Status of Insecticide Resistance in Sitophilus oryzaein Andhra Pradesh

Insecticide resistance to malathion and dichlorvos was monitored in Sitophilus oryzae in strains collected from maize stocks of Andhra Pradesh. Among the four populations of S. oryzae collected from Jeedimetla, Guntur, Amudalavalasa and Amberpet, the former three strains exhibited resistance to malathion and dichlorvos. The degree of resistance acquired to malathion varied with a high level of resistance (7.8 folds) in Jeedimetla compared to Guntur(2.8 folds) and Amudalavalasa strain (2.7 folds) at LC50. On the basis of resistance factors at LC50 for the test population against a susceptible strain (Amberpet), the four strains were categorized as resistant strain (Jeedimetla) and moderately resistant strain (Guntur and Amudalavalasa). Whereas, on the basis of resistant factors at LC90 Amudalavalasa strain was categorized as resistant. Guntur and Jeedimetla strain as moderately resistant. Resistance to dichlorvos varied with strains of S. oryzae with a higher degree of resistance in Guntur strain (9.0 folds) than Jeedimetla strain (4.9folds) and low level of resistance in Amudalavalasa strain (1.4 folds) at LC50 It is the first report of acquired resistance in S. oryzae to dichlorvos. On the basis of resistance factors at LC50, Guntur strain has been categorized as resistant. Jeedimetla strain as moderately resistant and Amudalavalasa strain as slightly resistant to dichlorvos. Whereas on the basis of resistant factors at LC90, Jeedimetla strain was categorized as resistant and Amudalavalasa strain as slightly resistant to dichlorvos. High variability in resistance was recorded at both the levels of resistance i.e. LC50 and LC90.