Testing biological control agent compatibility: Cyphocleonus achates and Larinus minutus on diffuse knapweed

While weed biological control success is typically achieved with one agent, multiple agents are invariably introduced. Biological control agents that share a host–plant may interact either directly or indirectly through changes in host–plant quality. Negative interactions could reduce the impacts of the agents on the density of their host–plant while positive interactions (facilitation) could improve biological control success.In the Okanagan Valley of British Columbia, Canada, initial declines in the invasive rangeland weed, diffuse knapweed (Centaurea diffusa) were attributed to the introduction of the weevil Larinus minutus. A second weevil, Cyphocleonus achates has recently become common on diffuse knapweed. We sought to determine if the recent increase of C. achates could threaten the success of L. minutus. We considered whether L. minutus colonisation or performance remained the same when C. achates was present, and whether the two agents acted independently to reduce plant performance.Neither changes in colonisation rates nor competitive interactions were apparent between C. achates and L. minutus. Both insects reduced plant performance and, for all metrics, the reduction in plant performance by one species was independent of the second. The two agents appear to be compatible and both should contribute to the control of diffuse knapweed. To assess how biological control agents interact requires understanding both their competitive interactions and their joint effects on the shared host.     

The lesser of two weevils: physiological responses of spotted knapweed (Centaurea stoebe) to above- and belowground herbivory by Larinus minutus and Cyphocleonus achates

The physiological responses of plants to variable levels of root and shoot herbivory in the field may yield valuable insights regarding potential compensation or tolerance responses for herbivory. In an infestation of Centaurea stoebe (spotted knapweed) located in the Colorado foothills, we measured physiology, biomass, and flower production of individual plants undergoing a natural range of herbivory by the above- and belowground biological control insects, Larinus minutus and Cyphocleonus achates. Over the growing season, net carbon assimilation rate, transpiration, stomatal conductance, and intercellular leaf [CO₂] (C _i) all decreased, while water use efficiency increased. The decrease in these physiological traits was due to an increase in the intensity of L. minutus damage over time; effects of C. achates root damage to plant physiology, including transpiration were only marginally significant. The effects of these two species on plant physiology were not interactive, and Larinus minutus was found to exert larger negative effects on this invasive plant in terms of plant physiology and potential reproductive output than C. achates. While previous studies have shown C. achates to have significant negative effects on population densities of spotted knapweed, the addition of Larinus minutus to the suite of insects used in biological control of spotted knapweed should facilitate continued or enhanced reduction in densities of this noxious weed.     

Plant community changes after the reduction of an invasive rangeland weed, diffuse knapweed, Centaurea diffusa

The expected outcome of weed control in natural systems is that the decline of a dominant weed will result in an increase in diversity of the plant community but this has seldom been tested. Here we evaluate the response of the plant community following the decline of diffuse knapweed (Centaurea diffusa) in six different pastures at White Lake, BC, Canada over five years. This period followed the establishment, spread and high levels of attack by the introduced European weevil, Larinus minutus, as part of a biological control program. Knapweed declined immediately before and during the study period, but, contrary to expectations, the species richness and diversity of the rangeland plant community did not increase. The absolute cover of native and introduced forbs and grasses increased following knapweed decline, but only the introduced grasses showed a consistent increase in cover relative to the other life-forms. However, unlike in other studies, the native plants dominated the study site. We conclude that the changes in plant communities following successful biological control are variable among programs and that the impact of replacement species must be evaluated in assessing the success of ecological restoration programs that use biological control to manage an undesirable weed.     

Impact of the belowground herbivore and potential biological control agent, Ceutorhynchus scrobicollis, on Alliaria petiolata performance

We studied the influence of the root-crown weevil Ceutorhynchus scrobicollis on its host plant Alliaria petiolata, a European biennial herb that is currently invading much of temperate North America. Varying timing of attack and herbivore densities in a common garden allowed to assess seasonality of plant response, density-dependence of impact, and the effect of intraspecific competition on C. scrobicollis recruitment (number of F₁ generation adults emerged). Data collected in the common garden were compared with data collected at field sites. C. scrobicollis is a common weevil in Europe, frequently attaining high attack levels on its host plant. In the common garden, weevil attack decreased plant survival by up to 43%, reduced plant height by 54%, increased the number of shoots by up to four–fold and delayed seed ripening, but had no significant negative effect on seed production. Plants infested in spring allocated less biomass to aboveground plant parts, and remained smaller than plants attacked in autumn, indicating that the latter were able to partly compensate for weevil attack. Increasing weevil density rarely had an effect on A. petiolata performance, and did not increase F₁ recruitment, suggesting strong intraspecific competition. At field sites, C. scrobicollis attack is spread over a long time period, which probably alleviates intraspecific competition. In summary, attack by the root-crown feeding weevil, C. scrobicollis, can substantially reduce growth and survival of A. petiolata. If introduced as a biological control agent into North America, C. scrobicollis is likely to decrease the fitness and competitive superiority of A. petiolata.     

Aphid suitability for the predatory hoverfly Episyrphus balteatus altered with elevating atmospheric CO2 and sinigrin

The fitness of natural enemies should be altered in response to changes in herbivore quality induced by the impact of increased atmospheric CO₂ levels on plants. We studied the effect of different CO₂ levels on the aphid predator Episyrphus balteatus DeGeer fed either specialist or generalist aphids reared on either of two host plants under laboratory conditions. In the host plant that contains sinigrin (black mustard), elevated CO₂ increased the sinigrin content of both host plant and the specialist aphid, but reduced the already very low levels in the generalist aphid. Predator development time increased with elevated CO₂, while fecundity decreased. Consequently, individual fitness decreased slightly with increasing atmospheric CO₂. Sinigrin significantly decreased fecundity and increased development time of the predator. As a result, fitness was significantly lower too. The consumption rate was influenced significantly by plant and prey solely and the interactions of host plant × prey type and CO₂ level × prey type. Further research on the effects of climate change parameters (e.g. greenhouse gases such as CO₂, ozone (O₃) and nitrogen dioxide (NO₂), etc.) separately and jointly under controlled environmental conditions will help to understand the nature and direction of their effects on natural enemies as part of the tritrophic system.     

Interaction between Medicago truncatula and Pseudomonas fluorescens: Evaluation of Costs and Benefits across an Elevated Atmospheric CO2

Soil microorganisms play a key role in both plants nutrition and health. Their relation with plant varies from mutualism to parasitism, according to the balance of costs and benefits for the two partners of the interaction. These interactions involved the liberation of plant organic compounds via rhizodeposition. Modification of atmospheric CO₂ concentration may affect rhizodeposition and as a consequence trophic interactions that bind plants and microorganisms. Positive effect of elevated CO₂ on plants are rather well known but consequences for micoorganisms and their interactions with plants are still poorly understood. A gnotobiotic system has been developed to study the interaction between Medicago truncatula Jemalong J5 and the mutualistic bacteria Pseudomonas fluorescens strain C7R12 under two atmospheric CO₂ concentrations: ambient (365 ppm) versus enriched (750 ppm). Costs and benefits for each partner have been determined over time by measuring plant development and growth, the C and N contents of the various plant parts and the density of the bacteria in rhizosphere compartments. Following the increase in CO₂, there was a beneficial effect of P. fluorescens C7R12 on development, vegetative growth, and C/N content of M. truncatula. Concerning plant reproduction, an early seed production was noticed in presence of the bacterial strain combined with increased atmospheric CO₂ conditions. Paradoxically, this transient increase in seed production was correlated with a decrease in bacterial density in the rhizosphere soil, revealing a cost of increased CO₂ for the bacterial strain. This shift of costs-benefits ratio disappeared later during the plant growth. In conclusion, the increase in CO₂ concentration modifies transiently the cost-benefit balance in favor of the plant. These results may be explained either by a competition between the two partners or a change in bacterial physiology. The ecosystem functioning depends on the stability of many plant-microbe associations that abiotic factors can disrupt.     

Influences of two life‐history stages of the weevil,Larinus minutus,on its host plant Centaurea diffusa

1. We hypothesised that, for weed biological control, using species that feed as both adults and larvae would be advantageous. Here, we test the impacts of adult and larval feeding of Larinus minutus (Col.: Curculionidae), a successful biological control agent of diffuse knapweed, Centaurea diffusa (Asteraceae), in British Columbia, Canada. 2. At one site, the observations of the present study showed that the intensity of adult weevil feeding did not predict the level of larval attack. Experiments found no evidence for plant‐mediated competition between the life‐history stages. 3. At two sites and in two years, experimental manipulations of adult and/or larval feeding damage were conducted and plant performance measured. Neither antagonistic nor synergistic interactions occurred, but at each of the two sites a different life‐history stage was responsible for reduction in the number of seeds produced by the plants. 4. Although one of the two different feeding modes was redundant at each site, the ability of adults and larvae to reduce plant performance in different areas makes the species effective in a wider range of environments.     

How do biological control and hybridization affect enemy escape?

Two mechanisms often linked with plant invasions are escape from enemies and hybridization. Classical biological control aims to reverse enemy escape and impose top-down population control. However, hybridization has the potential to alter interactions with enemies and thus affect biological control. We examined how introductions of biological control agents affect enemy escape by comparing specialist enemy loads between the native and introduced ranges of two noxious weeds (spotted and diffuse knapweed; Centaurea stoebe L. and C. diffusa Lam.) that have been the targets of an extensive biological control program. Hybrids between spotted and diffuse knapweed are often found within diffuse knapweed sites in North America, so we also compared enemy loads on plants that appeared morphologically like diffuse knapweed and hybrids. Finally, we tested the preference for diffuse knapweed, hybrids, and spotted knapweed of one of the agents thought to be instrumental in control of diffuse knapweed (Larinus minutus; Curculionidae). In North America spotted knapweed has largely escaped its root herbivores, while seedhead herbivore loads are comparable in the introduced and native ranges. Diffuse knapweed exhibited seedhead herbivore loads five times higher in the introduced compared to native range. While this pattern of seedhead herbivory is expected with successful biological control, increased loads of specialist insect herbivores in the introduced range have rarely been reported in the literature. This finding may partially explain the better population control of diffuse vs. spotted knapweed. Within North American diffuse knapweed sites, typical diffuse knapweed and hybrid plants carried similar herbivore loads. However, in paired feedings trials, the specialist L. minutus demonstrated a preference for newly created artificial hybrids over North American diffuse knapweed and for European diploid spotted knapweed over North American tetraploid spotted knapweed. Overall though, hybridization does not appear to disrupt biological control in this system.     

Effects of Rhinoncomimus latipes on the growth and reproduction of Persicaria perfoliata,an invasive plant in North America

Native to Asia, mile-a-minute Persicaria perfoliata, is an invasive weed in North America, and the weevil Rhinoncomimus latipes is a host-specific insect agent which occurs widely in China. We conducted a common garden experiment to compare P. perfoliata plant responses of native and invasive populations to herbivory by the weevils from different origins. We found weevils from Hunan, Hubei and Heilongjiang Provinces had strong, moderate and weak ability to suppress host plant, respectively. Weevils from Hunan and Hubei Provinces had stronger impact on the growth of both native and invasive plant populations than the weevil from Heilongjiang Province. The losses in seed output of invasive plants were also significantly greater than natives in the weevil treatments. Our results suggested that the weevil population from Hunan Province may be the most suitable for the control of mile-a-minute, while the population from Heilongjiang Province may be the least suitable due to climate matching.     

Strong effects of hydrologic environment and weak effects of elevated CO<Subscript>2</Subscript> on the invasive weed <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> and the biocontrol beetle <Emphasis Type="Italic">Agasicles hygrophila</Emphasis>

Global change, such as elevated CO₂, may alter interactions between invasive plants and biocontrol agents, impacting biocontrol efficacy. Here, we conducted four experiments in Texas, USA to test how elevated CO₂ influences an invasive plant (Alternanthera philoxeroides) and its interactions with an introduced biocontrol beetle (Agasicles hygrophila) in terrestrial (well-watered) and flooded environments. We grew plants for 9 months in ambient or elevated CO₂ (800 ppm) chambers in continuously flooded or well-watered conditions. In no-choice trials, flooding increased leaf toughness and decreased beetle consumption but beetles only oviposited on ambient CO₂ leaves. In choice trials, beetles preferred to feed and oviposit on terrestrial plants but were also less likely to damage elevated CO₂ leaves. Caged beetle populations were larger in terrestrial conditions than aquatic conditions for a second set of plants grown in the chambers. With a third set of plants grown in the ambient or elevated CO₂ chambers, damage for plants placed in the field (aquatic setting) was higher for plants grown in terrestrial conditions vs. flooded conditions at ambient CO₂. Our results suggest that elevated CO₂ will have minor effects on the efficacy of this biocontrol agent by decreasing oviposition and number of leaves damaged, and hydrologic environment may affect invasive plant performance by altering herbivore oviposition and feeding preferences. A broader understanding of the effects of global change on biocontrol will help prevent and manage future spread of invasive plants.     

Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado

Abundances and interactions among biological control insects and their effects on target invasive plants were monitored within the flower heads and roots of diffuse knapweed, Centaurea diffusa, and in spotted knapweed, Centaurea stoebe, along the Colorado Front Range. Flower weevils, (Larinus species) and root-feeders (Cyphocleonus achates and Sphenoptera jugoslavica) were released on knapweed that already supported biological control gall flies (Urophora species). At a single monitoring site, seed production by C. diffusa declined from 4400 seeds m⁻² in 1997 to zero seeds m⁻² on the monitoring sites in 2006, while the flowering stem density of C. diffusa declined from a peak of almost 30 stems m⁻² in 2000 to zero stems m⁻² in 2006. The average abundance of Urophora and Larinus in flower heads fluctuated independently during the 2001–2006 interval, while the relative abundance of C. achates and S. jugoslavica in roots exhibited a weak inverse relationship that appeared driven by climate effects. The relative abundance of insects on a population of C. stoebe was monitored for five years as Larinus species and C. achates became established on spotted knapweed that already supported Urophora species. Spotted knapweed seed production on our monitoring site declined from 4600 seeds m⁻² in 2003 to zero seeds m⁻² in 2006. Unlike C. diffusa, substantial numbers of rosettes of C. stoebe remained present. Larinus consumed almost all Urophora encountered in C. diffusa, and consumed about 40% of the Urophora in co-infested flower heads of C. stoebe (ca. 10–15% of the total Urophora population). No negative correlations between the relative densities of flower head and root-feeding insects were observed. The effects of these insects on target plants have produced results consistent with the ‘cumulative stress hypothesis’ for biological control of Centaurea species.     

Elevated CO2 and aboveground–belowground herbivory by the clover root weevil

Predicted increases in atmospheric carbon dioxide (CO₂) concentrations are expected to increase primary productivity in many terrestrial ecosystems, which could lead to plants becoming N limited. Studies suggest that legumes may partially overcome this by increasing biological nitrogen fixation. However, these studies have not yet considered how these changes may be affected by the altered dynamics of insect herbivores feeding on the plant. This study investigated how elevated CO₂ (700 μl l^?1) affected the clover root weevil (Sitona lepidus), a significant pest of white clover (Trifolium repens). Adults feed on leaves aboveground where they lay eggs; soil-dwelling larvae initially feed on root nodules that house N₂-fixing bacteria. Foliar C:N ratios rose by 9% at elevated CO₂, but the biggest responses were observed belowground, with increases in root mass (85% greater) and nodule abundance (220% more abundant). Root C:N ratios increased significantly from 10.95 to 11.60 under elevated CO₂, which increased even further to 13.13 when nodules were attacked by larval S. lepidus. Adult S. lepidus consumed significantly more leaf tissue at elevated CO₂ (0.47 cm² day^?1) compared with ambient CO₂ (0.35 cm² day^?1), suggesting compensatory feeding, but laid 23% fewer eggs at elevated CO₂. Even though fewer eggs were laid at elevated CO₂, 38% more larvae were recovered suggesting that larval survival was much better under elevated CO₂. Increased larval abundance and performance at elevated CO₂ were positively correlated with the number of nodules available. In conclusion, reduced foliar quality at elevated CO₂ was generally disadvantageous for adult S. lepidus living aboveground, but extremely beneficial for S. lepidus larvae living belowground, due to the enhanced nodulation. Climate change may, therefore, enhance biological nitrogen fixation by T. repens, but potential benefits (e.g. provision of N without chemical fertilizers) may be undermined by larger populations of S. lepidus larvae belowground.     

Effect of elevated CO2 on tropical soda apple and its biological control agent Gratiana boliviana (Coleoptera: Chrysomelidae)

Rising atmospheric CO₂ levels could have drastic effects on the performance of invasive weeds and their insect herbivores. Despite the importance of biological control as an effective management tool for environmental weeds, there have been few studies on the potential impact of climate change on the future efficacy of biological control. The objective of the present study was to evaluate the effect of elevated CO₂ on the performance of tropical soda apple Solanum viarum (Solanaceae) and its biological control agent Gratiana boliviana (Coleoptera: Chrysomelidae). We established three levels of CO₂ in environmental growth chambers: ambient (400 ppm), medium (580 ppm) and high (780 ppm). Plants growing at the high level had greater biomass compared to those growing at the ambient and medium levels of CO₂. Leaf water content and the amount of leaf nitrogen were reduced at high compared to ambient or medium CO₂ levels. G. boliviana immature survival and developmental time were negatively affected at high CO₂ but not at medium or ambient levels. Adults were lighter and smaller when reared at the high CO₂ level compared to ambient and medium treatments, while adult fecundity was higher at the medium CO₂ level. Leaf area consumed by fifth instars was lower when feeding on plants grown at the high CO₂ level either inside a Petri dish or on potted plants. These results suggest that beetle performance may be diminished under future climate. However, further studies should incorporate other factors such as temperature and precipitation as well as the evolutionary potential of herbivores and plants to adapt to a changing climate.     

Impact of climate change on grassland production and soil carbon worldwide

The impact of climate change and increasing atmospheric CO₂ was modelled for 31 temperate and tropical grassland sites, using the CENTURY model. Climate change increased net primary production, except in cold desert steppe regions, and CO₂ increased production everywhere. Climate change caused soil carbon to decrease overall, with a loss of 4 Pg from global grasslands after 50 years. Combined climate change and elevated CO₂ increased production and reduced global grassland C losses to 2 Pg, with tropical savannas becoming small sinks for soil C. Detection of statistically significant change in plant production would require a 16% change in measured plant production because of high year to year variability in plant production. Most of the predicted changes in plant production are less than 10%.     

Effects of climate change scenarios on Tempranillo grapevine (Vitis vinifera L.) ripening: response to a combination of elevated CO2 and temperature, and moderate drought

Greenhouse experiments were conducted to investigate the impact of predicted climate change (elevated CO₂, 700 μmol CO₂ mol^?1 air vs. ambient; elevated temperature, 28/18°C vs. 24/14°C, day/night; and partial irrigation, 40% of field capacity vs. well-irrigated) on grape berry quality characteristics during ripening. Grapevine (Vitis vinifera L. cv. Tempranillo) fruiting cuttings were used as experimental plant material. Climate change shortened the time between grape veraison and full maturity. At harvest time, many of the grape quality parameters determined were affected by the different grape maturity. The data were re-grouped according to total soluble solids to factor out changes due to the shortened time to maturity, and the effects on grape quality were then re-examined. Under current CO₂ and temperature conditions, partial irrigation decreased berry malic acid concentration and facilitated anthocyanins extractability. Elevated CO₂ and temperature decreased berry malic acid and total anthocyanins potential in well-irrigated plants and increased tonality index, irrespective of water availability. In partial irrigation conditions, elevated CO₂ and temperature hindered the anthocyanins extractability. In summary, results indicate that climate change (elevated CO₂, high temperature and partial irrigation) affects phenology and berry quality.     

Do invasive alien plants benefit more from global environmental change than native plants?

Invasive alien plant species threaten native biodiversity, disrupt ecosystem functions and can cause large economic damage. Plant invasions have been predicted to further increase under ongoing global environmental change. Numerous case studies have compared the performance of invasive and native plant species in response to global environmental change components (i.e. changes in mean levels of precipitation, temperature, atmospheric CO₂ concentration or nitrogen deposition). Individually, these studies usually involve low numbers of species and therefore the results cannot be generalized. Therefore, we performed a phylogenetically controlled meta‐analysis to assess whether there is a general pattern of differences in invasive and native plant performance under each component of global environmental change. We compiled a database of studies that reported performance measures for 74 invasive alien plant species and 117 native plant species in response to one of the above‐mentioned global environmental change components. We found that elevated temperature and CO₂ enrichment increased the performance of invasive alien plants more strongly than was the case for native plants. Invasive alien plants tended to also have a slightly stronger positive response to increased N deposition and increased precipitation than native plants, but these differences were not significant (N deposition: P = 0.051; increased precipitation: P = 0.679). Invasive alien plants tended to have a slightly stronger negative response to decreased precipitation than native plants, although this difference was also not significant (P = 0.060). So while drought could potentially reduce plant invasion, increases in the four other components of global environmental change considered, particularly global warming and atmospheric CO₂ enrichment, may further increase the spread of invasive plants in the future.     

Autumn growth of three perennial weeds at high latitude benefits from climate change

In autumn, agricultural perennial weeds prepare for winter and can store reserves into creeping roots or rhizomes. Little is known about influence of climate change in this period. We tested the effect of simulated climate change in autumn on three widespread and noxious perennial weeds, Elymus repens (L.) Gould, Cirsium arvense (L.) Scop. and Sonchus arvensis L. We divided and combined simulated climate change components into elevated CO₂ concentration (525 ppm), elevated temperatures (+2–2.5°C), treatments in open‐top chambers. In addition, a control in the open‐top chamber without any increase in CO₂ and temperature, and a field control outside the chambers were included. Two geographically different origins and three pre‐growth periods prior to the exposure to climate change factors were included for each species. All species increased leaf area under elevated temperature, close to doubling in E. repens and quadrupling in the dicot species. E. repens kept leaves green later in autumn. C. arvense did not benefit in below‐ground growth from more leaf area or leaf dry mass. S. arvensis had low levels of leaf area throughout the experiment and withered earlier than the two other species. Below‐ground plant parts of S. arvensis were significantly increased by elevated temperature. Except for root:shoot ratio of C. arvense, the effects of pure elevated CO₂ were not significant for any variables compared to the open‐top chamber control. There was an additive, but no synergistic, effect of enhanced temperature and CO₂. The length of pre‐growth period was highly important for autumn plant growth, while origin had minor effect. We conclude that the small transfer of enhanced above‐ground growth into below‐ground growth under climate change in autumn does not favour creeping perennial plants per se, but more leaf area may offer more plant biomass to be tackled by chemical or physical weed control.     

内蒙古地带性针茅植物对CO2和气候变化的适应性研究进展

收集了1992—2013年关于模拟CO2浓度升高及气候变化(温度升高、降水变化)对内蒙古地带性草原群落的5个建群种针茅植物(贝加尔针茅、本氏针茅、大针茅、克氏针茅、短花针茅)影响的实验研究结果表明,模拟CO2浓度升高、增温和增雨将提高针茅植物的光合作用和株高生长,但CO2处理时间延长会导致光合适应;温度和降雨变化将改变针茅植物的物候进程,但物种之间反应有差异;CO2浓度升高有助于针茅植物生物量增加,增温和干旱则相反,CO2浓度升高对干旱的影响具有补偿作用;干旱和涝渍胁迫将提高针茅植物植株C/N,CO2浓度升高将加剧水分胁迫下针茅植物植株C/N的增加效应,导致牧草品质下降。由于当前在适应性指标、针茅植物对气候变化协同作用的适应机理及其敏感性研究等方面存在的不足,导致目前无法全面比较各针茅植物对CO2和温度、降水变化的响应差异及其敏感性,因而无法预测未来在全球变化背景下,这几种针茅植物的动态变化及其在地理分布上的迁移替代规律。为科学应对气候变化,未来应加强内蒙古地带性针茅植物的适应性指标、针茅植物对多因子协同作用的适应机理及敏感性研究。