Effects of Number of Test Insects, Exposure Period, and Posttreatment Interval on Reliability of Fumigant Bioassays11

Abstract

Studies were designed to determine the reproducibility of re- sults from standardized laboratory fumigation tests using an 80:20 (CCI4:CS2 by vol.) fumigant in empty 20-liter bottles at 80° F. The test insects were adults of rice weevil ( Sitophilus orlyza (L.)), confused flour beetle ( Tribolium confusum Duv.), and saw-toothed grain beetle ( Oryzaephilus surinamensis (L.)) and three age groups of immature rice weevils. Analysis and evaluations showed that (1) the optimum number of adult insects used in each test cage was 50; (2) rice weevil and confused flour beetle should be held at least 10 days and sawtoothed grain beetle 15 days after fumigation for final mortality counts; (3) the descending order of resistance to the fumigant was rice weevil pupae, eggs, and larvae, and adult saw-toothed grain beetles, confused flour beetles, and rice weevils, and (4) variation in results was unpredictable although variations were greatest with rice weevil adults and smallest with rice weevil larvae. Results of parallel experiments conducted on different days were highly variable in some instances, but replicates within a given day's tests were reasonably consistent. Adult insects demonstrated greater variation following longer exposures to the fumigant than following shorter exposures. conducted weevils responded erratically following the shorter exposures but varied least of all groups in the 48-hour exposures. Sub lethal treatments of immature rice weevils retarded their rate of development. LC values with their respective confidence intervals were determined. The observed LC50 and LC90 values associated with 24- hour exposures were 290 and 370 µM./1. (micromoles per liter) for confused flour beetle adults, 165 and 220 µM./1.for rice weevil adults, 125 and 500 µM1./1. for rice weevil eggs and young larvae, 155 and 345 µM./1. for young and middle-aged rice weevil larvae and 375 and 865 µM1./1.for older rice weevil larvae and pupae. I.C values for 4- and 8-hour exposures were greater, and those for 48-hour exposures were smaller, than the 24-hour I.C values.