Ecology Letters, (2006) 9: 443–450 doi: 10.1111/j.1461-0248.2006.00896.x LETTER Biological control agents elevate hantavirus by subsidizing deer mouse populations Abstract Dean E. Pearson* and Ragan M. Biological control of exotic invasive plants using exotic insects is practiced under the Callaway assumption that biological control agents are safe if they do not directly attack non-target Division of Biological Sciences, species. We tested this assumption by evaluating the potential for two host-specific University of Montana, biological control agents (Urophora spp.), widely established in North America for Missoula, MT 59812, USA spotted knapweed (Centaurea maculosa) control, to indirectly elevate Sin Nombre *Correspondence and present hantavirus by providing food subsidies to populations of deer mice (Peromyscus address: Dean E. Pearson, Rocky maniculatus), the primary reservoir for the virus. We show that seropositive deer mice Mountain Research Station, (mice testing positive for hantavirus) were over three times more abundant in the USDA Forest Service, 800 E. Beckwith Ave., Missoula, presence of the biocontrol food subsidy. Elevating densities of seropositive mice may MT 59801, USA. E-mail: increase risk of hantavirus infection in humans and significantly alter hantavirus ecology. [email protected] Host specificity alone does not ensure safe biological control. To minimize indirect risks to non-target species, biological control agents must suppress pest populations enough to reduce their own numbers. Keywords Biological control, Centaurea maculosa, disease ecology, food subsidies, hantavirus, indirect effects, invasive species, non-target effects, Peromyscus maniculatus, Urophora. Ecology Letters (2006) 9: 443–450 agents (Urophora spp.) widely established across western INTRODUCTION North America to control spotted knapweed (Centaurea Exotic plant invasions threaten the biological diversity of maculosa Lam.), indirectly increase the Sin Nombre natural ecosystems around the world (Wilcove et al. 1998, hantavirus by providing food subsidies to populations of Mack et al. 2000). Classical biological control, the introduc- its native rodent reservoir. tion of exotic organisms to control exotic invasive species, is The gall flies Urophora affinis (Frauenfeld) and Urophora a promising strategy that has proven effective at controlling quadrifaciata (Meigen) were first introduced into North exotic pests once they become widely established in natural America in the early 1970s as biological control agents for ecosystems (McFadyen 1998; Gurr & Wratten 2000). spotted (C. maculosa) and diffuse knapweed (Centaurea diffusa However, the introduction of exotic organisms for Lam.) (Harris 1980a), exotic forbs that aggressively invade biological control entails risks to non-target species arid habitats of western North America (Sheley et al. 1998). (Simberloff & Stiling 1996; Louda et al. 1997; McEvoy & Adult Urophora lay eggs within immature flowerheads of Coombs 2000; Strong & Pemberton 2000). For example, Centaurea where the resulting larvae induce gall formation control agents with broad host ranges sometimes attack (Harris 1980a). The larvae overwinter within the seedheads native species, causing deleterious non-target effects from September to June, then emerge as adults and repeat (Simberloff & Stiling 1996; Louda et al. 1997; Stiling the cycle (Story et al. 1992). Urophora have remained host 2002). To reduce this threat, rigorous screening for host- specific since their introduction over 30 years ago and have specificity is conducted before introduction of biological been shown to substantially reduce seed production in the control agents for weed control (McEvoy 1996). However, two Centaurea species (Harris 1980b). However, seed this does not prevent agents from indirectly impacting non- reductions have not effectively controlled these weeds target organisms through food web interactions (Holt & (Maddox 1982; Stanley 2005) which continue to spread to Hochberg 2001; Pearson & Callaway 2003). Here we new locations and increase in abundance. As a result, provide evidence that two host-specific biological control Urophora now infest C. maculosa and C. diffusa populations Ó 2006 Blackwell Publishing Ltd/CNRS. No claim to original US government works 444 D. E. Pearson and R. M. Callaway across western North America and have become as in areas with high C. maculosa abundance (mean cover superabundant as their prolific hosts, occurring at densities > 20%). The two high C. maculosa grids at each site were many times greater than in their native Europe (Myers & originally selected to represent high and medium infesta- Harris 1980). tions of C. maculosa. However, comparisons of C. maculosa The abundance and availability of Urophora larvae during cover estimates showed no difference between the high fall, winter and spring make them a valuable food resource and medium grids, so these were pooled to form the high for many native consumers. Deer mice (Peromyscus manicu- C. maculosa category. Each grid was comprised of 100 latus Wagner), in particular, readily exploit this novel food sampling stations at 10 m spacing in a 10 by 10 array. At source (Pearson et al. 2000). Within C. maculosa-invaded each site, grids were placed in areas with similar grasslands, deer mice switch microhabitats and shift their topography and vegetation composition and separated by diet to exploit the seasonally available larvae (Pearson et al. x ¼ 3500 m (SD ¼ 5400) to ensure independence among 2000). As a result, in areas with abundant C. maculosa, grids. Each site was trapped at the same time each year in Urophora larvae make up ‡ 50% of the deer mouse diet spring (April to May) 2001–2003. Spring was the focus during most months of the year and 85% of their diet during because Urophora food subsidies are most likely to affect key winter months when these mice typically experience a SNV through increased overwinter survival of mice population decline associated with scarce native food (Ortega et al. 2004, D. Pearson and R. Fletcher, unpub- resources (Pearson et al. 2000). This exploitation of Urophora lished results). Increased overwinter survival should reduce larvae has increased survival of mice and doubled deer the risk that mouse populations and SNV will be locally mouse populations in habitats invaded by C. maculosa extirpated (e.g. Boone et al. 1998). It may also facilitate the (Ortega et al. 2004, D. Pearson and R. Fletcher, unpublished horizontal transmission of the virus through higher results). overwinter mouse densities and winter aggregations of The significant direct effects of Urophora food subsidies mice in C. maculosa stands (Pearson et al. 2000; Ortega et al. on deer mouse populations may translate into significant 2004, D. Pearson and R. Fletcher, unpublished results). indirect effects on other organisms through food web Spring is also significant for SNV transmission since SNV interactions (Pearson & Callaway 2003). For example, deer seasonally peaks in spring (Bennett et al. 1999; Calisher mice are the primary reservoir for the Sin Nombre virus et al. 1999; Douglass et al. 2001) and HPS cases increase at (SNV) (Childs et al. 1994), which causes the deadly this time (Mills et al. 2002). hantavirus pulmonary syndrome (HPS) in humans. Thus, food subsidies that elevate deer mouse populations may Urophora density and host abundance increase the prevalence of SNV and thereby elevate the risk of humans contracting HPS (Mills et al. 1999; Yates et al. Overwintered mice captured in spring are affected by 2002). To test the hypothesis that Urophora indirectly Urophora produced the previous summer, because Urophora increase SNV incidence through food subsidies to deer produced during the growing season are only available to mice (food subsidy hypothesis), we compared deer mouse mice as larvae during the period from the fall after eggs are abundance and SNV incidence between deer mouse laid until the following spring when the mature flies leave populations in areas with high and low abundance of the flowerheads (Pearson et al. 2000). Therefore, to deter- C. maculosa and Urophora. mine the amount of Urophora available as a food resource to overwintering mice sampled during the springs of 2001– 2003, we sampled Urophora produced during the summers of METHODS 2000–2002. In 2001 and 2002, we estimated Urophora densities at the end of the growing season in 0.5-m2 quadrat Study sites and overall design frames systematically located at 33 sampling stations across We compared deer mouse abundance and SNV incidence each grid. At each station, we counted all C. maculosa stems, between locations with high and low C. maculosa infesta- determined the number of seedheads per stem for 10 tions, representing correspondingly high and low Urophora randomly selected stems and estimated the number of densities, for 3 years (2001–2003) at eight replicate sites in Urophora larvae per seedhead by dissecting 20 seedheads bluebunch wheatgrass (Pseudoroegneria spicata) and rough randomly selected from the 10 stems. This allowed us to fescue (Festuca scabrella) grassland types across western estimate the density of Urophora per 0.5 m2 in 2001 and Montana. Replicate sites represented distinct mountains or 2002 and to back-estimate Urophora densities in 2000. The valleys and were separated by major drainages and spread extrapolation of Urophora densities in 2000 was necessary over > 5000 km2. We placed three 1-ha grids at each because we initiated mouse sampling in 2001 but the