Low Persistence of Bacillus thuringiensis Serovar israelensis Spores in Four Mosquito Biotopes of a Salt Marsh in Southern France

Abstract

We studied the persistence of Bacillus thuringiensis serovar israelensis (Bti) in a typical breeding site of the mosquito Ochlerotatus caspius in a particularly sensitive salt marsh ecosystem following two Bti-based larvicidal applications (Vectobac 12AS, 1.95 L/ha). The treated area was composed of four larval biotopes that differed in terms of the most representative plant species (Sarcocornia fruticosa, Bolboschoenus maritimus, Phragmites australis, and Juncus maritimus) and the physical and chemical characteristics of the soil. We sampled water, soil, and plants at various times before and after the applications (from spring to autumn, 2001) and quantified the spores of B. thuringiensis (Bt) and Bacillus species. The B. cereus group accounted for between 0% and 20% of all Bacillus spp. before application depending on the larval biotope. No Bti were found before application. The variation in the quantity of bacilli during the mosquito breeding season depended more on the larval biotope than on the season or the larvicidal application. More bacilli were found in soil (10(4)-10(6) spores/g) than on plant samples (10(2)-10(4) spores/g). The abundance in water (10(5) to 10(7) spores/L) appeared to be correlated to the water level of the breeding site. The number of Bti spores increased just after application, after declining; no spores were detected in soil or water 3 months after application. However, low numbers of Bti spores were present on foliage from three of the four studied plant strata. In conclusion, the larvicidal application has very little impact on Bacillus spp. flora after one breeding season (two applications).

Bacillus thuringiensis isolates entomopathogenic for Culex quinquefasciatus (Diptera: Culicidae) and Anticarsia gemmatalis (Lepidoptera: Noctuidae)

“…Some of our new B. thuringiensis isolates from both soil and insects produced high mean percentage mortality in C. quinquefasciatus, A. gemmatalis or both, confirming the reports of various authors that both soil and insects are good sources for new entomopathogenic B. thuringiensis strains (Hansen et al, 1997;Schnepf et al, 1998;Kim, 2000;Cavados et al, 2001;Quesada-Moraga et al, 2004;Hajaij et al, 2005;Hernandez et al, 2005).…”

Fate of Bacillus thuringiensis subsp. israelensis in the Field: Evidence for Spore Recycling and Differential Persistence of Toxins in Leaf Litter

“…Unfortunately, the sensitivity of this approach is often too low to detect slight modifications of toxicity, as B. thuringiensis usually does not exhibit a high level of residual activity (36). Second, spores, which usually persist longer than the larvicidal activity, can be detected in field samples by culturing them on petri dishes (18,46) or by PCR and quantitative PCR, allowing the detection of potential recycling events (12,17). Finally, the fate of Cry toxins, which are mainly responsible for the toxicity of B. thuringiensis, has often been studied by using enzyme-linked immunosorbent assays (ELISAs) (1,29).…”

Using the Bio-Insecticide Bacillus Thuringiensis Israelensis in Mosquito Control

“…Depending on all these factors, Bti toxicity was shown to decrease with highly variable patterns in the field, from a few days to several weeks. In contrast, very few studies looked at the fate of Bti spores in the environment after spraying (Hajaij et al, 2005;Tilquin et al, 2008), and no studies exist so far on the fate of Bti toxins in the environment. Reduced toxicity over time does not mean that Bti spores and toxins are quickly eliminated in the environment: they may accumulate, or even proliferate in soil, or in decaying vegetation at the bottom of mosquito breeding sites (Tilquin et al, 2008;de Melo-Santos, 2009).…”