Laboratory evaluatlons were conducted for 8 weeks to compare the residue levels
maln!ained in water treated with eight chlorinated ~.olyethylene pellet formulations of
chlorpyrifos.The eight formulations were of four Particle sizes, each at two chlorpyrifos
concentrations. All Pellets were cylindrical in shape, had a specifIC gravily :> 1.0 and
each was evaluated at dosages of 1.0 ppm and 2.0 ppm in laboratory test jars. In the laboratory tests. average weekly residues for all formulations increased
significantly through week 4. leveled off through week 7, and showed a second
significant increase at week 8: and Ihe 2.0 ppm dosllge resulted in significantly higher
residues than the 1.0 ppm dosage. except at week 1 after treatment. For each
rormulation there was no signlficanl differences due to percent chlorpyrifos therein. The effects of low-level concenlration of chlorpyrifos on the development of CUlex
p < /em>lp < /em>iens qu;nquefasciatus (Say) populations from the time of hatching until adult
emergence. The low-level concentrations were determined below the LCso (0.9 ppb)
by treating laboratory test jars with various numbers of polyvinyl chloride (PVC) petrelS
containing 0.1 percent chlorpyrifos. Control average of the laboratory populations
were 47.3 % in jars containi"g average residues of < 0.10 ppb; 72.2 % in jars
containing average residues of 0.14 ppb; and 9~1.7 % in jars containing average
residues of 0.32 ppb.