Resistance comparison of domesticated silkworm (Bombyx mori L.) and wild silkworm (Bombyx mandarina M.) to phoxim insecticide

In this study, the resistance difference to phoxim between Bombyx mori L. and Bombyx mandarina M was investigated. For the both silkworm species, the whole body of each larval were collected, and on the third day of the 5(th) instar, the brain, midgut, fat bodies, and silk gland were collected for enzymatic activity assay of acetylcholinesterase (AChE). Our results showed that in the early larval stages, the resistance difference to phoxim was not significant between the two species. However, in the 4(th) and 5(th) instar, the resistance differences showed significant increase. When compared to B. mori L, the LC50 of B. mandarina was 4.43 and 4.02-fold higher in the 4(th) and 5(th) instar, respectively. From the 1(st) to 5(th) instar, the enzymatic activities of AChE of B. mandarina were 1.60, 1.65, 1.81, 1.93 and 2.28-fold higher than that of B. mori, respectively. For the brain, midgut, fat body, and silk gland on the third day of the 5(th) instar, the enzymatic activity ratios of B. mandarina to B. mori were 1.90, 2.23, 2.76, and 2.78, respectively. The AChE-I-50 values of B. mori and B. mandarina detected by eserine method were 5.02 x 10(-7) and 5.23 x 10(-7) mol/L, respectively. Thus, our results indicate that the higher enzymatic activities of AChE and the insensitivity to specific inhibitor of the enzyme might be the underlying mechanisms for higher phoxim resistance in B. mandarina.