INSECTICIDAL ACTIVITY AND BIODEGRADATION OF THE TOXIN FROM BACILLUS-THURINGIENSIS SUBSP KURSTAKI BOUND TO HUMIC ACIDS FROM SOIL

The equilibrium adsorption and binding of the active toxin from Bacill us thuringiensis subsp, kurstaki, toxic to lepidopteran larvae, to hum ic acids extracted from two forest and two cultivated soils, as well a s the insecticidal activity and the biodegradation of the bound toxin. were studied. From 75 to 85% of the toxin added was rapidly adsorbed to the humic acids at equilibrium, and adsorption to a constant amount of humic acids increased with the concentration of the toxin until a plateau was reached. Differences in total acidity and in the content o f phenolic groups of the humic acids appeared to be primarily responsi ble for differences in the amounts of toxin bound (45-80% of the adsor bed toxin) after extensive washing with distilled water. The content o f carboxyl groups and the degree of polymerization (E4/E6) did not app ear to influence significantly the differential binding. Bound humic a cid-toxin complexes were toxic to larvae of the tobacco hornworm (Mand uca sexta). The lethal concentration necessary to kill 50% of the larv ae (LC50) Of the bound toxin was comparable with that of the free toxi n, indicating that the binding of the toxin to humic acids did not aff ect its insecticidal activity. The bound toxin did not support the gro wth of a mixed microbial culture from soil, although the free toxin wa s rapidly utilized as a carbon and energy source for growth, indicatin g that binding of the toxin to humic acids reduced its biodegradabilit y. The result of these studies indicate that the toxins from B. thurin giensis introduced in transgenic plants and microbes could persist, ac cumulate, and remain insecticidal in soil as a result of binding to hu mic acids, as well as on clays, as previously described. This persiste nce could pose a hazard to non-target organisms and enhance the select ion of toxin-resistant target species. (C) 1998 Elsevier Science Ltd. All rights reserved.