Adult Emergence of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) in Stored Rice Varieties and Pulses with Broken Grains

The rice weevil, Sitophilus oryzae, is a serious pest of stored paddy and rice. Much information on its development during the storage of traditional and new improved rice varieties is not found in Sri Lanka. During milling, some percentage of rice is broken, and broken rice generally enhances the development of insects. However, the ability of progeny development of S. oryzae in the presence of broken rice particles is unknown. Hence, this study was conducted to assess the progeny development of S. oryzae in different rice and pulse media with a variable percentage of broken grains. Locally-available rice varieties, Red Kekulu, Red Samba, Red Heeneti, Kuruluthuda, Suwandel and the pulses, cowpea, greengram and blackgram having different broken percentages were tested for progeny adult emergence at  2, 4 and 6 weeks following initial infestation. The number of adults emerged varied with the grain type, broken percentage and the duration following initial infestation. Greengram (0-1.5) and blackgram (0-0.75) had very low adult emergence while Red Heeneti (0-191.75) and Red Kekulu (0.25-194.67) had the maximum adult emergence. The increase of the broken percentage initially increased the adult emergence and then decreased. The study concluded that S. oryzae adult emergence followed a trend of initial increasing and decreasing trend at the latter part in the samples with increasing percentages of broken rice. The adult emergence was higher at intermediate duration following infestation than the first and last durations. Future studies are needed to determine the reasons underlying the variation in the adult emergence across different grain types and broken percentages.