Effects of a habitat-altering invader on nesting sparrows: An ecological trap?

Many invading species impact native species through predation, parasitism or competition, while others affect natives indirectly by restructuring their habitat. How invasive plants affect native animals, and to what extent native animals respond to changes in their habitat and the novel selection pressures that follow, is not well known. We investigated the impacts of a habitat-altering invader, the Atlantic cordgrass Spartina alterniflora, on the nesting success of Alameda song sparrows (Melospiza melodia pusillula), a California Species of Special Concern, in tidal marshes in three sites in San Francisco Bay. Date of laying was the most influential factor in determining daily survival rate of nests, but whether the nest was placed in exotic Spartina was the most important ecological variable. Nests placed in exotic Spartina had a success rate that was 30% lower than those placed in native vegetation. Nests in exotic Spartina were significantly more likely to fail due to tidal flooding than were nests placed in native vegetation, because the densest stands of exotic Spartina occurred at significantly lower elevations relative to the tides. Our results suggest that exotic Spartina may be an ecological trap for song sparrows in San Francisco Bay, attracting birds to nest sites that are often destroyed by tidal flooding.     

Effects of simulated herbivory and resource availability on the invasive plant, Alternanthera philoxeroides in different habitats

In biological control programs, the insect natural enemy’s ability to suppress the plant invader may be affected by abiotic factors, such as resource availability, that can influence plant growth and reproduction. Understanding plant tolerance to herbivory under different environmental conditions will help to improve biocontrol efficacy. The invasive alligator weed (Alternanthera philoxeroides) has been successfully controlled by natural enemies in many aquatic habitats but not in terrestrial environments worldwide. This study examined the effects of different levels of simulated leaf herbivory on the growth of alligator weed at two levels of fertilization and three levels of soil moisture (aquatic, semi-aquatic, and terrestrial habitats). Increasing levels of simulated (manual) defoliation generally caused decreases in total biomass in all habitats. However, the plant appeared to respond differently to high levels of herbivory in the three habitats. Terrestrial plants showed the highest below–above ground mass ratio (R/S), indicating the plant is more tolerant to herbivory in terrestrial habitats than in aquatic habitats. The unfertilized treatment exhibited greater tolerance than the fertilized treatment in the terrestrial habitat at the first stage of this experiment (day 15), but fertilizer appears not to have influenced tolerance at the middle and last stages of the experiment. No such difference was found in semi-aquatic and aquatic habitats. These findings suggest that plant tolerance is affected by habitats and soil nutrients and this relationship could influence the biological control outcome. Plant compensatory response to herbivory under different environmental conditions should, therefore, be carefully considered when planning to use biological control in management programs against invasive plants.     

The Effects of Flavonoid Allelochemicals from Knapweeds on Legume–Rhizobia Candidates for Restoration

Russian knapweed ( Acroptilon repens ) and Spotted knapweed ( Centaurea maculosa ) are allelopathic weeds invasive in North American grasslands. Both species contain at least one phytotoxic flavonoid root exudate with demonstrated negative influences on other plants. Previous findings indicated that Silky lupine ( Lupinus sericeus ), among other legumes, was relatively resistant to Spotted knapweed invasion and allelochemistry. We hypothesized that legume species may exhibit resistance to flavonoids in knapweed root exudates and may serve as candidate species for management efforts. Because legumes form symbiotic relationships with rhizobia, these bacteria must also be evaluated for allelochemical resistance before legumes can be recommended for restoration. In this study, we examined four legume species for effects of 7,8-benzoflavone (from Russian knapweed) and (±)-catechin (from Spotted knapweed) on rhizosphere interactions involving legume roots and associated rhizobia. Pure cultures of four rhizobia strains exhibited varied responses when grown with 7,8-benzoflavone or (±)-catechin. Alfalfa ( Medicago sativa ) and its bacterial symbiont, Sinorhizobium meliloti , exhibited allelochemical resistance that varied with (±)-catechin concentration when grown in vitro. Four legume species were grown under greenhouse conditions. Plants that were inoculated and nodulated generally exhibited no response to 7,8-benzoflavone or (±)-catechin treatments. Plants that were not inoculated exhibited stronger responses. Therefore, inoculation and nodulation may confer resistance to allelochemicals. These results, when coupled with previous research and field observations, suggest that legumes may not be susceptible to knapweed allelopathy and may be good choices in restoration of knapweed infestations when inoculated, particularly on sites with low soil nitrogen.     

Insect herbivores associated with the introduced weed Bidens frondosa L. (Asteraceae) in Korea,and their potential role as augmentative biological control agents

The annual herb Bidens frondosa L., native to North America, is an invasive weed. Currently no information is available on the insect herbivores associated with this weed in Korea. A survey was carried out at two‐weekly intervals from May to October 2008 at two sites, and the incidence and abundance of various insect herbivores studied. A total nine species of insects was recorded and among them the defoliating caterpillar Hadjina chinensis (Wallengren) (Lepidoptera) was the only species known to have host plants restricted to genus Bidens. Further host specificity studies are required to evaluate the potential of this insect as a candidate for augmentative biological control agent for B. frondosa in Korea. All other insect species are either polyphagous or known crop pests.     

Clonal spread of invasive Ludwigia hexapetala and L. grandiflora in freshwater wetlands of California

Ludwigia hexapetala and L. grandiflora are recent, aggressive invaders of freshwater wetlands in California. To assess the relative role of sexual versus clonal reproduction in invasive spread, we used AFLP markers to genotype 794 ramets of L. hexapetala sampled from 27 populations in three watersheds of California, and 150 ramets of L. grandiflora from five populations in a fourth watershed. We then used two analytical methods, similarity thresholds and character incompatibilities, to distinguish genotypic variation within genets (clones) from variation between genets. Our analyses revealed extremely limited genotypic and genet variation in invasive L. hexapetala and L. grandiflora within California. Within L. hexapetala, 95% of the ramets analyzed represented a single genet. The genet was the only one detected in 20 populations. The remaining seven populations contained two to nine genets. Within L. grandiflora, all ramets were of only one genotype. Thus, invasive spread within and between populations, and across watersheds, appears to be almost exclusively clonal and brought about by the dispersal of vegetative propagules. The extremely low seedling recruitment indicates that management should target vegetative dispersal and growth.     

Changes in the nucleotide pools in leaves and buds of tobacco plants upon treatment with 2,4-dichlorophenoxyacetic acid

Tobacco ( Nicotiana tabacum L. cv. Samsun) plants were treated once with 2,4-dichlorophenoxyacetic acid (2,4-D) at the 8-leaf stage. The effect of the herbicide on leaf metabolism was followed over 7 days by determination of the ribonucleotide pools, including NAD⁺, NADP⁺ and UDP-sugars, by high-preformance liquid chromatography. 2,4-D treatment resulted in large changes in the nucleotide concentrations, the magnitude and sign of which were dependent upon the leafage. The nucleotide pools decreased in the apical tissue, but increased strongly in the mature leaves with the highest relative increase in the oldest leaf tested. The time course of the changes revealed a maximum on day 5 after 2,4-D treatment. The increase in the adenine nucleotide pools, energy charge and the NADVNADP⁺ ratio are interpreted to indicate a stress situation. The different responses of young, mature and senescent tissue to the synthetic auxin could reflect their different inherent sensitivity due to the natural auxin gradient.     

Enhanced suppression of plant growth through production of L-tryptophan-derived compounds by deleterious rhizobacteria

Plant-growth-suppressive activity of deleterious rhizobacteria (DRB) may be due to production of metabolites absorbed through roots. Auxins produced in high concentrations in the rhizosphere by DRB contribute to reduced root growth. Selected DRB able to produce excessive amounts of auxin compounds for suppression of weed seedling growth may be effective for biological control of weeds. The objectives to this study were to assess the ability of DRB originating from weed seedlings to synthesize auxins from L-tryptophan (L-TRP), determine effects of DRB with or without L-TRP on seedling root growth, and characterize auxins produced from L-TRP using high performance liquid chromatography (HPLC). Auxins expressed as indole-3-acetic acid (IAA)-equivalents were produced by 22.8% of the DRB tested based on a colorimetric method. Under laboratory conditions, a DRB isolate classified as Enterobacter taylorae with high auxin-producing potential (72 mg L^–1 IAA-equivalents) inhibited root growth of field bindweed (Convolvulus arvensis L.) by 90.5% when combined with 10^–5 M L-TRP compared with non-treated control. Auxin derivatives produced by E. taylorae from L-TRP in broth culture after 24 h incubation identified by HPLC included IAA (102 g L^–1), indole-3-aldehyde (IALD; 0.4 g L^–1), and indole-3-lactic acid (ILA; 7.6 g L^–1). Results suggest that providing L-TRP with selected auxin-producing DRB to increase phytotoxic activity against emerging weed seedlings may be a practical biological control strategy.     

AreAltica carduorumandAltica cirsicola(Coleoptera: Chrysomelidae) Different Species? Implications for the Release ofA. cirsicolafor the Biocontrol of Canada Thistle in Canada

Altica carduorumandAltica cirsicolaare two species of leaf-feeding beetle (Coleoptera: Chrysomelidae) which appear to be morphologically indistinguishable and reportedly hybridize in the laboratory. A European population ofA. carduorumwas previously screened for host-plant specificity and released in North America for the control of Canada thistle,Cirsium arvense.A population ofA. cirsicolafrom China is currently being considered as a biocontrol agent forC. arvenseand, as a different beetle species, must be screened using host-specificity tests similar to those used forA. carduorum.IfA. carduorumandA. cirsicolaare, in fact, one species, the screening requirements forA. cirsicolacould be significantly reduced. Hence, we investigated the taxonomic relationship betweenA. carduorumandA. cirsicolausing morphometric analyses, hybridization experiments, and DNA fingerprinting using random amplified polymorphic DNA (RAPD) analysis. Discriminant function analyses indicate thatA. carduorumandA. cirsicolacannot be reliably distinguished by their morphologies, and interspecific matings produce fertile F₁offspring. However, because interspecific matings produce significantly fewer offspring than intraspecific matings, and because of clear differences in their DNA profiles, we conclude thatA. carduorumandA. cirsicolaare separate species. This study serves to highlight the value of genetic analyses in taxonomic studies and their role in biological control programs.     

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.