The topical toxicities of carbofuran and terbufos insecticides were determined on three species of corn rootworms. The average LD(,50) values ((mu)g/g) for carbofuran were 12.68 and 1.46 on western corn rootworm (WCR) larvae and adults, 9.20 and 1.56 on northern corn rootworm (NCR) larvae and adults, and 23.14 and 1.06 on southern corn rootworm (SCR) larvae and adults, respectively. The average LD(,50) values ((mu)g/g) for terbufos were 1.08 and 4.74 on WCR larvae and adults, 7.34 for NCR adults, and 3.70 and 4.30 on SCR larvae and adults, respectively. The toxicities of five analogues of each were determined on WCR adults in 1981. The LD(,50) values ((mu)g/g) are: 3-hydroxycarbofuran, 10.45; 3-ketocarbofuran, 9.69; 7-phenolcarbofuran, > 768.07; 3-hydroxy-7-phenolcarbofuran, > 660.78; 3-keto-7-phenolcarbofuran, > 657.82; terbufos sulfoxide, 3.11; terbufos sulfone, 3.87; terbufoxon, 3.81; terbufoxon sulfoxide, 5.15; terbufoxon sulfone, 5.95;Rootworm acetylcholinesterase (AChE) activity was studied using whole body homogenates. Analysis of Lineweaver-Burk plots of velocity versus acetylthiocholine (ASCh) concentration yielded K(,m) ((mu)M) and V(,max) ((mu)mole ASCh hydrolyzed/min/g) values of 9.28 and 1.02 for SCR larvae, 20.04 and 1.58 for WCR larvae, and 23.00 and 0.47 for WCR adults, respectively. Substrate inhibition was seen at ASCh concentrations greater than 25 (mu)M for SCR larvae and greater than 50 (mu)M for WCR larvae and adults. The K(,ss) ranged from 369 to 794 (mu)M. Dithiobisnitrobenzoic acid in the range 31 to 1,242 (mu)M did not inhibit AChE. A plot of AChE activity versus larval weight was linear with a positive slope indicating that as the larvae increase in size, the AChE per larva also increases proportionally;In vitro addition of the LD(,50) concentration of carbofuran to SCR larval homogenate produced an 82% inhibition of AChE activity within five minutes. After six hours, inhibition was 94%. The in vivo application of carbofuran to WCR adults caused 52% inhibition after one hour. Total AChE recovery in the surviving 50% was complete by 48 hours;The in vitro addition of the LD(,50) concentration of terbufos to SCR larval homogenate produced no inhibition of AChE activity indicating that no conversion of terbufos to terbufoxon occurred. The in vivo application of terbufos to WCR adults caused a maximum inhibition in surviving insects at 42% at two hours post treatment. The surviving 50% rapidly regain 26% of the lost activity, having only 16% inhibition of AChE at eight hours.