Larval susceptibility of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), to Bacillus thuringiensis H serovars isolated in Japan

Differential toxicity of Bacillus thuringiensis strains and their crystal toxins against high-altitude Himalayan populations of diamondback moth, Plutella xylostella L

BACKGROUND

Diamondback moth, Plutella xylostella (L.), is a major insect pest of crucifers in the biodiversity-rich north-western Indian Himalayan hills. The present investigation was aimed at determining the susceptibility pattern of P. xylostella populations collected from different locations of this region to autochthonous and standard Bacillus thuringiensis strains.

RESULTS

Among the reference as well as indigenous B. thuringiensis strains tested, sub spp. kurstaki HD-1, kurstaki HD-73, galleriae HD-8, local galleriae/colmeri strain BtOa1 and some of their Cry1 class toxins were found to be highly toxic. Surprisingly, the sub sp. tolworthi HD-125, local tolworthi strain BtHa1 and Cry9 class toxins were found to be non-toxic. Midgut homogenate from fourth-instar larvae was found to activate 130 kDa protoxin from the local tolworthi strain BtHa1 into 68 kDa toxin, but failed to exert any larval mortality, probably owing to lack of receptor binding.

CONCLUSION

The present study provides valuable baseline susceptibility data for the deployment of B. thuringiensis-based control methods, as well as for future monitoring of development of resistance in P. xylostella to B. thuringiensis in this ecologically sensitive region.

Occurrence of parasporin-producing Bacillus thuringiensis in Vietnam

A total of 63 Bacillus thuringiensis isolates were recovered from urban soils of Hanoi, Vietnam. Of these, 34 were identified to 12 H serogroups. None of the isolates showed larvicidal activities against three lepidopterous insects. Three isolates belonging to the two serovars, colmeri (H21) and konkukian (H34), were highly toxic to larvae of the mosquito Aedes aegypti. Parasporal inclusion proteins of four isolates exhibited cytocidal activities against HeLa cells. Immunologically, proteins of four isolates were closely related to parasporin-1 (Cry31Aa), a parasporal protein that preferentially kills human cancer cells. Haemolytic activities were associated with parasporal proteins of the three mosquitocidal isolates but not with those of the four cancer-cell-killing isolates. PCR experiments and nucleotide sequence analysis revealed that the genes of four anti-cancer isolates are closely related to the gene parasporin-1 (cry31Aa) but are dissimilar to those of the three other existing parasporins. Our results suggest that the soil of northern Vietnam is a good reservoir of parasporin-producing B. thuringiensis.

Microbial ecology of Bacillus thuringiensis: fecal populations recovered from wildlife in Korea

A total of 34 fecal samples, collected from 14 species of wild mammals in Korea, were examined for the occurrence of Bacillus thuringiensis. The organism was detected in 18 (53%) samples. Among the three food-habit groups, herbivorous animals yielded the highest frequency (69%) of samples positive for B. thuringiensis, followed by omnivorous animals (50%). Of the six fecal samples from carnivorous animals, only one sample contained B. thurin giensis. Among 527 isolates belonging to the Bacillus cereus - B. thuringiensis group, 43 (8%) were assigned to B. thurin giensis on the basis of the formation of parasporal inclusions. Of the 43 isolates, 13 were serologically allocated to the nine H-antigenic serotypes: H3ad (serovar sumiyoshiensis), H15 (dakota), H17/27 (tohokuensis/ mexicanensis), H19 (tochigiensis), H21 (colmeri), H29 (amagiensis), H31/49 (toguchini/muju), H42 (jinghongiensis), and H44 (higo). Other isolates were untestable or untypable by the 55 reference H antisera available. Insecticidal activity was associated with 23% of the fecal populations: three isolates killed larvae of the silkworm, Bombyx mori (Lepidoptera), and seven exhibited larvicidal activity against the mosquito, Aedes aegypti (Diptera). There was no larvicidal activity against the three lepidopterous insects: Plutella xylostella, Spodoptera exigua, and Spodoptera litura. The overall results suggest that wild animals in Korea are in contact with naturally occurring B. thuringiensis at high frequencies through the daily food intake of plants.