Common Name: Rice Weevil
Scientific Name: Sitophilus oryzae (Linnaeus)
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.
Adults about 1/8″ (2.5-3.5 mm) long. Color dull reddish brown, elytra (wing covers) usually with 4 faint reddish to yellowish paler marks. Punctures or pits on thorax round or irregularly shaped, deep, midline of pronotum usually puncture-free. Wings fully developed, can fly.
Male’s snout/beak shorter, wider, with more distinct punctures than that of female’s. Larva legless. Color creamy white with brownish-black head. Thickened in middle (humpbacked), and relatively smooth. Labial palp usually with 7 or 8 sensory papillae (tiny fingerlike projections).
(1) Granary weevil (Sitophilus granarius) lacks any pale area on elytra (wing covers), punctures (pits) on thorax elongated in shape.
(2) Maize weevil (S. zeamais) tends to be slightly smaller (length 2.3-2.9 mm) with punctures usually present along midline of pronotum, but identification (vs. rice weevil) is left to experts because it is based on genitalia, prefers sorghum, rice, and especially wheat (rice weevil prefers sorghum, rice, and especially corn).
(3) Broadnosed grain weevil (Caulophilus oryzae) with a short broad snout, snout length about twice width or less.
The rice weevil female bores a hole into the kernel of grain and lays an egg, sealing the opening with a gelatinous material. Females can lay 300-400 eggs in their lifetime but egg laying si sporadic during the winter time, with less activity the cooler the temperature. There are 3-4 instars which require ana average of 18 days for development. The pupal stage requires an average of 18 days for development.
The pupal stage requires an average of 6 days (range 3-9) and upon transformation, the adult insect will remain within the kernel for 3-4 days until it hardens and matures. The life cycle (egg to egg) may be as short as 32 days in the summer. The adult may live for 3-6 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 field and the infestation continues through storage. 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 80-86F (27-30C), 75-90% relative humidity, and grain of 13.5-17.6% moisture content. When disturbed, it will draw its legs up to its body and play “possum.” Adults can fly and are attracted to lights.
Follow the standard control procedures for stored product pest. Extremes of heat and cold can effectively kill all stages.