Search

Eurasian seed-head weevils, Larinus obtusus Gyllenhal and Larinus minutus Gyllenhal (Coleoptera: Curculionidae), were released annually from 2004-2013 across Wisconsin for biological control of the invasive forb spotted knapweed ( Centaurea stoebe L. ssp. micranthos (Gugler) Hayek). In 2014, Larinus spp. presence/absence surveys were conducted at 18 previous releases sites between 7/18/2014 – 8/16/2014 from 2007 (n=3), 2008 (n=3), 2009 (n=3), 2010 (n=6), 2011 (n=1) and 2012 (n=2). We found a significant, linear relationship between time since release and the natural log of the observed dispersal rates of Larinus spp. (F 1,10 = 18.8, P = 0.002, R 2 = 0.65), suggesting an increasing dispersal rate through time. Modeled result suggested the following relationship: LN (Dispersal distance in km) = 0.258*time since release + 0.741. Because the model intercept (i.e. dispersal rate) did not pass through zero at age zero, which was expected from a biological basis, I warn against extrapolating the modeled dispersal rate to the period from release (t=0) to the end of Year One. Applying these dispersal rates to all 326 previous Larinus spp. releases in Wisconsin we predict that by 2017 low level populations of Larinus spp could be found state-wide.

The morphology of two introduced knapweed feeders Larinus obtusus Gyllenhal, 1835 and Larinus minutus Gyllenhal, 1835 is investigated. The external and internal morphological characters are described for both sexes. The most promising discriminative characters including male and female terminal segments and genitalia structure are illustrated.

Introductions of biological control agents are unique field experiments to examine the response of novel organisms to new environments. Post-release monitoring is particularly challenging where closely related and morphologically similar biological control agents have been released. Two closely-related phytophagous weevils, Larinus minutus and L. obtusus, were introduced as biological control agents for two major rangeland weeds, diffuse knapweed, Centaurea diffusa and spotted knapweed, C. stoebe subsp. micranthos in North America. The release of the Larinus species coincided with a decline in C. diffusa abundance in many areas. However, it was not clear whether both species of Larinus had become established as they are morphologically very similar. We asked: (1) Could genetic markers be used to identify both the weevil species? (2) Do both weevil species attack both knapweed species in Canada? (3) Are the levels of genetic diversity of the introduced populations of the weevils similar to those of a European source population? (4) Is genetic mixing between the weevil species occurring? Both microsatellite and CO1 mtDNA markers distinguish between the two weevil species. Larinus obtusus was only found on spotted knapweed, while L. minutus was more widely distributed and attacked both weed species. The relatively large initial introductions of weevils to British Columbia (4,300 L. minutus and 5,500 L. obtusus) together with rapid population growth and frequent redistribution among sites has been sufficient to maintain levels of genetic diversity. Results from small samples showed that haplotype, nucleotide diversity and allelic richness of the introduced populations were comparable to those of a European population. We found no evidence for hybridization in the introduced populations.

Spotted knapweed, Centaurea stoebe L. (Asteraceae), is an invasive perennial forb that has become economically and ecologically damaging in North America. The weevil Larinus minutus Gyllenhal (Coleoptera: Curculionidae), a biological control agent of invasive knapweeds, was introduced to 37 sites in northwest Arkansas since 2008 as part of a biological control program for spotted knapweed. In 2011 and 2012, 25 of these release sites were surveyed to monitor how L. minutus infestation rates changed in relation to distance and time from release. The initial L. minutus introductions at these sites occurred from 2008 to 2011. Transects were used from the point of initial weevil introduction to establish sampling quadrats in which capitula were collected to document weevil presence and infestation rates. The mean maximum distance of weevil colonization and mean local abundance (within the first 50 m from the release point) were calculated in relation to time (yr) since release. Five sites had > 10 quadrats in both sampling years and were analyzed with an exponential decay function to model localized population growth and spread. Annual increases in mean local abundance and maximum distance of colonization were observed. These results were substantiated by localized growth (at 3 sites) and spread (at 2 sites) that occurred at the 5 sites analyzed with the exponential decay function. These findings suggest that in years following L. minutus introduction, assuming similar release strategies and environmental conditions, consistent increases in weevil infestation rates and spread from the release site may be expected in about half the sites at which populations establish. USDA-APHIS-PPQUnited States Department of Agriculture (USDA) We thank Edward Gbur Jr. and Fred Stephen for their input on experimental design, and Elizabeth Long for her feedback on the manuscript. This work was supported in part through a Cooperative Agreement with USDA-APHIS-PPQ.

Interactions among seed-feeding insects are well noted in the literature, with many of these interactions being asymmetrical and causing a disadvantage for one of the species involved. While often effective, the use of the cumulative stress approach to biological control (where multiple natural enemy species are released) may increase the risk of interaction, which may lead to less effective biological control programs. Spotted knapweed ( Centaurea stoebe ssp. micranthos (Gugler) Hayek) is an invasive plant that causes damage in rangelands and pastures. In Arkansas, two biological control agents for spotted knapweed, Larinus minutus Gyllenhal and Urophora quadrifasciata Meigen, are established. Both species attack the capitula of spotted and diffuse knapweeds. There is potential for interactions between these species, and this could affect the efficacy of the biological control program. To evaluate these potential interactions, we collected spotted knapweed capitula weekly from three sites in Arkansas from 2010 to 2012. Capitula were monitored for emergence of L. minutus and U. quadrifasciata , and the effect on seed reduction was analyzed. Over all three years, L. minutus and U. quadrifasciata were present in 35.8% (492) and 22.8% (313) of the capitula, respectively. We determined that the occurrences of L. minutus and U. quadrifasciata were not independent of each other, and that L. minutus is more effective at reducing spotted knapweed seed production than U. quadrifasciata within individual capitula . Seed production in capitula with both natural enemies present was no different from capitula with L. minutus.

Larinus (Larinomesius) minutus Gyllenhal (fig. 85) Larinus minutus Gyllenhal, 1835 Records. CRI* ČER ČRK* DNI DON KHE* KHM* KHR KRO* LUG ODE* POL ZAK ZAP* ŽIT* [ Lukjanovitch 1928: 250; Penecke 1932: 351; Shapiro 1940a: 52; Medvedev 1950b: 77; Kharakoz 1955: 15; Barsov et al. 1998: 150; Mazur 2002: 231 ]. Distribution. South Europe, West Asia; introduced to North America (G��ltekin & Fremuth 2013). Material examined. 143 specimens (ZIN, SIZK, KUMN, NSPU, ZMUM, IMC) from 15 provinces were studied. Cherkasy Prov.: 3 spec. (ZMUM) Korsun-Shevchenkivskyi, bank of Dnieper River, on Centaurea arenaria, Ter-Minasian leg., 23.vii.1933; 1 spec. (ZMUM) Cherkasy Distr., Verguny, A.F. Kryshtal leg., 8.vi.1946; Crimea: 1 spec. (KUMN) Rybachye near Alushta, S.I. Medvedev leg., 21.vii.1972; 2 spec. (KUMN) Yedi Quyu Distr., 21.vii.1972; 1 spec. (SIZK) Armyansk, S. V. Volovnik leg., 12.v.1983; 1 spec. (SIZK) Simferopol, A. Bazhenov, 19.vi.1899; 2 spec. (ZIN) Kerch, Kirichenko leg., 10.v.1902; 1 spec. (ZIN) İsl��m Terek Distr., Agarmish Mt., Kirichenko leg., 9.vi.1906; 1 spec. (ZIN) Shchebetovka [= Otuzi]. near Feodosia, Kirichenko leg., 18.vi.1901; 1 spec. (IMC) Bilohirsk Distr., Karasivka, I. V. Maltsev leg., 21.vii.1976; 1 spec. (IMC) Saky Distr., Skvortsivka, I. V. Maltsev leg., 1.vi.1954; 1 spec. (SIZK) Yedi Quyu Distr., Kazantip Cape, S. V. Volovnik leg., 18.v.1983; 3 spec. (SIZK) Bilohirsk Distr., Grushivka, 11.vi.1957; Kherson Prov.: 4 spec. (SIZK) Kherson, Potemkinsky Island, 3.viii.1966; 1 spec. (ZIN) Tsiurupynsk (= Alyoshky), lower of Dnieper River, D. Znoyko leg., 22.vii.1926; 1 spec. (IMC) Kherson, Maly Potemkinsky Island, Rafalsky leg., 22.vii.1964; Khmelnytskyi Prov.: 1 spec. (ZIN) Kamianets-Podilskyi, V. & I. Jakubowskij leg., 1905) Yelisavetgrad (= Kropyvnytsyi), Jacentkowskij leg., 1903; Odesa Prov.: 1 spec. (ZIN) Kodyma, G. Bazhenov, 19.vi.1902; 1 spec. (IMC) Odesa, Otrada Beach, I. V. Maltsev leg., 25.vi.1970; 1 spec. (IMC) Savransky Distr., Polyanetske, I. V. Maltsev leg., 25.vi.1970; 1 spec. (SIZK) Ovidiopol Distr., Bugaz, V. S. Solodovnikova leg., 21.vi.1966; 3 spec. (SIZK) Bilhorod-Dnistrovskyi Distr., Manashi, glade in woodland, 14.vi.1966; Zaporizhia Prov.: 1 spec. (FRC) Zaporizhia, S.I. Alexeenko, 7���8.v.2010; Zhytomyr Prov.: 6 spec. (IMC) Malin, on Centaurea, A. Bogachev leg., 6.vii.1944. Biology. Primary steppes, ruderal habitats. Adults occur in V���IX. Oligophagous on Centaurea spp. (Asteraceae) (Groppe 1990; Volovnik 1996a)., Published as part of Yunakov, Nikolai, Nazarenko, Vitalij, Filimonov, Rostislav & Volovnik, Semyon, 2018, A survey of the weevils of Ukraine (Coleoptera: Curculionoidea), pp. 1-494 in Zootaxa 4404 (1) on pages 384-385, DOI: 10.11646/zootaxa.4404.1.1, http://zenodo.org/record/3766751, {"references":["Gyllenhal, L. (1835) [new taxa]. In: Schoenherr, C. J., Genera et species curculionidum, cum synonymia hujus familiae. Species novae aut hactenus minus cognitae, descriptionibus a Dom. Leonardo Gyilenhal, C. H. Boheman, el entomologis aliis illustratae. Tomus tertius. - Pars prima. [1836], Roret, Parisii and Fred. Fleischer, Lipsia, 505 pp. https: // doi. org / 10.5962 / bhl. title. 8952","Lukjanovitch, Th. K. (1928) Spisok Cleoninae Poltavskoy gubernii (Coleoptera, Curculionidae). Zbirnyk Poltavskogo derzhavnogo muzeiu, 1, 1 - 4. [in Russian]","Penecke, K. A. (1932) Die Curculioniden (Russelkafer) Fauna der Bucovina. Nachtrage und Berichtigungen. Buletinul Facultatii de Stiinte din Cernauti, 5 (2) [1931], 347 - 356.","Shapiro, D. S. (1940 a) Entomofauna travianogo pokrova opushek polezashchitnykh polos. Trudy nauchno-issledovatelskogo zoologo-biologicheskogo instituta Kharkovskogo gosudarstvennogo universiteta. Sektor ekologii, 8, 47 - 67. [in Russian]","Medvedev, S. I. (1950 b) K voprosu o proiskhozhdenii entomofauny parkov Askanii-Nova. Trudy nauchno-issledovatelskogo instituta biologii Kharkovskogo gosudarstvennogo universiteta, 14 (15), 67 - 88. [in Russian]","Kharakoz, A. V. (1955) Entomofauna travianistogo pokrova Veliko-Anadolskogo lesa. Nauchnye zapiski Dnepropetrovskogo universiteta, 48, 151 - 164. [in Russian]","Barsov, V. A., Smirnov, M. E. & Antonets, N. V. (1998) Materialy k inventarizatsii zhestkokrylykh Dneprovsko-Orelskogo zapovednika. In: Rol okhoroniuvanykh pryrodnykh terytorii u zberezhenni bioriznomanittia. Fitosociocenter, Kaniv, pp. 146 - 151.","Mazur, M. (2002) The distribution and ecology of weevils (Coleoptera, Nemonychidae, Attelabidae, Apionidae, Curculionidae) in western Ukraine. Acta zoologica cracoviensia, 45 (3), 213 - 244.","Gultekin, L. & Fremuth J. (2013). Tribe Lixini. In: Lobl, I. & Smetana, A. (Eds.), Catalogue of Palaearctic Coleoptera. Vol. 8. Brill, Leiden, pp. 456 - 472. https: // doi. org / 10.1163 / 9789004259164","Groppe, K. (1990) Larinus minutus Gyll. (Coleoptera: Curculionidae), A Suitable Candidate for the Biological Control of Diffuse and Spotted Knapweed in North America. CABI Institute of Biological Control, Delemont, 30 pp.","Volovnik, S. V. (1996 a) Distribution and ecology of some species of Cleoninae (Coleoptera, Curculionidae) III. Genus Larinus Germ. Entomologicheskoe Obozrenie, 1995, 74 (2), 314 - 321. [in Russian, translated in Entomological Review, 75 (1), 10 - 19]"]}

highlights � Elevated CO2 (but not temperature) increased plant fitness but also agent density. � Biological control agent thus tempered positive CO2 effect on plant fitness. � Both elevated CO2 and temperature hastened plant phenology. � Faster plant phenology under elevated CO2 made plants more attractive to agents. � Plant and agent phenologies may be better matched with climate change. graphical a bstract abstract Predicted increases in atmospheric CO2 and temperature may benefit some invasive plants, increasing the need for effective invasive plant management. Biological control can be an effective means of managing invasive plants, but the anticipated range in responses of plant-insect interactions to climate change make it difficult to predict how effective biological control will be in the future. Field experiments that manipulate climate within biological control systems could improve predictive power, but are challeng- ing to implement and therefore rare to date. Here, we show that free air CO2 enrichment in the field increased the fitness of Centaurea diffusa Lam., a problematic invader in much of the western United States. However, CO2 enrichment also increased the impact of the biological control agent Larinus minutus (Coleoptera: Curculionidae) on C. diffusa fitness. C. diffusa plants flowered earlier and seed heads devel- oped faster with both elevated CO2 and increased temperature. Natural dispersal of L. minutus into the experimental plots provided a unique opportunity to examine weevil preference for and effects on C. dif- fusa grown under the different climate change treatments. Elevated CO2 increased both the proportion of seed heads infested by L. minutus and, correspondingly, the amount of seed removed by weevils. Warming had no detectable effect on weevil utilization of plants. Higher weevil densities on elevated CO2 plants reduced, but did not eliminate, the positive effects of CO2 on C. diffusa fitness. Correlations between plant development time and weevil infestation suggest that climate change increased weevil infestation by hastening plant phenology. Phenological mismatches are anticipated with climate change in many plant-insect systems, but in the case of L. minutus and C. diffusa in mixed-grass prairie, a better phenological match may make the biological control agent more effective as CO2 levels rise.

Centaurea stoebe L. ssp. micranthos (Gugler) (spotted knapweed) is an invasive plant that has been the target of classical biological control in North America for more than four decades. The seedhead-feeding weevils Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae) are two of the most-widely released C. stoebe control agents, and have more recently been introduced into the eastern US. While there have been many studies focusing on their ability to impact C. stoebe in the western US and Canada, there have been few studies from eastern North America, and basic knowledge of important aspects of their biology is lacking. Here we report on the phenology and dispersal of L. minutus and L. obtusus in Michigan. We regularly sampled two established Larinus spp. populations in southern Michigan in 2012 and 2013, and found that while adult abundance fluctuates during the growing season, they remained at easily detectable levels from mid-June through the end of August. We also used previously established populations of L. minutus and L. obtusus released in 1996 ( n = 1), 2007 ( n = 2), and 2010 ( n = 5) to determine how dispersal of Larinus spp. into the surrounding landscape changes with time since release. Populations of Larinus spp. weevils showed little dispersal for 2 years post-release . However, after initial establishment dispersal rates increased rapidly, resulting in average dispersal rates that increased exponentially with time since release. These findings can inform future biological control release and sampling programs for Larinus spp. in eastern North America.

Spotted knapweed (Centaurea stoebe ssp. micranthos (Gugler) Hayek) is an invasive plant from Eurasia that degrades pastures and rangelands in the western United States and Canada. A biological control program for spotted and diffuse (C. diffusa Lamarck) knapweeds was initiated in the 1960s, with 13 species of insects introduced and established in the United States and Canada. The program has largely been considered a success and Larinus minutus Gyllenhal (Coleoptera: Curculionidae) is considered to be one of the key agents responsible for the reduction of knapweeds. Spotted knapweed expanded its range into the southeastern United States, but biological control was not used against the weed in this region until recently. Larinus minutus were released in 6 counties in Arkansas from 2008 through 2012. Weekly sweep-net sampling to monitor weevil activity showed establishment of the weevils in 5 counties. The numbers of L. minutus captured at each site increased from 2010 to 2012. Larinus minutus in Arkansas was univoltine and sex ratios were 1:1, except during flowering, when the proportion of females captured increased. Eggs were not present in females until 1 week after flowering of spotted 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.

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.

The root weevil Cyphocleonus achates (Fahr.), and the seed-head weevil, Larinus minutus Gyll. were introduced into North America as part of a classical biological control program against spotted (Centaurea stoebe L. ssp. micranthos (Gugler) Hayek) and diffuse knapweed (Centaurea diffusa Lamarck). The impact of C. achates alone, or in combination with L. minutus, has not been assessed on diffuse knapweed. In a field study, densities of C. achates were initially manipulated to measure its impact on the density, size, shape, and reproductive potential of diffuse knapweed. Subsequently L. minutus was released at a constant rate across all experimental treatments to determine how attack by this species was affected by previous impacts of C. achates. C. achates reduced plant density by 58%, height by 29%, seed-head diameter by 4.6% as well as the number of seed heads per plant through its effect on plant height. Rosettes attacked by C. achates also produced more flowering stems than unattacked plants and these plants produced fewer seed heads than unattacked plants after accounting for plant size. The reductions in seed-head size had a small negative effect on attack rates by L. minutus but this was generally balanced by a positive effect of reduced seed-head numbers on L. minutus attack rates. After accounting for direct impacts of root herbivory, attack rates by L. minutus were still significantly lower in C. achates plots. We conclude that the negative effects of C. achates on diffuse knapweed outweigh the potential negative interactions with L. minutus and that the two species will likely have additive effects when released in combination at diffuse knapweed sites.

Summary 1. The distribution of herbivores among plant patches may be an important factor determining plant population persistence. The resource concentration hypothesis proposes that herbivores are more abundant per unit plant at higher host plant densities and this has been found to occur in many systems. However, the opposite pattern, resource dilution, in which the herbivores are more abundant in low-density patches and situations in which the number of insect herbivores per unit plant remains constant, also occurs. 2. We developed a simulation model to explore how the distribution of insects per plant affects plant population decline and persistence. We varied the numbers of plants per patch and the distribution pattern, i.e. whether insects were found in a resource concentration distribution, a resource dilution distribution or a distribution in which insect abundance increased linearly with plant density. 3. Resource concentration resulted in longer persistence of plant populations. Plant populations declined more rapidly with either weak resource dilution or directly proportional insect distribution patterns. As the intensity of resource concentration increased, the decline in plant population density was reduced, and plant persistence increased because of increasing variance in insect load. Under strong resource dilution, increasing variance in the insect load also led to a reduction in plant population decline and an increase in plant persistence. 4. We complement our model with field data from the diffuse knapweed, Centaurea diffusa biocontrol system. We compared the relationship with plant density of a successful biocontrol agent, Larinus minutus, and an unsuccessful one, Urophora affinis. Larinus minutus density was directly proportional to plant density, while U. affinis showed a resource concentration pattern with higher rates of attack in high-density patches. 5. Synthesis: Patterns of insect distribution with host plant density will alter the extent to which patches of differing plant densities decline or persist. Resource concentration promotes persistence of the insect–plant system because increased herbivore pressure in high-density patches leads to negative density-dependent plant growth. Weak resource dilution and a distribution of insects that is directly proportionate to plant density can accelerate plant population decline. Strong resource dilution leads to positive density dependence with higher population growth in large patches. Our simulation model and field data demonstrate that the relationship between insect distribution and plant densities can influence plant population dynamics and has implications for choices of weed biological control agents.

The role of spotted knapweed phenology on the attack rate of two seed-head insects Urophora affinis and Larinus minutus was assessed in a series of field studies at four study sites in south-eastern British Columbia, Canada. Slow or later developing knapweed plants had more seed heads that contained only single or multiple U. affinis whereas early or faster developing plants had more seed heads containing L. minutus alone or in combination with U. affinis. L. minutus did not distinguish between seed heads with or without U. affinis larvae when laying eggs. However, seed heads with multiple U. affinis present, produced fewer L. minutus adults than expected. The probability of single or multiple U. affinis galls being present increased with seed-head diameter but was not affected by seed-head height. Attack by L. minutus increased with seed-head diameters >5 mm and was lower at plant heights above 50 cm. These results demonstrate two mechanisms that enable U. affinis to successfully coexist with L. minutus: differences between the species in their response to the developmental phenology of knapweed heads, and increased survivorship of U. affinis in heads with multiple U. affinis galls through niche interference competition. These mechanisms provide a possible explanation for the persistence of U. affinis populations on spotted knapweed, in spite of high levels of within seed-head mortality that have been observed with increasing L. minutus populations.

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...

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.

The biological control program for diffuse knapweed, Centaurea diffusa Lamarck, a Eurasian plant that has invaded large areas of grasslands in western North America, has gone on for over 35 years. This program involved the release of 12 biological control agents of which four are numerous and widely distributed; two species of Tephritid flies, Urophora affinis and Urophora quadrifasciata, the root boring beetle, Sphenoptera jugoslavica, and the most recently established weevil Larinus minutus. Field observations show that diffuse knapweed densities declined at sites in British Columbia Canada where the weevil L. minutus became established. Decline in diffuse knapweed density did not occur where densities of L. minutus were low. Field cage experiments showed that feeding by L. minutus damaged rosette leaves and bolting stems, and reduced seed production, seedling density and the density of rosette and flowering diffuse knapweed plants.

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.

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.

Spotted knapweed is an important weed of rangeland in the northwestern United States and western Canada, and has been the focus of considerable biological control efforts. Cyphocleonus achates, a Eurasian root weevil, has been released as a biocontrol agent against the weed in many areas of Montana and the Pacific Northwest. Spotted knapweed plant density was monitored over a 11-year period (1993–2004) at two sites in western Montana where C. achates was released. Spotted knapweed density declined significantly over time at both sites (99 and 77%, respectively), after C. achates numbers increased dramatically at both sites. The average annual population growth rate of C. achates during 1993–1995 was 14% at the two sites, compared to 10% during 1995–1998. The weevil population radially expanded a distance of 73 m per year at one study site compared to 99 m per year at the other site. The estimated C. achates population increase at the two sites was similar: in 1998, the C. achates population estimate for one site was 90,776 in a 23 ha area of occupation, compared to 97,173 in a 29 ha area at the other site. Evidence suggests that C. achates played a major role in the spotted knapweed population decline. Following the decline of spotted knapweed, two annual exotic weeds, Bromus tectorum and Descurainia sophia, became major components of the new plant community, with B. tectorum becoming the dominant plant. By 2004, B. tectorum comprised 89 and 50% of the replacement vegetation at the two sites.

The root weevil Cyphocleonus achates was introduced into western Montana in 1988 as a biological control agent against spotted knapweed (Centaurea maculosa), an exotic invasive weed from Eurasia. Spotted knapweed has declining in areas where C. achates was released, and experiments were conducted to evaluate the role this weevil may play in knapweed decline. Field experiments were conducted from 2001 to 2003, and from 2002 to 2004, with groups of 20 knapweed plants exposed to 8 pairs, 2 pairs, or no adult C. achates. Knapweed mortality and vigor of surviving plants were followed for two years. In the 2001–2003 plot experiment, mortality increased with exposure to increasing numbers of C. achates, and root and shoot biomass, and plant height declined in surviving plants. Treatment effects persisted after two years, even though second-year weevil abundance no longer correlated with original treatment levels. In the 2002–2004 experiment, knapweed mortality was relatively high in all treatment groups. A pattern of reduced knapweed vigor with increased exposure to weevils, similar to the 2001–2003 experiment, was observed. This is the first report on impacts by C. achates on spotted knapweed, supporting the hypothesis that knapweed decline at C. achates release sites is due at least in part to their attack.

Interspecific competition has been suggested as an explanation for the failure of some insects as biological control agents for weeds. Enclosure and exclusion cages were used, in southern British Columbia, Canada to evaluate the importance of interspecific competition between a seedhead weevil, Larinus minutus, and a gall-inducing fly, Urophora affinis, two biocontrol agents released against spotted knapweed in North America. At the seedhead scale, U. affinis, which is an inferior biological control agent based on knapweed seed mortality, was the superior competitor. Larinus minutus attack rates were significantly lower in the presence of U. affinis compared to release treatments where L. minutus was attacking alone. Reduced L. minutus attack rates were apparent in seed heads expected to contain both species, assuming insect distributions were random, but instead only contained U. affinis. L. minutus did not significantly affect U. affinis density. Although overall attack rates on knapweed seedhe...

Field studies were conducted to determine the competitive interactions between introduced biological control agents that attack the seed heads of spotted knapweed (Centaurea stoebe ssp. micranthos) and diffuse knapweed (Centaurea diffusa). Two weevils, Bangasternus fausti and Larinus minutus (Coleoptera: Curculionidae), were each paired with the previously established fly, Urophora affinis (Diptera: Tephritidae). Each species was released either alone or in pair-wise combinations inside screen cages placed over existing knapweed plants at six field sites in Montana and one in Oregon. Larinus minutus produced almost three times as many progeny on diffuse knapweed as on spotted knapweed. Larinus minutus reproduction was not affected by competition with U. affinis, but U. affinis reproduction was reduced by the presence of L. minutus (by 71% on spotted and 77% on diffuse knapweed). Bangasternus fausti reproduction generally was not affected by competition with U. affinis, nor was U. affinis affected...

Environ. Entomol. 34(1): 225Ð234 (2005) ABSTRACT The responses and impacts of Þve insect species that feed onCentaureadiffusa Lamarck, diffuse knapweed, to soil nitrogen and phosphorus additions were studied in grasslands east of the Colorado Front Range. We predicted that fertilization was unlikely to have a direct effect on herbivory but that if the insects preferentially select vigorous plants, increased tissue nitrogen, plant density, or plant mass resulting from fertilization would increase plant susceptibility to biological control insects. Fertilization caused a modest increase in the biomass of C. diffusa adults and the nutritive content of aboveground plant biomass. Larinus minutus Gyllenhal, a seed head weevil, was less abundant and exhibited reduced impacts on fertilized C. diffusa plants. Abundance of gall sy larvae, Urophora spp., was negatively insuenced by the presence of L. minutus and had no detectable effect on seed abundance. No positive or negative impacts of root feeders on knapweed were observed, but the root-feeding Cyphocleonus achates (Fahraeus) selected plants in fertilized plots, whereas the root phytophage Sphenoptera jugoslavica Obenberger was more common on knapweed in unfertilized plots. Our results indicate that these specialist biological control insects exhibit little preference based on plant biomass, density, or nutritive quality. However, variables that affect the timing of knapweed seed production, such as fertilization, mowing, and grazing, reduce insect impacts and may explain some of the regional variation in effectiveness of these insects as biological controls on knapweed.

Four species of biocontrol insects (knapweed root weevil, lesser knapweed flower weevil, spotted knapweed seedhead moth, and bronze knapweed root borer) were released at a diffuse knapweed site located about 10 km east of the Colorado Front Range. Two other biocontrol agents (banded gall fly and knapweed seed head fly) were already present at this site. Densities of rosettes and flowering plants, seedhead production per plant, and seeds per seedhead on mowed and unmowed areas were studied for 5 yr, 1997–2001. Of the six biocontrols, five (Urophora spp., bronze knapweed root borer, knapweed root weevil, and lesser knapweed flower weevil) obtained sizable densities relative to weed abundance. Diffuse knapweed declined from 8.3% in absolute cover in June 2000 to 1.9% by September 2001. Vegetation transects closest to the insect release areas showed the largest declines, with diffuse knapweed disappearing entirely on one transect. In contrast, diffuse knapweed abundance at a nearby prairie increased ...

The host specificity of the European seed-head weevil Larinus minutus was investigated between 1985 and 1989 as a potential biocontrol agent of Centaurea maculosa and C. diffusa in North America. The results of adult feeding, oviposition and larval development tests with 44, 47 and 72 plant species, respectively, in nine families demonstrate its limited host range with a preference for Centaurea species in the subgenera Acrolophus, Calcitrapa and Jacea. L. minutus accepted and developed normally in the North American biotypes of both, spotted and diffuse knapweed, and was therefore recommended for introduction into the USA and Canada. Since its approval in 1991, it has been established at some field release sites.

Substantial controversy surrounds the efficacy of biological control insects to reduce densities of Centaurea stoebe, a widespread, aggressive invasive plant in North America. We developed a graphical model to conceptualize the conditions required to explain the current contradictory findings, and then employed a series of manipulations to evaluate C. stoebe responses to herbivores. We manipulated soil nitrogen and competition in a field population and measured attack rates of a foliage and seed feeder (Larinus minutus), two gall flies (Urophora spp.), and a root feeder (Cyphocleonus achates), as well as their effects on the growth and reproduction of C. stoebe. Nitrogen limitation and competing vegetation greatly reduced C. stoebe growth. L. minutus most intensively reduced seed production in low-nitrogen soils, and removal of neighboring vegetation increased Larinus numbers per flower head and the percentage of flowers attacked by 15% and 11%, respectively. Cyphocleonus reduced flower production and aboveground biomass over two years, regardless of resources or competition. Our results, in conjunction with other published studies, demonstrate that positive, neutral, and negative plant growth responses to herbivory can be generated. However, under realistic field conditions and in the presence of multiple herbivores, our work repudiates earlier studies that indicate insect herbivores increase C. stoebe dominance.

Spotted knapweed (Centaurea stoebe) is found in over 3 million ha of rangeland and forests across North America, and evidence supporting the use of biological control as a regional method to reduce infestations and their associated impacts remains inconclusive. Several species of insects have been reported to reduce plant densities in some areas; however, rigorous studies that test combinations of these species and the influence of resource availability are lacking. We examined the singular and combined effects of herbivory by a root weevil (Cyphocleonus achates) and a flower head weevil (Larinus minutus) on the growth and flower production of C. stoebe. We also manipulated soil resource fertility as an additional factor that could explain the outcomes of contradictory biological control herbivore effects on C. stoebe. In a greenhouse study, herbivory by C. achates decreased flower production for plants across all resource environments. In a caged common garden study, the negative effects of herbivory also did not interact with soil nutrient status. However, the presence of plant competition further decreased knapweed growth, and the negative effects of concurrent herbivory by C. achates and L. minutus on plant biomass and flower production were additive. Derived within the context of variable levels of soil nutrient availability and competing vegetation, these results support the cumulative stress hypothesis and the contention that combined above- and belowground herbivory can reduce spotted knapweed densities and reduce the ecological and economic impacts of this species in rangelands of western North America.

Five insect biological control agents that attack flower heads of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, became established in western Montana between 1973 and 1992. In a controlled field experiment in 2006, seed-head insects reduced spotted knapweed seed production per seed head by 84.4%. The seed production at two sites in western Montana where these biological control agents were well established was 91.6-93.8% lower in 2004-2005 than 1974-1975, whereas the number of seed heads per square meter was 70.7% lower, and the reproductive potential (seeds/m(2)) was 95.9-99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m(2) compared with 19 seeds/m(2) at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m(2)), where the insects have been established for a shorter period. Urophora affinis Frauenfeld was the most abundant species at eight study sites, infesting 66.7% of the seed heads, followed by a 47.3% infestation by Larinus minutus Gyllenhal and L. obtusus Gyllenhal. From 1974 to 1985, Urophora spp. apparently reduced the number of seeds per seed head by 34.5-46.9%; the addition of Larinus spp. further reduced seed numbers 84.2-90.5% by 2005. Path analysis indicated that both Larinus spp. and U. affinis contributed significantly to reduction of seed production over the 30-yr period. Spotted knapweed density may not decrease significantly until the seed bank falls below a critical threshold.

Results of a field experiment in northern Greece designed to determine the host specificity of Larinus curtus, L. minutus and Urophora sirunaseva are discussed. Data on plant development are presented and observations on the occurrence of labeled adult weevils on seven test plant species are recorded. The results indicate a clear preference for the individual hosts, as well as a preference for different plant parts by both sexes. Rearings of all flowerheads formed during the experiment and dissections of capitulum samples give an impression of the diversity of their fauna and confirm the host specificity of the three species. Zusammenfassung Ergebnisse eines Freilandexperiments zur Untersuchung der Wirtsspezifitat von Larinus curtus Hochhut (Col., Curculionidae) und Urophora sirunaseva Hg. (Dipt., Tephritidae) als biologische Gegenspieler von Centaurea solstitialis L. (Asteraceae) und Larinus minutus Gyllenhal als ein biologischer Gegenspieler von C. maculosa Lam. und C. diffusa Lam. Es werden die Ergebnisse eines Freilandversuches zur Wirtswahl von Larinus curtus, L. minutus and Urophora sirunaseva in Griechenland diskutiert. Daten zur Pflanzenphanologie und Beobachtungen der Kafer auf den sieben angebotenen Testpflanzen werden prasentiert. Die Ergebnisse weisen auf eine deutliche Praferenz fur die jeweilige Wirtspflanze sowie auch auf Praferenzen beider Geschlechter fur unterschiedliche Pflanzenteile hin. Schlupfdaten aller wahrend des Versuchs verfugbaren Blutenkopfe und Sektionsdaten von Blutenkopfproben vermitteln einen Eindruck von der Diversitat der Blutenkopffauna und bestatigen die hohe Wirtsspezifitat der drei untersuchten Insekten.

The syntypes of 41 species of Larinus Dejean described by L. Gyllenhal and C. H. Boheman, housed at the C. J. Schoenherr collection in the Swedish Museum of Natural History, Stockholm and the Museum of Evolution, Uppsala University, are examined. Lectotypes are designated, a nomenclatural review performed, many previous synonyms confirmed and three new synonyms proposed: Larinus planus (Fabricius, 1792) (= Larinus rusticanus Gyllenhal, 1835 syn. nov. ); Larinus carlinae (Olivier, 1807) (= Larinus sulphurifer Boheman, 1843 syn. nov. ); Larinus minutus Gyllenhal, 1835 (= Larinus puncticollis Capiomont, 1874 syn. nov. ). Colour images for 15 lectotypes are presented.

The weevil, Larinus minutus oviposits in open flowers of diffuse and spotted knapweed [Centaurea diffusa and C. maculosa (Asteraceae)] and the larvae feed on developing achenes. The species is univoltine and overwinters as an adult in soft soil or among debris near the host plant. Overwintered adults appear on plants by mid-May. Oviposition begins at the end of June and the new generation starts to emerge by the end of July. Up to 130 eggs per female were laid in C. diffusa flowers in cages. One larva destroys 100% of the seeds in an infested seedhead. During the warm hours of the day (over 30°C) the adults hide under flowers or other parts of the plants and remain inactive.

Larinus minutus Gyllenhal (Coleoptera: Cur culionidae) is a univoltine weevil that feeds on the seeds of spotted and diffuse knapweeds Centau rea stoebe ssp. micranthos (Gugler) Hayek and C. diffusa Lamarck. After emerging from overwin tering sites in the leaf litter, adult weevils begin feeding on the vegetative portions of the plants. Adults, however, prefer to feed on flowers when they are available and development of beetle ova ries is dependent upon flower feeding (Groppe 1990). Females oviposit on newly opened flower heads (capitula). Two or 3 eggs can occur in each flower head, but only 1 larva usually develops in smaller capitula. Multiple larvae can survive in large spotted knapweed capitula (Groppe 1990). Under laboratory conditions (25°C), eggs hatch in 3-4 d (Groppe 1990). Larval development takes approximately 4 weeks and larvae go through 3 instars. Larvae feed on knapweed seeds and pu pate in the capitula, making a cocoon out of the seed head material (Kashefi & Sobhian 1998). Larvae can destroy up to 100% of the seeds in a capitulum (Kashefi & Sobhian 1998). In the West ern United States, adult weevils emerge in late Sep and feed on plants until winter, when the adults overwinter in leaf litter and emerge in the following Jun (Jordan 1995). Larinus minutus was first released into the United States in 1991 with collections from Greece and Romania (Story 2002). Although 12 other natural enemy species were introduced into the Western United States and Canada to control spotted and diffuse knapweeds, only recently has adequate suppression of some populations been seen (Myers 2004; Smith 2004). Myers (2007) sug gested that knapweed populations did not signif icantly decline until the establishment of L. minutus. Populations of L. minutus have been es tablished in Washington, Wyoming, Oregon, Mon tana, Minnesota, Colorado, and Indiana (Lang et al. 1996; Story 2002). No natural enemies of spotted knapweed have been released in Arkansas until the inception of this study. However, we found Urophora quadrifasciata (Meigen) (Diptera: Tephritidae) established throughout the range of spotted knapweed in the state in a survey in 2006 for knapweed natural ene mies. This seedhead galling fly was introduced from Russia into Canada in 1980 and has since been re distributed or spread on its own to several states in the northeastern and northwestern United States (Story 2002). Duguma (2008) found that U. quadri fasciata reduced the number of seeds produced by spotted knapweed by 44% late in the season (Aug), at a time when plants are more environmentally stressed. However, the fly did not significantly re duce the number of seeds produced earlier in the season, a time when knapweed is most robust (Du guma 2008). Thus, it is likely that U. quadrifasciata alone will not significantly suppress knapweed pop ulations in Arkansas, or stop its spread further into the southern United States.

Based on a study of the types of species described by J. Desbrochers in the genus Larinus Dejean, several nomenclatural changes are proposed. Lectotypes are designated (or Holotypes recognized) for the following species names: Larinus stricticollis Desbrochers, L. interruptus Desbrochers, L. breviusculus Desbrochers, L. villosiventris Desbrochers, L. villosicollis Desbrochers, L. distinguendus Desbrochers, L. senilis (Fabricius) var. rubripes Desbrochers, L. crassiusculus Desbrochers, L. obesulus Desbrochers, L. sacer Desbrochers, L. hierolosymae Desbrochers, L. cribricollis Desbrochers, L. elegans Desbrochers, L. densicollis Desbrochers, L. akbesianus Desbrochers, L. rugithorax Desbrochers, L. tauricus Desbrochers. Eight new synonymies are proposed: Larinus ursus (Fabricius, 1792) [= Larinus senilis (Fabricius) var. rubripes Desbrochers, 1892 syn. n. ]; Larinus fucatus Faust, 1891 [= Larinus sacer Desbrochers,1896 syn. n. ]; Larinus brenskei Faust, 1890 [= Larinus akbesianus Desbrochers, 1897 syn. n.]; Larinus crassiusculus Desbrochers, 1895 [= Larinus khnzoriani Ter-Minassian, 1962, syn. n.]; Larinus lederi Faust, 1889 [= Larinus cribricollis Desbrochers, 1896, syn. n.]; Larinus sturnus (Schaller, 1783) [= Larinus rugithorax Desbrochers, 1897, syn. n.]; Larinus sibiricus Gyllenhal, 1836 [=Larinus distinguendus Desbrochers, 1892 syn. n.]; Larinus cardopatii Lucas, 1849 [=Larinus densicollis Desbrochers, 1897 syn. n. ]. The name of Larinus hierolosymae Desbrochers, 1896 is corrected and become Larinus hierosolymae Desbrochers, 1896 [emendation]. Larinus obesulus Desbrochers, 1896 is assigned to the subgenus Cryphopus Petri, 1907. Larinus villosicollis Desbrochers, 1892 and L. hierosolymae Desbrochers, 1896 are placed in the subgenus Phyllonomeus Gistel, 1856. Larinus tauricus Desbrochers, 1897 belongs to the subgenus Larinomesius Reitter, 1924. At the present time, the types of three species are not located : Larinus rufipes Desbrochers, 1870, Larinus numidicus Desbrochers, 1892 and Larinus minutus Gyllenhal var. minutissimus Desbrochers, 1891.