Evaluation of long-term residual activity of insecticides against Acanthoscelides obtectus (Say) on common bean in laboratory tests

There are insufficient data about long-term (longer than six-months) residual effects of insecticides against Acanthoscelides obtectus (Say) as an important economic insect pest of stored bean. Therefore, we evaluated, in laboratory conditions, the insecticide potential of recommended rates of commercial formulations: organophosphate malathion (10 mg active ingredient - AI/kg), spinosyns bioinsecticide spinosad (1 mg AI/kg) and pyrethroid deltamethrins (0,25 mg AI/kg, with and without the synergist piperonil butoxide - PBO) after 210-, 265-, 310- and 365-days of post-treatment. Testing was performed on two types of beans with kidney- and cranberry-shaped kernels to evaluate the insecticide residual effects on the survival of weevils, progeny production, and related levels of kernels damage. Survival of weevils depends mainly on adults exposure/living period in treated beans and on the recovering period. Small differencies in weevils survival in each insecticide were recorded. We found increases in progeny and thus higher damage in beans, as manifested effects of sublethal doses only in malathion treated bean. In spinosad treated bean we recorded low levels of progeny and damage, below the economic threshold. In deltamethrin treatments, we recorded weevils avoiding contact with beans, no progeny and no damages. The effect of bean type was very low or non existent. It can be concluded that bioinsecticide spinosad can provide long-term protection of common beans against A. obtectus, while organophosphate malathion has low control potential. Pyrethroid deltamethrins residues cause adults to avoid treated beans and no offspring to appear. The obtained results might have significant impact on better understanding of weevils behaviour in the presence of insecticide residues and on the creation of sustainable pest management programmes.