Biology and morphological analysis of rice weevil, Sitophilus oryzae (L.) (Coleoptera, Curculionidae) on different stored cereals

Abstract

Experimental rearing of Sitophilus oryzae (L.) in eight different cereals such as rice, wheat, oats, barley, sorghum, maize, finger millet and pearl millet has revealed that the pest can complete lifecycle in all of them. Rice and maize are found to be providing optimum living conditions to the pest and within them it can complete its lifecycle in 41 to 45 days, in maize with a mean shortest developmental duration of 40.72 days (egg 6.52, larva 28.45 and pupa 5.75 days). Maize was found to provide more viable living conditions than rice, as adults and developing stages of S. oryzae reared in it was significantly larger with higher adult longevity (female 18.60 days, male 12.56 days) compared to other cereals. In contrast, oats caused longest developmental period of 66.47 days (egg 13.70, larva 42.46, pupa 10.31 days), with female and male longevity reduced to 8.50 and 6.50 days respectively, and noticeable production of miniature adults due to developmental stress. Comparison with span of lifecycle in rice, rearing in wheat resulted delay of 6 days but pearl millet and barley caused delay of 12 days, finger millet resulted 16 days of delay in development while oats extended development by nearly 24 days beyond any cereal. In accordance with increase in developmental delay, body size decreased significantly across cereals, with smallest larval and adult dimensions recorded in oats. The sex ratio also varied, peaking in maize (M:F::1:1.92) and declining sharply in oats (M:F::1:0.074). Similarly, total life span was shortest in maize (female 59.42; male 54.07 days) and longest in oats (female 76.21; male 58.30 days). Morphometric observations showed that early and late instar larva measured 0.30-0.85 and 0.80-2.13mm respectively, pupae 2.58-3.77 and adults 2.60-4.53mm, with maize consistently producing the largest dimensions. Head capsule width of larval instars, a marker of health and viability prospective adults developed in maize, is significantly higher than those larvae developed in rice.