Geographical variation in larval susceptibility of the diamondback moth,Plutella xylostella(Lepidoptera: Plutellidae) toBacillus thuringiensisspore–crystal mixtures and purified crystal proteins and associated resistance development in India

Abstract

AbstractThe susceptibility of larvae of the diamondback moth,Plutella xylostellaLinnaeus to purified crystal proteins and spore–crystal preparations ofBacillus thuringiensiswas investigated for 13 populations from seven states in India. The LC50(μg ml−1, 48 h) values of Cry proteins for different populations ofP. xylostellaranged from 0.14–3.74 (Cry1Aa), 0.007–1.25 (Cry1Ab), 0.18–2.47 (Cry1Ac) and 0.12 – 3.0 (Cry1C). The LC50(mg (ai) l-1, 48 h) of spore-crystal preparations ranged from 0.02–0.98 (HD-1) and 0.06–2.14 (HD-73). Significantly higher LC50values for all tested toxins and strains were obtained with populations collected from Iruttupallam and Ottanchathiram in the southern state of Tamil Nadu, whereas some of the populations collected from the northern part of India were more susceptible than the susceptible IARI 17–65 population. The high levels of resistance in the Iruttupallam and Ottanchathiram populations to Cry1Ab suggested selection pressure by Cry1Ab, which is the predominant toxin inB.thuringiensisformulations used in India. Cry1Ab was found to be more toxic than the other toxins. The population from Iruttupallam showed increased resistance following selection with Cry1Ab in the laboratory (LC50from 1.25 to 4.31 μg ml−1over two generations) and also showed cross resistance to Cry1Aa and Cry1Ac. The resistance to Biobit®in the field population from Iruttupallam declined slowly; requiringc. 33 generations for an overall 10-fold decline in LC50when the insects were reared in the laboratory without exposure toB. thuringensis.