Residues of deltamethrin on wheat grain of 12% moisture content, stored in the laboratory at 25"C, remained constant within experimental error over a 15-month interval. However, the biological activity of the residues against Sitophilus oryzae (L.) declined to 78 % of the initial value at 3 months, and to 65 % at 15 months after application. Inactivation of a proportion of the intact residues is indicated.
One hundred unsexed adults of each of the following coleopterous species were added to 27 .3 kg of clean wheat: Sitophilus oryzae (L.) (rice weevil), Rhyzopertha dominica (F.) (lesser grain borer), Tribolium castaneum (Herbst) (rust-red flour beetle), and the flat grain beetles Cryptolestes pusillus (Schonherr), C. ferrugineus (Stephens), and C. pusilloides (Steele and Howe). For 15 months, measurements were made of population size and emigration from this wheat bulk. The highest populations recorded for each species were: 138 850, 65 680, 9 060, 47 000, 15 530, 11 940, respectively, and total emigrant numbers during the period were: 774 682, 278 094, 123 784, 335 588, 39 070, 9 352, respectively. Such high numbers of emigrants in relation to relatively low source populations emphasize the significance of small amounts of infested grain as sources of infestation of large grain bulks, and hence the importance of grain hygiene. The proportion of females in the populations of four species changed significantly from 0.50 as populations developed. Emigration behaviour was influenced most commonly by temperature, insect numbers, and wheat age, but these factors did not act uniformly on all species. Negligible numbers of emigrants returned to the food source. Low numbers caught in food traps away from the source were consistent with these not being attractive but traps near the shed walls caught greater numbers than expected.
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