Toxicity of Carbaryl and Diazinon to Gypsy Moth Larvae: Changes in Relation to Larval Growth12

Abstract

Laboratory tests with larval instars of Porthetria dispar (L.) showed a continued increase in tolerance to topical applications of carbaryl or diazinon with larval growth, with the result that 5th instars tolerate 25 × as much carbaryl and 50 × as much diazinon as 2nd instars at the LD50 level; on a weight basis, the tolerances are 1.9 and 4.5 ×, respectively. A substantial increase in tolerance with growth also occurred within instars (2nd and 3rd) but this is correlated largely with size. Neither piperonyl butoxide nor 2, 6-dichlorobenzyl-2-propynyl ether is effective against 2nd instars, but with subsequent instars, the activity increases greatly so that by the 5th instars only 1/5 the usual dose of carbaryl is needed to kill a larva when applied in combination with the propynyl synergist; this amount of carbaryl is less than that needed kill 4th instars. The increase in tolerance to carbaryl and diazinon with larval growth, as well as increase in synergist efficacy, correlates with increases in mixed-function oxidases in advanced instars.