Rice weevil
The rice weevil is a stored product pest which attacks seeds of several crops, including wheat, rice, and maize.
Description
The adults are usually between 3 and 4.6 mm long, with a long snout. The body color appears to be brown/black, but on close examination, four orange/red spots are arranged in a cross on the wing covers. It is easily confused with the similar looking maize weevil. The maize weevil is typically somewhat larger than the rice weevil, but rice weevils as large as the largest maize weevils and maize weevils nearly as small as the smallest rice weevils have been found. Some external features can be used to differentiate the vast majority of adults, but the only reliable features are on the genitalia. Both species can hybridize. The genitalic structure of hybrids is unknown.| Rice weevil | Maize weevil |
| Longitudinally elliptical punctures on pronotal dorsum | Punctures on pronotal dorsum typically nearly circular, rarely elliptical |
| Pronotal punctures are separated by a flat, median, longitudinal puncture-free zone | Pronotal punctures are nearly equally spaced apart, and pronotum typically has no median puncture-free area |
| Less than 20 pronotal punctures along the approximate midline, running from neck to scutellum | More than 20 pronotal punctures along the approximate midline, running from neck to scutellum |
| Scutellar elevations typically closer together compare to their longitudinal length | Scutellar elevations typically farther apart compared to their longitudinal length |
| Scutellar elevations typically extend longitudinally approximately more than halfway down the scutellum | Scutellar elevations typically extend longitudinally approximately halfway down the scutellum |
| Proepimera meets behind the fore coxae and along the posterior edge, has a distinct notch along the posterior edge at the site of the meeting point | Proepimera meets behind the fore coxae and has a barely discernible notch along the posterior edge at the site of the meeting point |
| Male aedeagus is smooth and shiny on the dorsal surface | Male aedeagus has two dorsal, longitudinal grooves |
| Epipharyngeal rods of larvae have virtually the same width throughout | Epipharyngeal rods of larvae tapering apically |
| Lateral lobes of Y-shaped sclerite of female genitalia not tapering and rounded at apex | Lateral lobes of Y-shaped sclerite of female genitalia tapering and pointed at apex |
| Less than 5 sensory organs at the tip of the labial palps of larvae | More than 5 sensory organs at the tip of the labial palps of larvae |
Biology
Adult rice weevils are able to fly, and can live for up to two years. Females lay 2-6 eggs per day and up to 300 over their lifetime. The female uses strong mandibles to chew a hole into a grain kernel after which she deposits a single egg within the hole, sealing it with secretions from her ovipositor. The larva develops within the grain, hollowing it out while feeding. It then pupates within the grain kernel and emerges 2–4 days after eclosion.Male S. orzyae produce an aggregation pheromone called sitophilure to which males and females are drawn. A synthetic version is available which attracts rice weevils, maize weevils and grain weevils. Females produce a pheromone which attracts only males.