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

A total of 1700 Japanese strains of Bacillus thuringiensis, belonging to at least 47 H serogroups, were examined for insecticidal activity against lar vae of the diamondback moth, Plutella xylostella. The high-level toxicity w as associated with 612 isolates (36.0%). Of these, 608 isolates (99.3%) fel l into 13 H serogroups belonging to the low-numbered H serotypes, H1-H10. C onversely, most isolates belonging to the high-numbered serotypes (>H10) ha d little or no larvicidal activity; only one isolate of the serovar japonen sis H23 was active. P. xylostella larvae were susceptible to 89.8% of the s erovar morrisoni H8a:8b strains and 85.7% of galleriae H5a:5b strains. High values of 60-80% were also obtained in six serovars (thuringiensis H1, ale sti H3a:3c, kurstaki H3a:3b: 3c, kenyae H4a:4c, aizawai H7, and tolworhi H9 ), while relatively low values of <60% in two other common serovars, sotto H4a:4b and darmstadiensis H10a:10b. Five selected isolates, belonging to H serovars other than kurstaki and aizawai, were 10-60 times less toxic than the reference strain HD-1 (serovar kurstaki). Parasporal inclusion proteins of these strains were immunologically unrelated to those of the strain HD- 1 and the aizawai type strain.