Studies on some aspects of bioassay of stomach poisons

Abstract

Using DDT dust as insecticide and third and fourth instar larvae ofEuproctis lunata Walk. as test insect the following fundamental aspects of the bioassay of stomach poisons have been investigated. Starvation and resistance.—It has been indicated that, contrary to previous notions, resistance of E. lunata larvae decreases with increase in period of starvation. The apparent increase in resistance due to starvation is obviously due to the fact that prolonged periods of starvation (24–48 hours) make theE. lunata larvse somewhat lethargic and these lethargic larvae actually eat lesser quantity of both poisoned and unpoisoned food and therefore their percentage mortality is also low. Body weight and resistance.—It has been shown that resistance to insecticide increases with increase in body weight. An increase in amount of poisoned sandwich eaten with increase in body weight has been observed, but the amount of poison ingested per gm. body weight of insect varied within a comparatively narrow range and showed no definite order of variation. It has been suggested that more or less constant amount of DDT per gm. body weight begins to have an inhibitory effect on further feeding. Several smaller doses versusone large dose.—It has been shown that a comparatively larger dose given to larvae in one instalment is more effective than the same dose given in several instalments of smaller doses. It is suggested that the detoxification of some of the poison may be the cause of this phenomenon.