Biomass reduction of Euphorbia esula/virgata by insect/bacterial combinations.

Abstract

Biological control efforts against the perennial invasive Euphorbia esula/virgata in North America have left 30-50% of all treated sites without impact after 10-15 years. Those efforts focused almost exclusively on insect releases. Much evidence is available indicating that soil biotic factors affect both invasiveness and biocontrol effectiveness. The authors have shown that soilborne bacteria and fungi are linked to biomass reductions or mortality in conjunction with insect damage. To understand factors possibly affecting synergistic interaction of the insects with plant pathogens shown to cause rapid weed mortality, predominant bacteria associated with the flea beetle Aphthona flava Guill. (Coleoptera: Chrysomelidae) released to control E. esula/virgata L. in western North America, were isolated and identified. Two Euphorbia-infested sites with widely differing levels of impact 8-10 years after insect release were sampled. From the site that exhibited rapid, sweeping declines in Euphorbia density, 6 of 12 isolates were Bacillus spp., 4 were coryneform species and 2 were Pseudomonad aceae. Bacteria isolated from the Cottonwood site included some species often associated with the biocontrol of soilborne plant pathogens. The results of tests for a range of hydrolytic enzymes showed that the two groups differed in the frequency of isolates positive for such enzymes as cellulase and xylanase. Two isolates from each location representative of predominant bacterial species and their range of traits were selected for testing on E. esula/virgata in combination with Aphthona spp. After 35-37 weeks, two isolates positive for cellulase from the Knutson Creek site caused significant (P = 0.05) dry weight reductions of E. esula/virgata plants of 64% and 6%, respectively, in combination with Aphthona spp. One of the two isolates from the Cottonwood site, also positive for cellulase production, caused a 60% reduction in dry weight compared with the control.

Rhizosphere bacterial communities associated with insect root herbivory of an invasive plant, Euphorbia esula/virgata .

“…Despite apparent insect damage-based stimulation of overall microbial biomass, Stenotrophomonas spp., well-known as antagonistic to plant pathogens, was most favored. There were also Gram-positive bacteria that have shown a capacity in combination with insects to reduce biomass of E. esula/ virgata (Caesar and Kremer, 2008). Spurge infestations with both of these characteristics have persisted well after other infestations of E. esula/virgata have been dramatically reduced in density at TRNP.…”

“…The alterations in soil microbial community structure also have strong implications for the possibility of restoration of native plant communities. Analyses of Gram-positive rhizosphere bacterial communities, which contain isolates shown to cause 28-65% reductions in the biomass of E. esula/virgata (Caesar and Kremer, 2008) associated with Aphthona spp. herbivory were the subject of a companion study intended to be published separately.…”