Although when UV-irradiated seven most toxic strains of Bacillus sphaericus lost their viabilities between 2.5 -4.5 min, their larvicidal activity was protected for longer periods. Benzaldehyde, cinnamaldehyde, salicylaldehyde, methylene blue and yeast extract showed good protective effect for spore viability and larvicidal activity from UV inactivation in B. sphaericus. This protective effect has also been confirmed by SDS-PAGE analyses whereby the 42 kDa and 51 kDa toxic proteins bands did not disappear following UV treatment.
Although some strains of Bacillus sphaericus are pathogenic on mosquito larvae, the most toxic strains differ from each others by their ability to produce 42 kDa and 51 kDa toxic proteins in the parasporal crystal as in the case of strains 2362, 1881, 1593 and 2297. The toxic crystal proteins produce rapid effects on midgut cells of mosquito larvae and intoxication occurs (CHARLES 1987). DAVIDSON (1988) has also shown the importance of the receptor binding site in the susceptibility of mosquito species of these toxins.
The effect of biotic and abiotic factors on the viability and mosquito larvicidal activity of Bacillus thuringiensis var. israelensis and B. sphaericus has been summarized by MULLA (1990) and LACEY (1990). One of the important environmental factors affecting the larvicidal activity of these bacteria is sunlight or ultraviolet light. UV-light drastically reduces the viability of B. sphaericus 1593 spores although larvicidal activity of spore suspensions remains unaffected (BURKE et al. 1983). Besides, moreover, in another study, while B. sphaericus 1593-4 has been shown to lose its larvicidal activity by direct sunlight, the activity of B. thuringiensis H-14 remains stable (MULLIGAN et al. 1980). It has also been shown that B. sphaericus spores and toxin are protected from solar radiation within larval cadavers (DES ROCHERS and GARCIA 1984). Stability of both spore and toxin of B. sphaericus and B. t. israelensis against pH, high temperature and UV irradiation was enhanced by encapsulation with carboxymethylcellulose, alginate and carrageenan (KAPPUSAMY et al ELǬN DQGÖKTEMER ELǬN et alÇÖKMÜș DQGELǬN ,WKDVDOVREHHQ VKRZQWKDWWKHDGGLWLRQRIVRPHZDWHUVROXEOH89DEVRUELQJ.RPSRXQGVSURORQJHGWKHLQ VH.WL.LGDOUHVLGXDOD.WLYLW\ of B. thuringiensis (MORRIS 1983).
The most toxic strains of B. sphaericus used in this study belong to three different serogroups: serotype 5a5b with strains 2362, 1593 and 1881; serotype H6 with strains IAB59, IAB460 and IAB467; serotype 25 with strain 2297 (DE BARJAC 1990), which produce parasporal crystals toxic to mosquito larvae (DAVIDSON and YOUSTEN 1990). Susceptibility of the mentioned toxic strains of B. sphaericus to UV light has not been compared in detail yet. Because of the importance of the recycling potential of pathogenic B. sphaericus strains (DES ROCHERS and GARCIA 1984, CHARLES andNICHOLAS 1986;CORREA and YOUSTEN 1995), our aims in this study were to determine the spore viability and larvicidal activity of seven most toxic strains of B. sphaericus exposed to UV-light and their protection by some UV-absorband chemicals.