Common Name: Rice weevil/Granary weevil
Scientific Names: Sitophilus oryzae/Sitophilus granarius
The name rice weevil suggests that this beetle was primarily associated with whole grain rice early on. As the term weevil indicates, this insect belongs to the family Curculionidae, known as snout beetles because of the narrowed, forward projecting head and tiny mandibles. The rice weevil is considered by many to be the most important stored product pest. It has worldwide distribution but primarily in warm climates. In the United States, it is usually confined to stored grain north of North Carolina, but is widely distributed in field and stored grain in the south.
The granary weevil, like the rice weevil, has been a major pest for centuries. It occurs worldwide in the cooler climates. In the United States, it is usually found breeding from and including, North Carolina northward.
Adult rice weevils are about 1/8 of an inch long, dull reddish-brown and usually have 4 faint reddish to yellowish paler marks on the elytra (wing covers). There are numerous round pits on the thorax. Rice weevils have fully developed wings and are able to fly.
Adult granary weevils are about 1/8 to 1/4 of an inch long and usually shining reddish-brown but sometimes nearly black. The elytra (wing covers) lack any pale marks. The pits on the thorax are elongated, not round. The wings are greatly reduced; therefore, this weevil cannot fly.
The larva of both species is legless, creamy white with brownish-black head, thickened in the middle (humpbacked) and relatively smooth.
The Maize weevil (Sitophilus zeamais) tends to be slightly smaller than the rice weevil but is so similar that reliable identification should be left to an entomologist equipped with a microscope.
The rice weevil female chews a hole into a kernel of grain and lays an egg, sealing the opening with a gelatinous material. Females can lay 300 to 400 eggs in their lifetime but egg-laying is sporadic during the wintertime, with less activity at cooler temperatures. There are 3 to 4 instars (growth stages), which require an average of 18 days for development. The pupa stage requires an average of 6 days (range 3 to 9) and upon transformation, the adult insect will remain within the kernel for 3 to 4 days until it tans (hardens) and matures. The life cycle (egg to egg) may be as short as 32 days in the summer. The adult may live for 3 to 6 months.
The granary weevil female, like that of the rice weevil, chews a hole into a kernel of grain and lays an egg, sealing the opening with a gelatinous material. The egg hatches in a few days and the larva feeds on the internal portion of the kernel. A female can lay more than 200 eggs under favorable conditions (range 36 to 254). There are 4 larval instars, which require 19 to 34 days for development. Pupation occurs within the kernel and requires 5 to 16 days. The entire life cycle (egg to egg) may be as short as 1 month during the summer or as long as 5 months during the winter, being very dependent upon the temperature. There are usually 4 generations per year. The adults can live for 7 to 8 months.
The rice weevil is usually confined to stored grain north of North Carolina. In the south, adults fly from stored grain to infest the new grain crop in the filed and the infestation continues through storage. Rice weevils occasionally are delivered to pet shops and super markets in birdseed containing infested sunflower seeds. Therefore, rice weevils are sometime brought into the home in birdseed. It has been recorded attacking corn, wheat, rice, beans, nuts, cereals, rye, buckwheat, stored cotton, wheat products of all kinds and grapes. In addition, it will feed on apples and pears. Optimum conditions for rice weevil activity are 80 to 86°F, 75 to 90% relative humidity and grain of 13.5 to 17.6% moisture content. When disturbed, it will draw its legs up to its body and play possum. Adults can fly and are attracted to lights.
Because its wings are not functional, the granary weevil is confined to stored grain and is primarily transported by man. The larva typically requires a whole kernel for development but can develop in caked grain material. It attacks all kinds of grains and grain products. When disturbed, it draws its legs up to its body and pretends to be dead. Adults are not attracted to light.
Cultural Control & Preventative Measures
Let the buyer beware: Bags of bird seed and packages of whole grain cereal and snack foods should be carefully inspected for evidences of rice and granary weevils and other stored product pests before purchase. If infested products are discovered on shelves, they should be bagged and discarded immediately. The beetles can be removed from activity sites using a vacuum cleaner fitted with a hose attachment.
In commercial settings, infested product should either be removed and discarded or returned/shipped back to the source along with a description of the infestation as being the reason for return.
Once infested product has been removed and the remaining insects and food debris have been vacuumed or wiped clean, a Rottler technician can perform a crack and crevice treatment using a non-residual aerosol insecticide to eliminate adult weevils that may be concealed in structural seams and crevices of cabinets and shelving.
In food warehouses and mills infested with rice and granary weevils, the facility may need to be heat-treated, igated, or treated with a ULV (fog) application of synergized pyrethrum to destroy the adult weevils.