H. Hoffmann, Eds.), pp. 480 – 481. Univ. of Capetown, Rondebosh, South Africa. Mueggler, W. F., and Stewart, W. L. 1980. Grassland and shrubland habitat types of Western Montana. USDA-FS Intermountain Forest and Range Experiment Station. Ogden, UT. Neter, J., Wasserman, W., and Kutner, M. H. 1985. “Applied Linear Statistics Models,” 2nd ed. Irwin, Homewood, IL. Rees, N. E. 1994. The Aphthona pilot study. In “Proceedings of the Leafy Spurge Symposium. Fargo ND: North Dakota Cooperative Extension Service,” pp. 21–24. North Dakota State University, Fargo, ND. Rees, N. E., Quimby, P. C., Jr., Piper, G. L., Coombs, E. M., Turener, C. E., Spencer, N. R., and Knutson, L. V. 1996. “Biological Control of Weeds in the West.” Western Soc. Weed Sci., Bozeman, MT.
Relationships among Edaphic, Climatic, and Vegetation Conditions at Release Sites and Aphthona nigriscutis Population Density James S. Jacobs, 1 Roger L. Sheley, 2 Neal R. Spencer, 3 and Gerry Anderson 4 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana 59717 Received July 18, 2000; accepted April 17, 2001
Key Words: Euphorbia esula; leafy spurge; noxious weed management; biological control.
The role of site conditions on the success of leafy spurge control by Aphthona nigriscutis is poorly understood. Our objective was to determine the relationships among the climatic and edaphic conditions of A. nigriscutis release sites, A. nigriscutis population density, leafy spurge cover and density, and grass cover. We sampled 13 field sites in eastern Montana where A. nigriscutis had been established for 6 to 8 years. Sampling was done in June and July of 1998 along a randomly determined transect from the center of each release point to densely infested leafy spurge. Data were analyzed by step-down regression procedures using A. nigriscutis density, leafy spurge cover and density, cover of grass, forbs, litter, and bare ground as the dependent variables. Independent variables included vegetative and insect data sampled as well as site conditions. Average annual precipitation was the only edaphic or climatic site characteristic that influenced A. nigriscutis density; the beetle population density increased as precipitation increased. There was a negative association between A. nigriscutis and number of leafy spurge flowering stems and leafy spurge cover. Grass, litter, and bare ground cover increased as A. nigriscutis numbers increased. Within the range of site conditions sampled in this study, successful establishment and population expansion of A. nigriscutis appears to be related to the cover of leafy spurge, grass, forbs other than leafy spurge, litter, and bare ground. This information may aid in site selection for release of this important biological control agent or in creating conditions for optimum establishment. ©
INTRODUCTION
Biological control is important for the management of some rangeland weed species such as leafy spurge (Euphorbia esula L., Euphorbiaceae) because management using herbicides is rarely cost-effective, and herbicide use is restricted in riparian areas where this invasive weed often dominates. This long-lived, deeprooted perennial weed is rapidly spreading in the United States and Canada (Lajeunesse et al., 1999) and has an economic impact of $130 million each year in Montana, Wyoming, and South Dakota (Bangsund et al., 1993; Leitch et al., 1994). Development of an effective biological control program for leafy spurge requires an understanding of the relationships among the weed, the control agent, and the site conditions for successful establishment and maintenance of biological control agents. Thirteen species of biological control insects for leafy spurge have been approved by the USDA for introduction into the United States (Rees et al., 1996). Six species of the root- and shoot-feeding flee beetles in the genus Aphthona are well established and believed to provide the greatest control of leafy spurge (Lym, 1998). Aphthona nigriscutis Fourdas (Coleoptera: Chrysomelidae), the first successful Aphthona spp. to establish, was introduced into Canada in 1983 and into the United states in 1989 (Rees et al., 1996). It is currently established in Colorado, Idaho, Montana, Nebraska, North Dakota, Oregon, and Washington. The success rate of A. nigriscutis establishment has been variable, and at many sites where the beetle has established, the impact on leafy spurge has been low (Lym, 1998). Rees (1994) reported that in 1991 only 14 of 34 A. nigriscutis releases established. Habitat type, soil conditions, and the density of spurge have been suggested to affect A. nigriscutis establishment. In Canada, A. nigriscutis established well on Stipa grassland habitat sites (Rees et al., 1996). A. nigriscutis is
2001 Academic Press
1 To whom correspondence should be addressed. Research Assistant Professor, Department of Land Resources and Environmental Sciences, Montana State University, Bozeman. Fax: (406) 994-3933. E-mail: [email protected]. 2 Associate Professor, Department of Land Resources and Environmental Sciences, Montana State University, Bozeman. E-mail: [email protected]. 3 Laboratory Director, Northern Plains Agricultural Research Laboratory, USDA-ARS, Sidney, MT. E-mail: nspencer@sidney. ars.usda.gov. 4 Co-Principal Investigator, TEAM Leafy Spurge, USDA-ARS, Sidney, MT. E-mail: [email protected].
1049-9644/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.
46
47
RELATIONSHIPS BETWEEN SITE CONDITIONS AND Aphthona nigriscutis
TABLE 1 Characteristics of 13 A. nigriscutis Release Sites Sampled Site
Date established
Number of insects released
1
7/6/90
1000
2
7/6/90
1000
3
7/6/90
500
4
7/2/91
1000
5
7/27/91
500
6
7/6/90
500
7
7/13/92
500
8
7/2/91
500
9
7/6/90
500
10
7/2/91
500
11
7/18/90
500
12
7/2/91
1000
13
7/2/91
500
Location 46°40⬘01.88 N 104°30⬘06.53W 46°38⬘07.36 N 104°32⬘42.87W 46°08⬘37.52 N 105°05⬘22.48W 45°28⬘47.17 N 105°20⬘18.97W 45°08⬘37.52 N 105°48⬘11.21W 48°35⬘52.89 N 109°25⬘27.00W 48°34⬘01.73 N 109°29⬘34.64W 48°33⬘19.06 N 108°32⬘34.67W 48°21⬘22.98 N 108°21⬘22.27W 48°21⬘14.18 N 108°21⬘21.74W 48°05⬘36.29 N 104°25⬘52.81W 48°13⬘21.26 N 104°12⬘26.56W 48°12⬘53.49 N 104°16⬘14.90W
Elevation (m)
Habitat type
Precipitation (mm)
% Sand
% Silt
% Clay
825
AGSP/AGSM
330
66.1
23.8
10.0
853
STGR/BOGR
380
67.2
26.4
16.3
853
STGR/BOGR
330
57.3
31.4
11.3
945
STGR/BOGR
380
6.6
40.9
52.5
1097
STGR/BOGR
280
73.7
17.6
8.8
792
STGR/BOGR
280
53.7
36.3
10.0
762
STGR/BOGR
280
51.2
36.3
12.5
792
STGR/BOGR
280
62.1
20.2
17.7
701
STGR/BOGR
280
45.8
32.8
21.4
701
STGR/BOGR
280
56.3
27.6
18.8
640
PODE/COST
330
31.8
45.5
22.7
701
STGR/BOGR
380
77.3
10.1
12.6
671
STGR/BOGR
380
58.5
25.2
16.4
reported to prefer sites with sandy, well-drained soils (Rees et al., 1996). However, Lym (1998) observed that this species did not establish on sites with soil texture greater than 80% sand. The probability of A. nigriscutis establishment is also believed to be reduced where leafy spurge density is greater than 320 stems/m 2 (Lym, 1998). An understanding of the relationships among edaphic and climatic conditions and vegetation at release sites and A. nigriscutis establishment may help determine where this biological control agent may establish and have the greatest impact on leafy spurge. Therefore, the objectives of this study were to determine the relationships among edaphic and climatic conditions at release sites, habitat type, and A. nigriscutis impacts on leafy spurge population density and cover, grass cover, forbs, litter, and bare ground. We hypothesize that A. nigriscutis populations and grass cover will be related to soil texture, precipitation, and leafy spurge densities. MATERIALS AND METHODS
Field sites. To quantify the relationships among site conditions, A. nigriscutis, leafy spurge, and grass, we intensively sampled 13 A. nigriscutis field release sites in eastern Montana, U.S.A. Only sites with successful establishment of A. nigriscutis were sampled. A. nigriscutis was released at these sites from 1990 to
1992. Sites were selected to represent a range in habitat types, elevation, 30-year average annual precipitation, and soil texture. Three habitat types that are commonly infested with leafy spurge were selected. They were Agropyron spicatum (Pursh) Scribn. & J.G. Sm./Agropyron smithii Rydb. (Mueggler and Stewart, 1980) cool-season grasslands, Stipa comata Trin. & Rupr./Bouteloua gracilis (Kunth) Lag. ex Griffiths (Mueggler and Stewart, 1980) mixed cool-season/ warm-season grasslands, and Populus deltoides Marsh. (cottonwood)/Cornus stolonifera Michx. (Hansen et al., 1996) river bottoms dominated by cottonwood. The elevation at the release sites ranged from 640 to 1097 m. Average annual precipitation ranged from 280 to 380 mm. The soil texture of the sites, determined by using particle-size analysis by a hydrometer (Gee and Bauder, 1986), ranged from 7 to 77% sand and 9 to 53% clay. Site conditions and the number of insects released for each site are listed in Table 1. Sampling. Sites were sampled from late-June to mid-July 1998 during peak adult A. nigriscutis emergence based on air degree-day accumulation (Hansen, 1996). Each site was divided into strata that were determined by distance from the original point of A. nigriscutis release. A transect was established from the point of release and extended in a randomly chosen
48
JACOBS ET AL.
TABLE 2 Regression Model Predicting A. nigriscutis Density a Regressor variable
Coefficient
P value
Intercept Precipitation Spurge cover ⴱ strata Grass cover Other forb cover Litter cover Bare ground cover
⫺57.0000 2.0433 ⫺0.5263 3.0867 ⫺2.1290 3.6130 2.4370
0.0013 0.0003 0.0076 0.0127 0.1099 0.0006 0.0179
a
P ⫽ 0.0001, R 2 ⫽ 0.49.
direction to an area of dense leafy spurge past the apparent point of A. nigriscutis expansion. The transect was divided into five strata of equal length. A. nigriscutis density was measured within each strata by sweeping 10 randomly located 1-m 2 plots of vegetation using 3 sweeps of a 381-mm sweep net. Insects captured in sweeps were placed into plastic bags, frozen at ⫺18°C, and subsequently counted in the laboratory. Leafy spurge density and cover, cover of grass, other forbs, litter, and bare ground were sampled in 10 randomly located 0.2- by 0.5-m frames within each strata. Leafy spurge flowering stems and stems without flowers were counted in each frame. Cover was visually estimated for leafy spurge, other forbs, grass, litter, and bare ground using six cover classes: 0 –5% (1), 6 –25% (2), 26 –50% (3), 51–75% (4), 76 –95% (5), and 96 –100% (6) of the ground covered (Daubenmire, 1970). Statistical analysis. The relationships among site characteristics, population density of leafy spurge, all cover measurements, and A. nigriscutis density were analyzed using the step-down linear regression procedure (Neter et al., 1985). Site characteristics used as predictor variables were precipitation, elevation, percentage sand, percentage silt, percentage clay, sand: silt ratio, sand:clay ratio, silt:clay ratio, and strata (distance from release point). Vegetation characteristics used as predictor variables when they were not the regressor variables were the population densities of leafy spurge flowering stems, leafy spurge vegetative stems, and total leafy spurge and leafy spurge cover, grass cover, cover of forbs other than leafy spurge, litter cover, and bare ground cover. Inverse and quadratic transformations were also tested. A combination of P value, model simplicity, and R 2 values was used to identify the best model for each step-down procedure. Scatter plots of the residuals versus the standardized predicted values were used to evaluate heterogeneity of variance for each model. Models presented are significant at the P ⱕ 0.05 level.
based on data from all 13 sites combined, used precipitation, the interaction of leafy spurge cover and distance from release, grass cover, litter cover, and bare ground cover as independent variables (Table 2). A. nigriscutis density was positively related to precipitation. Within the precipitation range of the sites in this study (280 –380 mm per year), for every 2.5-mm increase in precipitation, the model predicted an increase of 2 insects. Aphthona nigriscutis adult density was low farther from the release point where leafy spurge cover was high. There was a positive association between A. nigriscutis density and grass, litter, and bare ground cover. Leafy spurge density. The mean total leafy spurge density was 86 stems/m 2 over all sites and over all strata within the sites, and ranged from 0 to 790 stems/ m 2. The linear regression model predicting total leafy spurge density used habitat type, leafy spurge cover, grass cover, and cover of forbs other than leafy spurge as independent variables (Table 3). Of the habitat types sampled, a higher density of total leafy spurge stems was associated with flood plains dominated by cottonwood trees and cool-season grasslands compared to warm-season grasslands. The number of leafy spurge stems increased by about 82, 19, and 17 stems/m 2 for each increase in cover class of leafy spurge, other forbs, and grass, respectively. The mean vegetative leafy spurge stem density was 60 stems/m 2 over all sites and over all strata within the sites, and ranged from 0 to 730 stems/m 2. The model predicting leafy spurge vegetative stem density was very similar to the model predicting total stem density (Table 4). Leafy spurge vegetative stem density increased by 54, 22, and 19 stems/m 2 with each increase in the cover class of leafy spurge, other forbs, and grass, respectively. The mean number of leafy spurge flowering stems was 26 stems/m 2 over all sites and over all strata within the sites, and ranged from 0 to 230 stems/m 2. The number of flowering leafy spurge stems was best predicted by the ratio of clay to sand, the interaction of A. nigriscutis density and distance from release, and leafy spurge cover (Table 5). For each 1% increase of clay in the soil relative to sand, leafy spurge flowering
TABLE 3 Regression Model Predicting Total Leafy Spurge Density a
RESULTS
Aphthona nigriscutis density. The linear regression model for predicting A. nigriscutis population density,
a
Regressor variable
Coefficient
P value
Intercept Habitat type Spurge cover Grass cover Other forb cover
⫺158.0000 0.9724 81.6600 18.8700 22.2700
0.0001 0.0040 0.0001 0.0252 0.0115
P ⫽ 0.0001, R 2 ⫽ 0.78.
49
RELATIONSHIPS BETWEEN SITE CONDITIONS AND Aphthona nigriscutis
a
TABLE 4
TABLE 6
Regression Model Predicting Leafy Spurge Vegetative Stem Density a
Regression Model Predicting Leafy Spurge Cover a
Regressor variable
Coefficient
P value
Intercept Habitat type Spurge cover Grass cover Other forb cover
⫺33.0000 0.9598 53.9700 18.8700 22.2700
0.0001 0.0027 0.0001 0.0076 0.0023
P ⫽ 0.0001, R 2 ⫽ 0.65.
stem density decreased by 2 stems/m 2. The interaction between distance from release and A. nigriscutis density indicated that there were fewer leafy spurge flowering stems where there were more A. nigriscutis. As leafy spurge cover increased, the number of leafy spurge flowering stems increased. Leafy spurge cover. Leafy spurge cover was positively associated with precipitation, increases in clay relative to sand, and warm-season grass sites relative to cool-season grass sites and cottonwood flood plains (Table 6). Leafy spurge cover was positively associated with leafy spurge flowering stem density more than vegetative stem density. Leafy spurge cover was negatively associated with A. nigriscutis density as distance from release increased. Grass cover was best predicted by habitat type, A. nigriscutis density, leafy spurge flowering stem density, leafy spurge vegetative stem density, litter cover, and bare ground cover (Table 7). Higher grass cover was associated with cool-season grassland habitat types compared to warm-season grasslands or habitats dominated by cottonwood trees. Grass cover was positively associated with A. nigriscutis density. There was a negative relationship between grass cover and leafy spurge flowering stem density and positive relationship with leafy spurge vegetative stem density. Increases in litter and bare ground were associated with decreases in grass cover.
Regressor variable
Coefficient
P value
Intercept Precipitation Elevation Clay/sand ratio Habitat type Flowering spurge stem density Vegetative spurge stem density A. nigriscutis density ⴱ strata
0.0900 0.0681 0.0139 0.0334 ⫺0.0045 0.0257 0.0018 ⫺0.0048
0.8277 0.0130 0.1064 0.0847 0.0246 0.0001 0.0198 0.0094
a
P ⫽ 0.0001, R 2 ⫽ 0.93.
DISCUSSION
ern Montana. On the xeric sites sampled with average precipitation ranging from 280 to 380 mm annually, precipitation was the only climatic or edaphic factor that influenced A. nigriscutis density. Results showed that sites with high annual precipitation within this range were more likely to have greater populations of A. nigriscutis than low precipitation sites. Previous observations of A. nigriscutis establishment have not identified precipitation as an important influencing factor (Rees et al., 1996; Lym, 1998). In Canada, A. nigriscutis established well on Stipa grassland habitat sites (Rees et al., 1996). Our study was limited to three habitat types, one of which was a Stipa grassland habitat. While the beetles established well on the Stipa sites we sampled, there was neither an increase nor a decrease in A. nigriscutis density associated with the two other habitat types. Habitat type was more important in predicting leafy spurge density and cover, and grass cover. Soil drainage and texture have been reported to influence A. nigriscutis establishment (Rees et al., 1996; Lym, 1998). Our results indicated that, within the range of the sites sampled, there was no relationship between soil texture and the population density of A. nigriscutis. This supports the observations by Lym (1998) that soils with less than 80% sand will not reduce the probability of A. nigriscutis establishment.
We used adult beetle density as a measure of the success of A. nigriscutis populations at 13 sites in east-
TABLE 7
TABLE 5
Regression Model Predicting Grass Cover a
Regression Model Predicting Flowering Leafy Spurge Density a Regressor variable
Coefficient
P value
Intercept Clay/sand ratio A. nigriscutis density ⴱ strata Leafy spurge cover
⫺33.0000 ⫺1.9288 ⫺0.1521 29.5386
0.0001 0.0003 0.0056 0.0001
a
P ⫽ 0.0001, R 2 ⫽ 0.91.
Regressor variable
Coefficient
P value
Intercept Habitat type A. nigriscutis density Flowering spurge stem density Vegetative spurge stem density Litter cover Bare ground cover
5.9092 ⫺0.0188 0.0248 ⫺0.0126 0.0039 ⫺0.4769 ⫺0.6731
0.0001 0.0001 0.0116 0.0024 0.0283 0.0012 0.0012
a
P ⫽ 0.0001, R 2 ⫽ 0.44.
50
JACOBS ET AL.
The sand content of soils at our sites ranged from 6.6 to 77.3% (Table 1). Leafy spurge populations typically consist of large (ca 1 m) flowering stems and numerous smaller (⬍150 mm) vegetative stems in the understory. Our results showed no relationship between vegetative or total leafy spurge stem density and A. nigriscutis density. However, there was a positive relationship between flowering leafy spurge density and A. nigriscutis density that was dependent on the distance from the point of beetle release. While A. nigriscutis adults were present on the vegetative stems close to the release point, they tended to congregate on flowering stems at the edge of their apparent impact on leafy spurge. Lym (1998) reported that the probability of A. nigriscutis establishment was reduced by leafy spurge stem densities grater than 320 stems/m 2. The number of leafy spurge flowering stems on the sites in this study was 230/m 2 or less whereas the number of total stems was as high as 790 m ⫺2. We believe that flowering stem density may be important in determining optimum establishment sites for A. nigriscutis. In addition, fewer reproductive leafy spurge stems associated with A. nigriscutis confirm observations that this biological control agent may be important in reducing leafy spurge seed production and population spread. This is meaningful for sustainable management of this weed, particularly along streams and rivers where herbicide use is restricted and waterways help spread the seed. There are two possible explanations for the negative association between leafy spurge cover and A. nigriscutis density by distance from release point. The increase in A. nigriscutis density with distance from the release point resulted in a decrease in leafy spurge cover, implying A. nigriscutis as an important biological control agent. Alternatively, reduced leafy spurge cover with increasing distance from the point of release created conditions ideal for A. nigriscutis colonization. Creating conditions of low leafy spurge cover, or flowering stem density, through herbicide application, mowing, or sheep/goat grazing may create improved conditions for A. nigriscutis establishment and population growth (Lym et al., 1996). Previous research found little or no response in grass production to reductions in leafy spurge density at A. nigriscutis release sites (Kirby, 1996). We did not sample grass production; however, our results showed a positive relationship between grass cover and A. nigriscutis density and a negative relationship between grass cover and density of leafy spurge flowering stems. This implies a competitive shift favoring grasses over leafy spurge. We believe that the increases in litter and bare ground cover associated with increased population density of A. nigriscutis indicate
an opening of niches for potential occupation by desirable species. ACKNOWLEDGMENTS Support for this study was provided by USDA-ARS TEAM Leafy Spurge. Special thanks goes to Mark Gaffri, USDA-ARS, for his help in location and access to the study sites.
REFERENCES Bangsund, D. A., Baltezore, J. F., Leitch, J. A., and Leistritz, F. L. 1993. Economic impact of leafy spurge on wildland in Montana, South Dakota and Wyoming. Agricultural Economics Report No. 304. Dept. of Agricultural Economics, North Dakota State Univ., Fargo, ND. Daubenmire, R. 1970. Steppe vegetation of Washington. Washington Agric. Exp. Stn. Tech. Bull. No. 62. Gee, G. W., and Bauder, J. W. 1986. Methods of soil analysis, Part 1. Physical and mineralogical methods. Agronomy Monographs No. 9:383-411. Am. Soc. Agronomy. Hansen, P. L., Pfister, R. D., Boggs, K., Cook, B. J., Joy, J., and Hinckley, D. K. 1996. Classification and Management of Montana’s riparian and wetland sites. Montana Forest and Conservation Experiment Station School of Forestry, Univ. of MT, Missoula, MT. Miscellaneous Publication No. 54. Hansen, R. W. 1996. Development and application of phenological models for leafy spurge biological control agents. FY96 Annual Report, USDA-APHIS-PPQ Bozeman Biological Control Laboratory, Bozeman, MT. Kirby, D. 1996. Biological control of leafy spurge (Euphorbia esula) with flea beatles (Aphthona spp.). Abstr. Soc. Range Manage. 40, 40. Lajeunesse, S., Sheley, R. L., Duncan, C., and Lym, R. 1999. Leafy spurge. In “Biology and Management of Noxious Rangeland Weeds” (R. L. Sheley and J. K. Petroff, Eds.), pp. 249 –260. Oregon State Univ. Press, Corvallis. Leitch, J. A., Bangsund, D. A., and Leistritz, F. L. 1994. Economic effect of leafy spurge in the upper Great Plains: Methods, models, results. Agricultural Economics Report No. 316. Dept. Agricultural Economics, North Dakota State Univ., Fargo, ND. Lym, R. G. 1998. The biology and integrated management of leafy spurge (Euphorbia esula) on North Dakota rangeland. Weed Technol. 12, 367–373. Lym, R. G., Carson, R. B., Messersmith, C. G., and Mundal, D. A. 1996. Integration of herbicides with flee beetles, Aphthona nigriscutis, for leafy spurge control. In Proceedings of the IX International Symposium on Biological Control of Weeds (V. C. Moran and J. H. Hoffmann, Eds.), pp. 480 – 481. Univ. of Capetown, Rondebosh, South Africa. Mueggler, W. F., and Stewart, W. L. 1980. Grassland and shrubland habitat types of Western Montana. USDA-FS Intermountain Forest and Range Experiment Station. Ogden, UT. Neter, J., Wasserman, W., and Kutner, M. H. 1985. “Applied Linear Statistics Models,” 2nd ed. Irwin, Homewood, IL. Rees, N. E. 1994. The Aphthona pilot study. In “Proceedings of the Leafy Spurge Symposium. Fargo ND: North Dakota Cooperative Extension Service,” pp. 21–24. North Dakota State University, Fargo, ND. Rees, N. E., Quimby, P. C., Jr., Piper, G. L., Coombs, E. M., Turener, C. E., Spencer, N. R., and Knutson, L. V. 1996. “Biological Control of Weeds in the West.” Western Soc. Weed Sci., Bozeman, MT.