Supercooling capacity of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) on spotted knapweed: comparisons among plants, sites, time of season, and gall densities

Larval supercooling points of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) were compared among plants, six research sites in western Montana, four fall/winter time periods, and among gall densities. These two tephritid fly species are introduced biological control agents of spotted knapweed, Centaurea maculosa Lamarck, and diffuse knapweed, Centaurea diffusa Lamarck. Few differences in larval supercooling points for U. affinis and U. quadrifasciata were found among plants, and where differences were found, they were not consistent across fall/winter time periods. Significant differences in larval supercooling points were found among sites and across fall/winter time periods. No relationship was found between larval supercooling points and site elevation. Larval supercooling points of both U. affinis and U. quadrifasciata showed no relationship with the density of Urophora galls within spotted knapweed capitula. Mean larval supercooling points of U. affinis were consistently lower than those of U. quadrifasciata across sites and fall/winter time periods. In conclusion, temporal differences in temperature over the fall/winter time periods and microclimatic differences among sites appear to be the most important abiotic factors influencing the supercooling points in U. affinis and U. quadrifasciata.     

Is spotted knapweed (<Emphasis Type="Italic">Centaurea stoebe</Emphasis> L.) patch size related to the effect on soil and vegetation properties?

Spotted knapweed (Centaurea stoebe L. subsp. Micranthos (Gugler) Hayek) was first introduced in the 1890s from Europe into western North America, where it now occupies over three million hectares of rangeland and pasture in 14 states and two Canadian provinces, reducing forage production and causing economic damage. Despite many reported effects spotted knapweed can have on soils and native vegetation, it is not known whether patch size is correlated with these ecosystem-level effects. The objective of our study was to determine whether the effects of spotted knapweed on plant composition and soil properties was related to spotted knapweed patch size. We asked the following questions: (1) Are there differences in plant species richness and diversity between small and large knapweed patches? and (2) Do soil water and soil mineral nutrient properties change depending on knapweed patch size? Twenty-four knapweed patches, and paired natural grassland plots, were randomly selected within Lac du Bois Provincial Park, British Columbia, Canada. Knapweed patch size ranged from 6 to 366 m². Sampling and analysis revealed a significant effect of knapweed patch size on soil and vegetation properties. Soil P, soil temperature, and total dry plant biomass (g/0.25 m²) increased, while soil N, soil C, and soil moisture decreased with patch size. Since our results show that spotted knapweed patch size is related to degree of soil alteration, it is important to consider size of patch when modeling the impact of spotted knapweed in North America. Since large patches of spotted knapweed seem to have a proportionately greater effect on soil chemistry properties, large patches may move the system further away from a point where it is possible to restore the site to pre-invasion conditions.     

Establishment and Dispersal ofUrophora affinis(Diptera: Tephritidae) andMetzneria paucipunctella(Lepidoptera: Gelechiidae) in Southwestern Virginia

Two European gall-producing insects,Urophora affinisFrfld. (Diptera: Tephritidae) andMetzneria paucipunctella(Zeller) (Lepidoptera: Gelechiidae) were introduced into Virginia in 1986 for biological control of spotted knapweed (Centaurea maculosaLam.). Adults ofU. affinis(n = 2625) andM. paucipunctella(n = 450) were released at two sites in Montgomery County, Virginia, and their populations were monitored yearly by dissecting spotted knapweed flower heads. Beginning in 1992, knapweed samples collected at various distances from the release sites were checked for dispersal.U. affinisis well established and is spreading slowly. The number of larvae per flower head and the seed numbers are inversely related as plants with the greatest number of larvae per spotted knapweed head had the lowest number of seeds. Knapweed density has declined at one of the release sites which had the highest rate of infestation byU. affinis.Establishment of the moth,M. paucipunctella,is less certain as it has been recovered at a very low level from only one site.     

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.     

Role of plant phenology in mediating interactions between two biological control agents for spotted knapweed

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.     

Non-target effects of an introduced biological control agent on deer mouse ecology

Release of exotic insects as biological control agents is a common approach to controlling exotic plants. Though controversy has ensued regarding the deleterious direct effects of biological control agents to non-target species, few have examined the indirect effects of a ”well-behaved” biological control agent on native fauna. We studied a grassland in west-central Montana infested with spotted knapweed (Centaurea maculosa) to examine the effects of knapweed invasion and two gall flybiological control agents (Urophora affinis and U. quadrifasciata) on the native deer mouse (Peromyscus maniculatus). Stomach-content analysis revealed that Urophora were the primary food item in Peromyscus diets for most of the year and made up 84–86% of the winter diet. Stomach contents indicated that wild-caught mice consumed on average up to 247 Urophora larvae mouse^–1 day^–1, while feeding trials revealed that deer mice could depredate nearly 5 times as many larvae under laboratory conditions. In feeding trials, deer mice selected knapweed seedheads with greater numbers of galls while avoiding uninfested seedheads. When Urophora larvae were present in knapweed seedheads, deer mice selected microhabitats with moderately high (31–45% cover) and high knapweed infestation (≥46% cover). After Urophora emerged and larvae were unavailable to Peromyscus, mice reversed habitat selection to favor sites dominated by native-prairie with low knapweed infestation (0–15%). Establishment of the biological control agent, Urophora spp., has altered deer mouse diets and habitat selection by effecting changes in foraging strategies. Deer mice and other predators may reduce Urophora populations below a threshold necessary to effectively control spotted knapweed. Received: 04 May 1999 / Accepted: 14 August 1999     

Field cage assessment of interference among insects attacking seed heads of spotted and diffuse knapweed

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 by B. fausti on either host plant. There were extremely few cases of successful production of both weevil and fly in the same capitulum, which was probably because weevil larvae consume the developing flies. Both weevils increased the total proportion of seed heads infested on diffuse knapweed, and B. fausti increased it on spotted knapweed. However, the release of either weevil did not significantly further reduce seed production on either plant. The results and experimental design are discussed in light of the subsequent establishment and impact of these agents.     

Influence of seed head-attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period

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.     

Influence of Nutrient Availability on the Interaction Between Spotted Knapweed and Bluebunch Wheatgrass

Centaurea maculosa (Lam.) (spotted knapweed) reduces wildlife and livestock habitat biodiversity and increases erosion. Nutrient availability to plants may be used to accelerate succession away from spotted knapweed. Early‐successional plant communities often have high nutrient availability, whereas late‐successional communities are often found on lower nutrient soils. We hypothesized that removal of nutrients would change the competitive advantage from spotted knapweed to Pseudoroegneria spicatum (bluebunch wheatgrass) (late seral). In two addition series matrices, background densities of Secale cereale (annual rye) and Elymus elimoides (bottlebrush squirreltail) (3,000 seeds/m²) were used to remove nutrients from the soil. In another set of addition series matrices, nitrogen (33 kg/ha) or phosphorus (33 kg/ha) were added to the soil. Nutrient analysis of soil and vegetation indicated that annual rye and bottlebrush squirreltail reduced nutrient availability in soils. In another matrix, neither a background density nor nutrients were added. Data were fit into Watkinson's curvilinear model to determine the competitive relationship between bluebunch wheatgrass and spotted knapweed. This allowed comparison of the equivalence ratios (C) generated from each addition series. The C parameters are the per‐plant equivalent of bluebunch wheatgrass or spotted knapweed and can be interpreted as the ratio of intra‐to‐interspecific competition. The C parameters are also the equivalence ratio of the number of spotted knapweed it takes to have equivalent effect on bluebunch wheatgrass or the number of bluebunch wheatgrass having the equivalent effect on spotted knapweed. Without nutrient manipulation, spotted knapweed was more competitive than bluebunch wheatgrass. The C for bluebunch wheatgrass was 0.17, indicating that 0.17 knapweed plants were competitively equivalent to one wheatgrass. Annual rye changed the competitive balance in favor of bluebunch wheatgrass (C = 9.9). Addition of nitrogen, phosphorus, or the mid‐seral species did not change the competitive relationship between the two species. This preliminary study suggests that succession from spotted knapweed to late‐seral bluebunch wheatgrass community may be accelerated by altering resource availability.     

Effect of summer drought relief on the impact of the root weevil Cyphocleonus achates on spotted knapweed

A recent decline in spotted knapweed, Centaurea stoebe L. subsp. micranthos (Asteraceae), has been observed in parts of western Montana. The release of the biological control agent Cyphocleonus achates (Fahraeus) is thought to contribute to the decline, but persistent drought since at least 1999 may be an additional factor. We conducted outdoor plot experiments to test the relative impacts of C. achates weevils and summer drought relief on spotted knapweed survival and growth. Groups of spotted knapweed transplants were assigned to one of four weekly water addition treatments (no added water, and 0.25, 0.5 or full recovery of plant water deficit, where "deficit" refers to potential evapotranspiration minus rainfall) in May to August 2004 and June to August 2005 and to either exposure to or protection from C. achates. In June of each subsequent year (2005 and 2006), plants were harvested and growth attributes that reflect plant vigor were measured. Drought indices showed that throughout the time of the study until January 2006, western Montana was in drought alert or severe drought. Summer drought relief had no effect on aboveground biomass and plant height of knapweed plants in subsequent years, but feeding by C. achates larvae reduced these two measures of plant vigor. Knapweed plants resuming growth after the drought ended in spring 2006 were significantly larger than those resuming growth under drought conditions in spring 2005. Spring drought may reduce knapweed growth, but C. achates reduced knapweed growth regardless of drought conditions.     

Relationship between capitulum size and pre-dispersal seed predation by insect larvae in common Asteraceae

The evolution of a showy floral display as an advertisement to pollinators could simultaneously advertise the availability of resources to pre-dispersal seed-predators. The hypotheses tested here are that the incidence of seed predation by bud-infesting insect larvae in capitula of Asteraceae is positively related to (1) capitulum size among species, (2) capitulum size within species, (3) capitulum lifespan, and (4) the degree of flowering asynchrony on individual plants. Three populations of each of 20 common herbaceous species of Asteraceae from disturbed ground and grassland habitats were monitored for the presence of pre-dispersal, seed-eating insect larvae. Mean capitulum size (receptacle width) of each species was measured. In a sub-set of eight species, individual capitula were tagged to determine their flowering phenology and lifespan (from anthesis to seed shedding). From these data an index of flowering synchrony on individual plants was derived. Among species, the incidence of larval infestation increased with capitulum size. Small-flowered species such as Achillea millefolium were largely free of bud-infesting larvae, whilst large-flowered species such as Arctium minus were heavily infested. In three cases investigated in greater detail, bud infestation was found to increase with capitulum size within species, suggesting a potential for natural selection to favour smaller capitula. No relationship was found between infestation levels and either capitulum lifespan or degree of flowering synchrony, and there was no evidence that the relationship between capitulum size and infestation was confounded by correlations with these other features. The results support hypotheses 1 and 2, but not 3 and 4. It is suggested that the characteristic capitulum size of each species may represent a trade-off between the opposing selection pressures of pollinators and pre-dispersal seed predators.     

Effects of fragmentation of secondary broadleaf deciduous forests on populations of the near-threatened butterfly, Sasakia charonda (Lepidoptera, Nymphalidae), in central Japan

We examined the effects of fragmentation of secondary broadleaf deciduous forests (secondary forests) on populations of the near-threatened butterfly, Sasakia charonda, in central Japan. Regression analyses revealed that the number of overwintering larvae per host tree significantly increased when the area of secondary forest patches and the Isolation Index of the forest patch increased and the distance from secondary forest patches containing the focal host trees to the nearest secondary forest patch decreased. There was a significantly positive correlation between the number of overwintering larvae and the number of host trees in the neighborhood. The host trees were primarily distributed at the edges of secondary forests. From the results of the backward elimination method of multiple linear regression analysis, independent variables other than patch area were eliminated, and the standardized partial regression coefficient of the patch area was significant. This result suggested that a contiguous distribution of large secondary forest patches with many host trees is very important to conserving this butterfly species.     

Conditioning ewes and lambs to increase consumption of spotted knapweed

Spotted knapweed (Centaurea maculosa Lam.) is an invasive plant that alters species composition and grazing value of rangelands in the northwestern United States. The spread of invasive plants may be reduced by using livestock as a biological control. We determined if mature ewes and their lambs (n = 34 ewe/lamb pairs) consume more spotted knapweed when ewes and/or lambs are conditioned to fresh-cut spotted knapweed. Ewe/lamb pairs were randomly assigned to one of four conditioning treatments: ewes and lambs not conditioned to spotted knapweed (N), conditioned ewes with non-conditioned lambs (E), non-conditioned ewes with conditioned lambs (L), or conditioned ewes and lambs (both—B). Then, ewes and lambs were observed together for 5 days (Trial 1); 11 days later, lambs were observed for 4 days without their mothers (Trial 2). During conditioning, intake by conditioned and non-conditioned ewes and lambs varied over time (as-fed basis, treatment by day interaction; ewes P = 0.03; lambs P = 0.05). Overall, non-conditioned lambs (N, E) consumed more than conditioned lambs (L, B; P = 0.02). In Trial 1, N ewes consumed similar amounts of spotted knapweed and bromegrass (Bromus inermis Leyss.) as the E, L, and B ewes (P = 0.67). E ewes spent more time eating spotted knapweed than L ewes (P = 0.001), and E ewe/lamb pairs consumed more spotted knapweed than L ewe/lamb pairs (P = 0.02). In Trial 2, N lambs consumed less spotted knapweed than E, L, and B lambs (P = 0.06). L lambs consumed more than E lambs (P = 0.007). Conditioning ewes, lambs, or ewes and lambs did not increase time spent eating spotted knapweed when both grazed together in a drylot, but conditioned lambs, without their mothers present, consumed more spotted knapweed 11 days later than non-conditioned lambs. Conditioning lambs only in a group setting with their peers may have the greatest potential to enhance consumption of spotted knapweed, because of social facilitation and the predilection for young animals to try novel feeds.     

Hazard Assessment of Deer–Vehicle Collisions in Michigan

The relationship between deer–vehicle collision counts and vehicle miles traveled (VMT) is studied in this article using the Michigan (USA) crash database and categorized exposure data under different levels of data disaggregation. Negative binomial regression models were developed to establish the association between (human and deer) exposure and the frequency of deer–vehicle collisions. It is shown that VMT is nonlinearly correlated with the collision counts for most sets of circumstances except in rural areas. Observation of the association between the deer–vehicle collision count and the annual VMT across counties in Michigan reveals the unique characteristic of heteroskedasticity (i.e., variances of collision count are amplified with the increasing VMT). The regression results demonstrate that deer density stands out as the most significant exposure predictor in describing deer–vehicle collisions at the state level. It is recommended that differentiated exposure measurements for deer–vehicle collisions should be employed for three ecoregions in Michigan. Exposure predictors for deer–vehicle collisions appear to be more dependent on deer density in Michigan's Upper Peninsula and North Lower Peninsula ecoregions, while in the South Lower Peninsula ecoregion deer and human populations are equally important.     

松墨天牛成虫雌雄体型差异及蛹和成虫大小与越冬幼虫体重的关系

【目的】本研究旨在明确松墨天牛 Monochamus alteratus 成虫及越冬幼虫体型大小的差异,探讨该虫越冬后成虫体型大小和越冬幼虫体重大小关系及原因。【方法】于2014年5月1日至10月31日在浙江富阳野外诱捕松墨天牛,通过测量野外诱捕到的松墨天牛成虫体长、体宽,确定林间松墨天牛成虫体型的差异;松墨天牛越冬幼虫的采集和称重测量明确越冬幼虫的组成和体重大小差异;进而通过对越冬幼虫单头跟踪饲养至化蛹、羽化,确定越冬幼虫体重大小与发育所得蛹和成虫大小的关系,阐明林间松墨天牛成虫体型差异的原因。【结果】浙江富阳野外诱捕发现,松墨天牛的活动期间很长,从5月中旬到10月初一直能诱捕到松墨天牛成虫,高峰期在6和7月份。松墨天牛雌雄成虫体型差异很大,雌虫平均体长和体宽分别为20.59±0.19和6.59±0.06 mm;雄虫平均体长和体宽分别为19.90±0.26 和6.44±0.08 mm; 雌虫的平均体长明显高于雄虫,但二者体宽没有显著差异;并且雌雄成虫体长和体宽呈显著正相关。越冬幼虫的头宽测定表明头宽的变化很大,最小为2.20 mm,最大为4.24 mm,经比对越冬幼虫由4龄和5龄幼虫组成;幼虫体重差异大,平均体重为304.2 mg, 介于71.6~858.0 mg之间,其中5龄越冬幼虫显著重于4龄越冬幼虫。进一步将越冬幼虫单管饲养跟踪研究发现,越冬幼虫体重大小决定蛹和羽化后的成虫的大小,二者存在显著的正相关,并且由4龄越冬幼虫发育所得的蛹和成虫显著轻于由5龄幼虫发育所得的蛹和成虫。【结论】松墨天牛成虫体型差异很大,这与越冬幼虫体重差异相关;越冬幼虫的体重大小决定了其化蛹后的蛹重和羽化后成虫的体型和体重的大小;造成越冬幼虫体重差异的可能原因包括松墨天牛成虫扬飞周期长导致的产卵期长而使得天牛发育进度不一致以及寄主不同部位营养的差异。     

Plant size preference of Agapeta zoegana L. (Lepidoptera: Tortricidae), a root-feeding biological control agent of spotted knapweed

Agapeta zoegana L. (Lepidoptera: Tortricidae) is an oligophagous herbivore that was introduced to North America as a biological control agent of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek (often called Centaurea maculosa Lam.). Spotted knapweed is a perennial plant that usually increases in size each year. A previous field study reported that more larvae were found on larger plants and that infested plants tended to be larger than uninfested ones. Precisely quantifying the size-specific attack rate can help us model the impact of this agent on the weed population and better understand the interspecific interactions to improve the effectiveness of biological control. Field data were analyzed to determine the relative preference of attack for each size class of the host plant. Plants were classified based on root diameter at 2 cm below the root crown. Although small plants (<3 mm root diameter) were more abundant in the field population, the highest infestation rates occurred in large plants. Chesson’s electivity index was generally positive for root diameters >3.5 mm, indicating preferential attack of large plants. Because of its host-size preference, A. zoegana is expected to primarily affect large plants, which is contrary to previous expectations. Quantifying the insect’s direct impact is a difficult challenge, which may require several field seasons of measuring accumulated damage on individual plants. In order to complement the biological control agents already established, foreign exploration should focus on finding an agent that attacks young knapweed plants. These results also indicate that the efficiency of sampling roots in the field to detect the presence of A. zoegana can be improved by choosing only the largest plants.     

Genetic evidence for a single stock of the deep-sea teleost Beryx decadactylus in the North Atlantic Ocean as inferred from mtDNA control region analysis

Mitochondrial control region sequences of 141 alfonsino Beryx decadactylus sampled off the coast of South Carolina were compared with 164 sequences from B. decadactylus collected in the Azores for inferring population structure and demographic history of this deep-water teleost in the North Atlantic Ocean. Analysis of molecular variance showed that 100% of the genetic variation was found within populations, indicating an absence of population structure (Φ(ST) = -0· 003). Neutrality tests and mismatch distribution analyses of pooled sequences suggested that B. decadactylus in the North Atlantic Ocean have undergone population expansion. These results may indicate that transatlantic gene flow occurs, possibly through passive drift of larvae or adult migration. The potential of a shared stock between the eastern and western North Atlantic Ocean will need to be considered if a directed fishery for B. decadactylus were to develop in the U.S.A.     

Arbuscular mycorrhizal fungi enhance spotted knapweed growth across a riparian chronosequence

Arbuscular mycorrhizal fungi (AMF) mediate nutrient uptake that accelerates plant growth and reproduction. Thus, AMF may promote plant invasions often observed along rivers. We assessed the importance of AMF in improving growth of the invasive species, spotted knapweed (Centaurea stoebe), during succession of riparian vegetation along a flood plain in Montana, USA. We grew spotted knapweed with and without AMF in soils collected from riparian sites ranging from 1 to 72 years old and measured the plant’s growth response to AMF. We observed variability in relative effects of AMF, with greatest growth benefits in recently deposited alluvial sediments. We then separated effects of soil and inoculum source by growing spotted knapweed with soils and inocula collected from young or old sites and found that growth responses were greatest in young soils regardless of inoculum source. Our results demonstrate that AMF directly benefit growth of spotted knapweed, especially in soils that typify early successional sites on this alluvial flood plain.     

Geographic variation in the carotenoid plumage pigmentation of male house finches (Carpodacus mexicanus)

Geographic variation in both the colour and pattern of carotenoid plumage pigmentation displayed by males in two subspecies of house finches ( Carpodacus mexicanus frontalis and C. m. griscom ) was quantified. The extent of ventral carotenoid pigmentation (patch size) differed markedly between these two subspecies; frontalis males from the U.S. (New York, Michigan, California and Hawaii) displayed a medium patch extending from their throats to their lower bellies, while griscomi males sampled in Guerrero, Mexico displayed small patches restricted to their throats. Frontalis males sampled in Michigan and New York and griscomi males were relatively bright in colouration, while frontalis males sampled in Hawaii were relatively drab. Populations of frontalis in California showed substantial local variation in average male colouration: in two areas only 12 km apart males were as colourful and as drab as any population sampled. In aviary experiments in which they were fed either a plain seed diet or a diet supplemented with red carotenoid pigments during moult, males from all populations converged on a similar appearance, except that griscomi males attained a brighter plumage than frontalis males when their diet was supplemented with red pigments. Regardless of diet, the difference in patch size between frontalis and griscomi males persisted after moult in captivity. The author concludes that the difference in patch size between frontalis and griscomi males reflects genetic differences between these populations, but that the differences in the mean plumage colouration of males among populations reflect differences in the access that males have to carotenoid pigments during moult.     

Diffuse knapweed (Centaurea diffusa Lam.) seedling emergence and establishment in a Colorado grassland

Knapweeds (Centaurea spp.) are damaging invaders of grasslands and other North American rangelands. A field study was conducted to determine conditions that promote diffuse knapweed (C. diffusa) emergence and establishment in a native Colorado grassland (North America). Knapweed was planted in native grassland under treatments with different opening sizes, levels of competition, knapweed seed burial and season of seeding. There was no effect of opening size where competing natives were alive, but knapweed emergence in 5- and 15-cm openings was higher than 0-cm openings where natives were killed. Reducing competition reduced fall diffuse knapweed emergence, but did not affect spring emergence. Seed burial increased knapweed emergence, but the effect varied by season. Although diffuse knapweed emergence reached 35%, only four plants survived from 3,600 seeds. This native grassland did not prevent knapweed emergence or establishment, but both were so low that rapid knapweed invasion is unlikely.