Toxicity and development of resistance in Tribolium castaneum and sitophilus oryzae to certain selected insecticides

Abstract EN

The rust red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and the rice weevil, Sitophilus oryzae L.

(Coleoptera: Curculionidae) are considered the most important insect species which destruct the stored grain.

Application of synthetic insecticides for stored grain protection has been created resistance and cross-resistance in storage insects.

Laboratory bioassays were undertaken to assess the toxicity, resistance and cross-resistance of insecticides, chlorpyrifos, chlorpyrifos-methyl, malathion, pirimiphos-methyl, cyfluthrin, cypermethrin and methomyl, to T.

castaneum and S.

oryzae.

Two methods, exposure to treated surface and mixing with wheat grain as well as selective pressure and cross-resistance studies using LC50 were used.

Results obtained after 24h by thin film method revealed that pirimiphos-methyl was the premier insecticide to T.

castaneum with LC50 of 0.007 μg/cm2 .

While methomyl showed low toxicity.

On the other hand, chlorpyrifos-methyl was the most toxic compound to S.

oryzae with LC50 of 0.007 μg/cm2 .

For mixing with medium the order of toxicity obtained in this technique was different from that in thin film residue method.

The development of resistance in T.

castaneum indicated that methomyl failed to build up resistance after 4 generations since its resistance factor was 5 fold.

The highest level of resistance was induced by malathion followed by pirimipho-methyl with resistance factor, R.F., of 214.4 and 67.4-fold, respectively.

Selective pressure with chlorpyrifos for 4 generations did not succeed in building up more than 10.9 fold.

Data also, presented that all of the tested Op-resistant strains of T.

castaneum show only low level of cross-resistance to the pyrethroid insecticides cyfluthrin and cypermethrin at the LC50 level (R.F.

values ranged from 2.55 to 6.4 fold).

These resistant strains were more susceptible to the insecticide, chlorpyrifos-methyl than the susceptible strain (RF values ranged from 0.29 to 0.66).

It is concluded that the common insecticides, malathion and pirimipho-methyl have created high level of resistance compared to methomyl which was rarely used in grain protection.