A.R. Kalischuk, a, * R.S. Bourchier, a and A.S. McClay b

Transcription

1 Biological Control 29 (2004) Post hoc assessment of an operational biocontrol program: efficacy of the flea beetle Aphthona lacertosa Rosenhauer (Chrysomelidae: Coleoptera), an introduced biocontrol agent for leafy spurge A.R. Kalischuk, a, * R.S. Bourchier, a and A.S. McClay b a Agriculture and Agri-Food Canada, Avenue South, P.O. Box 3000 Lethbridge, Alberta, Canada T1J 4B1 b Alberta Research Council, Bag 4000, Vegreville, Alberta, Canada T9C 1T4 Received 9 January 2003; accepted 4 August 2003 Abstract Mixed populations of Aphthona lacertosa and Aphthona czwalinae were released at more than 50 locations in Alberta in Two and 3 years post-release, beetle populations were primarily A. lacertosa, with A. czwalinae forming less than 0.5% of the sampled populations. Beetle densities were moderate (10 70 beetles per m 2 ) or high (>70 beetles per m 2 ) at 14% and more than 60% of the sampled sites in 1999 and 2000, respectively. Larger beetles had greater instantaneous egg loads (r 2 ¼ 0:424; P ¼ 0:003). In 2000, the largest beetles were found at moderate density sites and there was a significant negative relationship between beetle size and the time taken to accumulate a degree day threshold of 1230 (for females: r 2 ¼ 0:678; P ¼ 0:001). Sites with the most rapid accumulation of degree days have the greatest potential for beetle population growth based on potential fecundity. Changes in leafy spurge percent cover, stem density, and canopy height from 1997 to 2000 were assessed across sites with low (<10 beetles per m 2 ), moderate, and high beetle densities in Sites with high beetle densities had significantly greater reductions of leafy spurge within 5 m of the release point than sites with low beetle densities (P < 0:017). Damage caused by the beetles at high-density sites was often visible as a halo-shaped patch of dead leafy spurge stems. The significant overall reduction of leafy spurge within release patches makes A. lacertosa a promising biocontrol agent for leafy spurge in Alberta. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Leafy spurge; Flea beetles; Aphthona lacertosa; Euphorbia esula; Biological control; Biocontrol 1. Introduction * Corresponding author. Present Address: Alberta Agriculture Food and Rural Development, 100, Avenue South Lethbridge, Alberta, Canada T1J 4V6. Fax: address: Andrea.Kalischuk@gov.ab.ca (A.R. Kalischuk). Leafy spurge, Euphorbia esula L. (Euphorbiaceae) is an introduced perennial weed infesting large areas of rangeland in western North American (Hansen et al., 1997). It was first reported in Alberta, Canada in 1933 (Haber, 1997) and is now estimated to cover more than 6000 ha in central and southern parts of the province (McClay et al., 1995). Leafy spurge was an early target for weed biological control because of the large scale of the infestations, the cost of controlling its rangeland populations with herbicides, and the availability of its natural enemies in its native range (Harris et al., 1985). Since 1970, 18 insects have been introduced into Canada for biological control of leafy spurge (Bourchier et al., 2002; Julien and Griffiths, 1998). Although there have been numerous releases over the past 30 years, there have been very few long-term, quantitative studies on the efficacy of weed biocontrol agents and of leafy spurge biocontrol agents in particular (McClay, 1995). Recently, biological control has been criticized because the outcomes of many biocontrol introductions have not been adequately assessed (Callaway et al., 1999; Louda et al., 1997; McEvoy and Coombs, 1999; Pearson et al., 2000). The most successful biological control agents of leafy spurge, in terms of population densities and redistribution, have been the flea beetles in the genus Aphthona /$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi: /j.biocontrol

2 A.R. Kalischuk et al. / Biological Control 29 (2004) (Bourchier et al., 2002). The larval and adult biology of each Aphthona species is similar and reviewed in Bourchier et al. (2002). Since 1978, five flea beetle species, Aphthona cyparissiae (Koch), Aphthona flava Guillebaume, Aphthona nigriscutis Foudras, Aphthona czwalinae Weise, and Aphthona lacertosa Rosenhauer have been released to control E. esula in Canada (Julien and Griffiths, 1998). Multiple species were introduced because of their differing habitat preferences (Gassmann and Schroeder, 1995). The most recent, wide-scale operational program for flea beetle redistribution in Alberta is for A. lacertosa, and the program was started in A single study documents the efficacy of these beetles in North America; Kirby et al. (2000) found that A. lacertosa, in combination with A. czwalinae, caused a significant reduction in leafy spurge at field sites in North Dakota, USA. More studies are needed and hence the overall purpose of this study was to assess the efficacy of an operational release program of A. lacertosa, in terms of establishment and impact on leafy spurge in Alberta, Canada. The specific objectives of this study were (1) to quantify A. lacertosa establishment and beetle densities 2 years post-release, (2) to compare environmental conditions and beetle attributes at high- and low-density beetle sites as a potential tool to predict establishment and (3) to assess the impact of A. lacertosa on leafy spurge in Alberta 2 to 3 years post-release. 2. Materials and methods 2.1. Beetle densities In 1997, Alberta Agriculture, Food and Rural Development (AAFRD) imported A. lacertosa from a field site at Valley City, North Dakota, USA, where beetles were collected on 24 and 25 June. AAFRD released the beetles at more than 90 sites over a period of 11 days between 27 June and 7 July. Releases were done in mesic prairie habitats and included coulee hillsides, alluvial floodplains, rangelands, and railway and road ditches. In 1999, 50 of the 1997 sites were located for monitoring. Most initial release densities (n ¼ 37) were of 1000 beetles per site but some releases (n ¼ 13) were of 2000 beetles per site. Site locations were recorded using a hand-held GPS (Garmin 12XL). Sites extended west to east from Coleman (49.6 N, W) to Medicine Hat (50.2 N, W) and south to north from Cardston (49.2 N, W) to Brosseau (53.5 N, W) (Fig. 1A). Aphthona lacertosa densities were monitored a single time at each site between 29 June and 29 July. Southern sites, located primarily along the west-east transect, were monitored first because they were expected to have earlier beetle emergence than northern sites. In 2000, 17 of the sites that were monitored in 1999 were chosen for intensive sampling (Fig. 1B). Sites were chosen to represent the broad geographical distribution of the sites across Alberta and to represent the diversity of beetle densities in Beetle densities were sampled approximately every 2 weeks from 5 June to 5 September. At all sites and in both years, beetle densities were assessed in a fixed area with a similar sampling effort. A garbage can with the bottom cut off (diameter 41.5 cm, area m 2 ) was placed over the plants 5 m from the release point in each of the cardinal directions. The area inside the garbage can was vacuumed using a modified leaf blower (STIHL BG75, high idle flow rate ¼ 0.25 m 3 / min) for 45 s. Vacuuming was vigorous and efforts were made to vacuum the leafy spurge shoots, the sides of the can, and the ground to include beetles that had jumped off the plants or fell to the ground. The vacuumed samples of beetles were collected in a fine-mesh stocking and placed on ice. At the laboratory, samples were frozen and then sorted to quantify the density of A. lacertosa. Mean beetle densities of A. lacertosa were plotted using the site coordinates. Mean beetle density in 1999 at each site was calculated using the number of beetles vacuumed within 5 m of the release stake, in each of the four cardinal directions for the sample date. In 2000, mean beetle density was calculated using the number of beetles vacuumed within 5 m of the release stake, in each of the four cardinal directions, for the sample date when beetles were most abundant. Sites were divided into categories of low (<10 beetles per m 2 ), moderate (10 70 beetles per m 2 ), or high (>70 beetles per m 2 ) beetle densities. The groupings best described the peaks in the frequency distribution of beetle densities in 1999 and 2000 (Fig. 2) Environment conditions and beetle attributes We used an unpublished degree-day model (Dr. Rich Hansen, USDA-APHIS, Bozeman, MT, USA) that was developed and tested using Montana populations of A. lacertosa that originated from the same source as the Alberta beetles (Valley City, North Dakota, USA). The model assumes a positive linear relationship between beetle development and temperature. Average cumulative degree days (CDD) are calculated using the half sine wave method (Allen, 1976) with January 1 as day 1 and a threshold temperature of 0 C. Based on field populations in Montana, peak densities of A. lacertosa were expected to occur at 1230 CDD (Rich Hansen, personal communication). Daily maximum and minimum temperatures were obtained from Environment Canada at 11 stations in Alberta and one station in British Columbia (Table 1). Aphthona lacertosa release sites were matched with the nearest weather station. At release sites where no station

3 420 A.R. Kalischuk et al. / Biological Control 29 (2004) Fig. 1. Location of 1997 releases of Aphthona lacertosa in Alberta monitored (A) in 1999 with mean beetle densities from a single sample date and (B) in 2000 with mean beetle peak densities from 2-week sampling. The approximate location of cities and towns are indicated and Calgary and Edmonton are included for geographical context. Fig. 2. Mean Aphthona lacertosa densities at biocontrol release sites in Alberta in 1999 (n ¼ 50 sites) and 2000 (n ¼ 17 sites). Sites were divided into those with low (<10 beetles per m 2 ), moderate (10 70 beetles per m 2 ), and high (>70 beetles per m 2 ) beetle densities. was within the vicinity (within 200 km), temperature data were interpolated between the two nearest weather stations. Interpolated data were calculated as arithmetic mean temperatures of the two stations. The degree day model was used to classify environmental conditions at release sites. Beetle body size and environmental conditions were compared with observed beetle densities to test if we could use size or environment to predict the establishment and population growth of a release site. Wing length was used as an estimate for body size; wing length is strongly correlated with body size for insects and birds (Lanciani and Le, 1995; Miller, 1997; Rodway, 1997). Wings were measured using an image analysis system (Kokko et al., 1996). Wings were removed from the beetles using fine tweezers and placed on a glass microscope slide in a drop of water. After soaking for a couple of minutes, the wings were spread flat using a fine bristled paintbrush and left to air dry. Microscope slides were positioned on a Shotz trans-illumination fluorescent light source beneath a Wild Photomakrosko M400 stereomicroscope that was fitted with a Hitachi HV- C20 video camera. The stereomicroscope was set with a magnification of 7 and digital grayscale images were acquired at a pixel resolution1 of pixels per mm. Wing length was measured as the distance between the intersection of the principle vein along the wing margin and the next nearest parallel vein to the tip of the wing (Fig. 3).

4 A.R. Kalischuk et al. / Biological Control 29 (2004) Table 1 Location of Environment Canada climate stations used to calculate cumulative degree days (CDD) at Aphthona lacertosa release sites in Alberta Town(s) nearest release sites Climate station Location ( N, W) Millarville Calgary 50.1, Camrose Camrose 53.0, Cardston, Magrath Cardston 49.2, Claresholm 50.0, Ft. Macleod Cardston + Claresholm Sherwood Park, Brosseau Edmonton 53.3, Lethbridge Lethbridge 49.7, Lloydminster 53.3, Hardisty Lloydminster + Red Deer Seven Persons, Medicine Hat Medicine Hat 50.0, Three Hills Olds 51.8, Pincher Creek, Lundbreck Pincher Creek 49.5, Red Deer Red Deer 52.2, Mirror, Ponoka Red Deer + Camrose Taber Taber 49.8, Bow Island Taber + Medicine Hat Coleman Crowsnest Pass 49.6, Fig. 3. Typical wings of Aphthona lacertosa male beetles (3 wings on the left) and female beetles (3 wings on the right). Scale bar ¼ 0.6 mm. Egg loads were estimated by dissecting and counting mature eggs in beetles that were collected on the peak sample date for each site. Total potential fecundity of A. lacertosa cannot be measured by dissecting the beetles because A. lacertosa are synovigenic. Wing length was tested as a predictor of instantaneous egg load for female beetles, in peak density samples, using a linear regression. Analysis of the correlation or interactions between beetle egg load and CDD data could not be conducted because of the small sample size of egg-bearing females that came from sites with similar CDD. Instead, at each site, the Julian date on which the cumulative degree days totaled at least 1230 CDD was tested as a predictor of beetle size using a weighted linear regression. Sites with peak beetle densities of 2 beetles per m 2 (n ¼ 4 sites) were not used in this analysis. The number of females sampled per site was the weighting factor. Beetle size was analyzed using a factorial ANOVA with sex (male or female) and beetle density (low, moderate, or high) as factors. Differences between treatment means were assessed using TukeyÕs HSD test (a ¼ 0:05) Beetle impact on leafy spurge Beetle impact on leafy spurge was assessed by analyzing the halo data. Halo presence or absence was recorded at all sites in 1999 and The relationship between 1999 beetle densities and the presence or absence of a halo was assessed using logistic regression. Leafy spurge attributes were measured at 17 sites in 1997, prior to the release of the biocontrol agent, and

5 422 A.R. Kalischuk et al. / Biological Control 29 (2004) again in Percent cover of leafy spurge was visually estimated and the number of vegetative and flowering shoots were counted using quadrats (50 20 cm) placed at 1, 3, and 5 m from the release point in each of the four cardinal directions. Average height of the leafy spurge canopy within 5 m of the release point was recorded in 1997 and again in The change in the canopy height of leafy spurge was tested against the fixed factor of beetle density in 2000 (low, moderate, and high) using ANOVA. Differences between treatment means were assessed using TukeyÕs HSD test (a ¼ 0:05). Changes in leafy spurge percent cover and stem count from 1997 to 2000, between sites with varying beetle densities (low, moderate, or high), and at 3 distances (1, 3, or 5 m) from the release point were assessed using split-plot ANOVA (Zar, 1999). The mean change per site of the percent cover and the number of stems (flowering and vegetative combined) of leafy spurge were calculated by pooling across the directions north, south, east, and west. Sample sizes varied as a result of 2000 drought conditions that caused some of the leafy spurge plants to dry out and crumble before measurements of percent cover or canopy height could be taken. Differences between treatment means were assessed using TukeyÕs HSD test (a ¼ 0:05). 3. Results Following their release, it was discovered that the beetles were mixed populations of A. lacertosa and A. czwalinae. The relative proportion of each species of beetle in the releases was unknown. The beetles are morphologically similar (LeSage, 1996) with both species being small and black, but are distinguishable by the color of the hind femur; that of A. czwalinae is dark-colored while the femur of A. lacertosa is yellow (LeSage, 1996). Beetle populations in Alberta consisted almost entirely of A. lacertosa. Aphthona czwalinae accounted for less than 0.5% (n ¼ 955) and 0.4% (n ¼ 833) of all the beetles collected from the release sites in 1999 and 2000, respectively. In 1999, there were no significant differences between mean beetle densities at sites where 1000 or 2000 beetles were released (Mann Whitney U ¼ 185; P ¼ 0:558). In 1999, sites with the highest densities of A. lacertosa were south of Calgary (51.7 N, W) (Fig. 1A). Beetle densities were moderate or high at 14% of the sites. Beetles were not found at 14 of the low-density sites on the single date that the site was monitored. High beetle density sites were at Lethbridge, Millarville, and Pincher Creek. At all 3 of these sites, the mean density of A. lacertosa was approximately 85 beetles per m 2. In 2000, beetles were found on at least one sample date at every site that was monitored. Peak beetle densities ranged from 2 to 250 per m 2 across sites and beetle densities were moderate or high at more than 60% of the sites (Figs. 1B and 4). All high-beetle-density sites were south of Calgary. Sites with low, moderate, and high beetle densities were visually distinct in the field. At low-density sites, there was no visible damage to leafy spurge shoots and no beetles were visible. Moderate density sites had visibly damaged leafy spurge shoots, and the beetles were easy to find. High-density sites were outbreak sites where the beetles were so numerous that leafy spurge shoots were completely covered with beetles and many leafy spurge shoots were completely defoliated by the beetles. Damage by the beetles at high-density sites was often visible as a halo, a patch of dead leafy spurge that usually extended a few meters around the point of release. All of the females that were collected on the date of peak beetle density were dissected. Only 19 of those females had eggs and those beetles came from 8 sites. There was a positive relationship between beetle size, estimated from wing length, and egg load; wing length accounted for 42% of the variation in instantaneous egg load (Fig. 5A). Larger beetles were found at sites that accumulated degree days earlier in the season. Sixtyeight percent of the variation in female wing length was accounted for by the date that the site reached the estimated peak CDD (Fig. 5B). Wing lengths of A. lacertosa females were significantly larger than males (F 1;397 ¼ 34:4; P < 0:001; Fig. 6A). Wing lengths were also significantly larger at moderate beetle densities compared to low- and highdensity beetle sites (F 2;397 ¼ 7:807; P < 0:001; Fig. 6B). The interaction between beetle sex and beetle density was not significant. Halos were present at 12 of the 50 sites monitored in The occurrence of a halo was positively related to beetle density (logistic regression: n ¼ 48; v 2 ¼ 74:96; P < 0:008). In 1999, there was more than a 50% probability of a halo at sites where mean beetle density was greater than 12 beetles per m 2 (i.e., moderate or high beetle density sites). Halos that were visible at sites in 1999 were still visible in The decrease in leafy spurge height from 1997 to 2000 was significantly greater (F 2;7 ¼ 11:25; P ¼ 0:007) at sites that had high beetle densities than at sites with lower beetle densities in 2000 (Fig. 7A). The reductions in percent cover and stem density of leafy spurge from 1997 to 2000 were significantly greater at sites that had more beetles in 2000 (Fig. 7B Percent cover, F 2;9 ¼ 8:4; P ¼ 0:009; Fig. 7C; Stem density: F 2;12 ¼ 5:9; P ¼ 0:017). Distance from the release point up to 5 m did not affect in changes in leafy spurge cover or stem density.

6 A.R. Kalischuk et al. / Biological Control 29 (2004) Fig. 4. Change in Aphthona lacertosa populations at 17 release sites in Alberta that were monitored during the summer of Latitude ( N) and longitude ( W) of the site and the near town is indicated at the top of each graph. Points represent actual mean beetle densities. The two top rows of graphs span from west to east across Alberta while the bottom rows span roughly south to north. 4. Discussion 4.1. Implications for leafy spurge biocontrol It appears that peak beetle emergence in 2000 may have been missed at a few of the more southern and western sites (Fig. 4). Alternatively, peak emergence may have occurred on or near the first date of collection. Lethbridge is typically warmer earlier than Ft. Macleod (Kalischuk, 2001) and peak emergence was not missed there (Fig. 4). The more eastern sites in Bow Island and Taber tend to warm up a bit earlier than Lethbridge, but differences between CDD tend to be less than 8 days apart between Lethbridge and the warmest sites in Medicine Hat (Kalischuk, 2001). Thus, analyses using peak beetle emergence data at the earlier warming sites should be comparable to peak beetle densities at other sites since all sites used estimates based on a 2-week sampling period. When comparing the effects of beetle density on size and potential fecundity, the largest beetles were found at the moderate-density beetle sites. This is contrary to the expectation that the largest beetles, which have the greatest potential fecundity (Fig. 5A), should be found at high-density beetle sites. However, once beetles reach very high densities, size may decline because of crowding or reductions in food quality (Slansky and Scriber, 1985). The largest beetles being associated with the sites that accumulate degree-days more rapidly (Fig. 5B) may be the result of those sites having higher food quality. Alternatively, higher temperatures may enable beetle larvae to feed more effectively and thus achieve their maximum size. The management implications of these results are that leafy spurge may be more quickly reduced at rapidly warming sites such as in southeastern Alberta, because those populations of A. lacertosa have the potential for the greatest population growth. The current study indicates that A. lacertosa provides some suppression of leafy spurge, 3 years post-release. A. lacertosa has successfully established and persists at

7 424 A.R. Kalischuk et al. / Biological Control 29 (2004) Fig. 5. (A) Relationship between Aphthona lacertosa wing length and instantaneous egg load (n ¼ 19) as measured from 8 sites in Alberta in Linear regression equation: number of eggs ¼ (wing length) ) 9.402, r 2 ¼ 0:424; P ¼ 0:003. (B) Mean wing length ( SE) of A. lacertosa females at sites in 2000 and the Julian date in 2000 that each site reached a cumulative degree date of 1230, the predicted date for peak beetle abundance. The number of beetle wings measured at each site is indicated. Weighted linear regression equation: wing length ¼ )00016(date) , r 2 ¼ 0:678, P ¼ 0:001. moderate to high densities at the majority of release sites in Alberta. Within 3 years, high densities of A. lacertosa significantly reduced leafy spurge within a localized area (within 5 m of the release point). The long-term prognosis for control of leafy spurge with A. lacertosa is positive, based on similar observations made for A. nigriscutis 3 years after their release in Alberta (McClay et al., 1995). Three years post-release, damage by A. nigriscutis at some of the release sites was visible and approximately 15 years post-release, leafy spurge has essentially disappeared from some of the A. nigriscutis sites (Andrea Kalischuk, personal observation). In fact, at one site in Edmonton, Alberta, A. nigriscutis reduced dense leafy spurge cover to less than 1% cover and less than 1 g per m 2 aboveground biomass in five years (McClay et al., 1995). At most of the A. nigriscutis release sites where control has been achieved, prior to beetle release, leafy spurge was the predominant vegetation that covered several hundred square meters. Fig. 6. Mean wing length ( SE) of Aphthona lacertosa in Alberta 2000 for (A) male and female beetles and (B) low, moderate, and highdensity beetle sites. Numbers are sample size, points with no lowercase letter in common are significantly different (P 6 0:05). Since beetle release, the leafy spurge has been reduced to an occasional stem within a more diversified prairie grassland and A. nigriscutis are still plentiful. Based on these observations, long-term evaluation of A. lacertosa release sites, particularly those with moderate or high beetle densities, will be critical to assess control of leafy spurge over larger scales. Leafy spurge reductions caused by A. lacertosa were not necessarily in the vicinity where the beetles had been released. Although beetles were released at a release point, the location of beetle damage was located in several places in the release patch. In the short term and on an individual plant basis, Kalischuk (2001) noted that A. lacertosa beetles actively aggregate and that plant morphology affects beetle distribution and feeding patterns. Twenty-four hours following their release, A. lacertosa were more likely to be found on flowering rather than vegetative shoots, and feeding damage was more likely to be found on leafy spurge shoots that were closer to the release point, shorter, and vegetative (Kalischuk, 2001). From a management perspective, this means that eradication of entire patches of leafy spurge may be difficult because of host plant preferences. However, elucidation of the behavior patterns of the flea beetles on a patch-scale will enable better recommendations for the future control of leafy spurge.

8 A.R. Kalischuk et al. / Biological Control 29 (2004) contribute to the development of biological control as a more predictive method of weed control. Acknowledgments We thank C. Kasahoff, K. Stromme, and J. Tansey (AAFRD) for work on beetle releases and initial field data collection; L. Van Heek and R. Wilson (AAFC) for assisting in subsequent field data collection; and numerous agricultural fieldmen and landowners for assistance in site location and access to sites. We also thank B. Lee (AAFC) for assistance in beetle wing measurements. Financial support was provided by Agriculture and Agri-Food Canada through the Federal Student Work Employment Program and a Matching Investment Initiative. This paper is LRC contribution number References Fig. 7. Mean change ( SE) in: (A) leafy spurge height, (B) percent cover, and (C) stem density from 1997, immediately prior to the release of the biocontrol agent Aphthona lacertosa, to Beetle density is the mean peak beetle density in The number of sites is indicated. Points with no lowercase letter in common within each category are significantly different (P < 0:05) Implications for future biocontrol releases Recently, there has been significant debate about the benefits and risks of biological control (Henneman and Memmott, 2001; Louda et al., 1997; Simberloff and Stiling, 1996; Wajnberg et al., 2001). The debate makes clear that future use of biological control will depend heavily upon our ability to predict the outcome of the planned releases. This study is the first step in a post hoc assessment of A. lacertosa on leafy spurge. It demonstrates that the biocontrol agent is behaving as predicted and reducing its target weed population at the scale of 5 m from the release stake. Additional research on the spread and impact of the beetles at patch- and landscape-scales will further contribute to the understanding of this particular system. Continued post hoc assessments of leafy spurge and other weeds are needed. For example, although biocontrol agents have successfully caused significant decreases in weed populations such as Dalmation toadflax (Linaria dalmatica (L.) Miller) (De Clerck-Folate and Harris, 2002) and houndstongue (Cynoglossum officinale L.) (De Clerck-Floate and Schwarzlander, 2002), there are few studies that document the patterns of weed decline or biocontrol agent behavior. Continued assessments of these biocontrol programs will subsequently Allen, J., A modified sine wave method for calculation degreedays. Environ. Entomol. 5, Bourchier, R., Erb, S., McClay, A., Gassman, A., Euphorbia esula (L.) (Leafy Spurge) and Euphorbia cyparissias (L.) (Cypress Spurge) (Euphorbiaceae). In: Mason, P., Huber, J. (Eds.), Biological Control Programmes in Canada CABI Publishing, Wallingford, Oxon, UK, pp Callaway, R., Deluca, T., Belliveau, W., Biological-control herbivores may increase competitive ability of the noxious weed Centaurea maculosa. Ecology 80, De Clerck-Floate, R.A., Schwarzlander, M., Cynoglossum officinale (L.) Houndstongue (Boraginaceae). In: Mason, P., Huber, J. (Eds.), Biological Control Programmes in Canada CABI Publishing, Wallingford, Oxon, UK, pp De Clerck-Folate, R.A., Harris, P., Linaria dalmatica (L.) Miller, Dalmation Toadflx (Scrophulariaceae). In: Mason, P., Huber, J. (Eds.), Biological Control Programmes in Canada CABI Publishing, Wallingford, Oxon, UK, pp Gassmann, A., Schroeder, D., The search for effective biological control agents in Europe: history and lessons from leafy spurge (Euphorbia esula L.) and cypress spurge (Euphorbia cyparissias L.). Biol. Control 5, Haber, E., Invasive exotic plants of Canada. Fact Sheet No. 9: Leafy Spurge. National Botanical Services, Canada, ON. Available from < Hansen, R.W., Richard, R.D., Parker, P.E., Wendel, L.E., Distribution of biological control agents of leafy spurge (Euphorbia esula L.) in the United States: Biol. Control 10, Harris, P., Dunn, P.H., Schroeder, D., Vonmoos, R., In: Watson, A.K. (Ed.), Leafy spurge. Monograph Series of the Weed Science Society of America, vol. 3, pp Henneman, M.L., Memmott, J., Infiltration of a Hawaiian community by introduced biological control agents. Science 293, Julien, M., Griffiths, M., Biological control of weeds A World Catalogue of Agents and their Target Weeds, fourth ed. CABI Publishing, Wallingford, UK. Kalischuk, A., Density and efficacy of the flea beetle Aphthona lacertosa (Rosenhauer), an introduced biocontrol agent for leafy spurge, in Alberta. M.Sc. Thesis, University of Lethbridge, Lethbridge, AB, 93 pp.

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