The Toxicity and Residual Effectiveness of Malathion and Diazinon Used for Protection of Stored Wheat1

Abstract

The relative susceptibilities of 17 species of stored-product insects to malathion and diazinon deposits (ppm) on wheat sprayed with acetone solutions of each insecticide were determined from LC values obtained in a series of toxicity tests. Mature larvae of 4 species of Trogodema were used; all other test insects were adults. Diazinon was more effective than malathion against Cryptolestes pusillus (Schonherr), flat grain beetle; Latheticus oryzae Water-house; Rhyzopertha dominica (F.), lesser grain borer; Sitophilus oryzae (L.), rice weevil; S. zeamais Motschulsky; Sitotroga cerealella (Olivier), Angoumois grain moth; Tribolium brevicornis (LeConte); T. castaneum (Herbst), red flour beetle; T. confusum Jacquelin duVal, confused flour beetle; Trogoderma inclusum LeConte; T. parabile Beal, and T. simplex Jayne. Diazinon was less effective against Doliema plana (F.); Oryzaephilus menator (Fauvel), merchant grain beetle; O. surinamensis (L.), saw-toothed grain beetle; and Sitophilus granarius (L.), granary weevil. At the LC50, T. glabrum (Herbst) was more susceptible to diazinon than malathion, but not at the LC95. Results from residual protect ant tests designed to evaluate the relative effectiveness of various concentrations of insecticide residues (ppm) on wheat in preventing infestations of 6 species of stored-product insects showed diazinon to be slightly more effective than malathion when considering counts of all insects, but the most important differences observed were in the response of species to the treatments involved. Tolerance of “bran beetles” to malathion was in the order of Tribolium castaneum > C. pusillus > O. surinamensis , and tolerance to diazinon was ill the order of O. surinamensis > T. castaneum > C. pusillus . Good protection against R. dominica demonstrated by malathion treatments, as compared with S. oryzae , suggested that susceptibility of R. dominica larvae may be an important factor in the use of grain protect ants for prevention of infestations of this species. Infestations of S. oryzae were greater in malathion-than in diazinon-treated samples of wheat. One species, S. granarius , failed to compete favorably in the test complex.