INSECTICIDAL EFFECTS OF ORGANOTIN(IV) COMPOUNDS ON PLUTELLA-XYLOSTELLA (LINNAEUS) LARVAE .1. TOPICAL APPLICATION TOXICITY AND ANTIFEEDANT EFFECT

Structure-activity relationship studies were conducted with early four th-instar larvae of a highly resistant strain of the diamondback moth, Plutella xylostella (Linnaeus) on (1) toxicity by topical appliction of 43 organotin compounds, and (2) the antifeedant effect of a selecte d number (17) of these compounds on treated Brassica chinensis (Chines e cabbage) leaves. The toxicity data revealed that the triorganotins ( R3SnX) were, without exception, more toxic than the commercial sample of malathion (84% active ingredient) used in the tests. Among the dior ganotins, phenylcyclopentyltin oxide proved to be as active as malathi on. Within the triorganotin series, the tricyclohexyltins were general ly more toxic than the triphenyltins, the most active tricyclohexyltin compound being (c-C6H11)3Sn(2-pyridinethiolato N-oxide) (LC50 0.03 mu g mul-1), which was almost 500-fold more active than malathion. The mo st active compound in the triphenyltin class was O,S-bis(triphenyltin) mercaptoacetate (LC50 0.30 mug mul-1). Variations in the anionic X gro up resulted only in marginal changes in activity in the (c-C6H11)3Sn s eries, but significant changes in activity were obtained with the Ph3S n compounds, especially the ring-substituted phenoxyacetates, (4-ZC6H4 )OCH2(O)COSnPh3. In the mixed triorganotin compounds an increase in ac tivity was observed when one of the phenyl groups in Ph3SnOH was repla ced by the p-chlorophenyl group. In the antifeedant tests, the tricycl ohexyltins were found to be generally more effective than the tripheny ltins. In most cases, antifeedant activity paralleled the toxicity by topical application trends in the (C-C6H11)3Sn series, but in the Ph3S n series an inverse trend was observed. The diorganotin compound (c-C5 H9)PhSnO exerted a relatively pronounced antifeedant activity which wa s comparable with that of a number of triphenyltin derivatives. It was established from histological studies of the mid-gut cross-sections o f the treated larvae that, in most cases, the organotins affected the columnar cells physiologically; an exception was noted for Ph3SnOC(O)C 6H4COOH-4 which, like malathion, caused severe morphological damage to the cell membrane.