Sitophilus zeamais and Sitophilus oryzae

Sitophilus zeamais (Motschulsky) and Sitophilus oryzae (L.)

Systematic position: Insecta, Holometabola, Coleoptera, Curculionidae.

Common names: Rice weevil, greater grain weevil and lesser grain weevil

Distribution: Cosmopolitan, especially in the warmer parts of the world.

Morphology: The larvae of both species are apodous, whitish, with brown heads, about 3-4 mm long. The adult S. zeamais is 3-4.0 mm in length, mostly brown-black. Elytra with 2 pairs of paler, reddish spots and >20 circular depressions on the pronotum, located along the longitudinal midline. The adults S. oryzae are 2.5-2.7 mm long, light brown, also with pronotal reddish spots but elytra with only < 20 punctures in longitudinal lines.

Life history: The adults may live for almost one year, laying several hundred eggs/female, raising 5-6 annual generations. The eggs are individually placed into small cavities in cereal grains which had been chewed by the female, and then sealed. The hatched larva feeds inside the grain, completing its development there in one to several months. The threshold of development is around 13°C, and the optimal temperatures are at 25-30°C. Females exude a sex pheromone to attract the males. Both genders of S. zeamais may fly and infest ripening grains in the field prior to harvest.

Economic importance: Both species of Sitophilus are major pests of several cereal crops, including barley, maize, oats, rye and wheat, as well as peas, cassava and fruits, like apples in storage. Under humid, unaerated store conditions, half the grain may be destroyed. As the larvae feed and tunnel within these commodities, their damage is usually not immediately noticed. However, emergence holes are large and rugged and the beetles may be seen on the commodities’ surface.


Monitoring: Traps baited with the male pheromone together with cracked wheat were very attractive to the beetles. The presence of emergence holes with rugged edges in grains indicates the pest’s presence.

Cultural methods: Harvesting maize as soon as it has reached maturity will reduce infestation in the field. Good store hygiene, including the removal and or burning of infested residues from former harvests. Spreading inert dusts on the grain reduces beetle numbers due to death by desiccation.

Aeration and Modified atmosheres: Aeration or chilling, which lower temperatures in storage facilities to >15°C during periods of warm ambient temperatures, can suppress pest numbers. Carbon dioxide (50% and 60%) killed all life stages of the weevil after an exposure of 10 days.

Irradiation: High-power microwave irradiation killed almost all S. zeamais stages infesting white wheat.

Plant resistance: Resistant maize varieties containing phenolic compounds slow the build-up of pest populations.

Chemical control: A combination of low dosages of vegetable oils and an organophosphate (pirimiphos-methyl) was highly toxic to immatures and adults of S. zeamais. These mixtures retained some controlling activity up to 60 days after application. However, resistance to pesticides has been reported from several regions. Essential oils applied as fumigants killed up to 100% of the pest population, and edible oils applied by contact was also very detrimental. Plant powders prepared from the leaves of five plant species killed over 50% of the exposed beetle adults.

Biological control: Common pteromalid parasitoids include Anisopteromalus calandrae, Lariophagus distinguendus and Theocolax elegans (Westwod). Based on a long-term experiment, a ratio of one A. calandrae to 30 beetles provided weevil control in a monthly parasitoid release program. A pyemotid may attack the beetles in store houses. Applications of an isolate of the entomopathogenic fungus Beauveria bassiana (Bals.-Criv.) Vuill. also resulted in good pest control in laboratory essays. Treatments with another entomopathogenic fungus, Metarhizium anisopliae (Metchnikoff) Sorokin, formulated with charcoal and oven ash, resulted in adult S. oryzae mortality rates of 73–86% mortality of 7 days post-treatment. Similar applications reduced damage rates to wheat to 0.5%, compared to damage rates in the control (6.0%).


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