Get to Know the Weevil: A Guide to Rice and Granary Weevils

Among the most notorious of pests that infiltrate post-harvest stored grain are weevils, specifically the rice weevil and the granary weevil. In this blog post, we will explore the characteristics, life cycles, feeding habits, signs of infestation, and effective control methods for these two common stored grain invaders.

Rice weevil (1)

Rice Weevil (Sitophilus oryzae)

The rice weevil is a small, snout beetle, measuring about 3/32” to 1/8” in length. It has a dull red-brown to black body with four distinct light red to yellow markings. Despite its tiny size, the rice weevil is equipped with functional wings, allowing it to fly to new food sources. Its thorax is densely pitted, and it is not tolerant of cold temperatures.

The rice weevil has a relatively quick development cycle from egg to adult, with a minimum duration of 28 days. This rapid reproduction rate can lead to swift population growth in ideal conditions, making control and prevention crucial.

Adult rice weevils feed on whole and broken kernels of grain. Unlike some other pests, they can infest grain while it is still in the field, though they are most commonly found in stored grain. The larvae grow from eggs deposited inside the kernels, causing internal damage that is often not visible until the newly formed adult has emerged.

One of the key signs of a rice weevil infestation is the presence of live adult weevils. Additionally, infested grain often has a "shot hole" appearance, indicating where larvae have exited the kernel.

GranaryWeevil (1)

Granary Weevil (Sitophilus granarius)

The granary weevil has a cylindrical body, shiny red-brown to black in color, and measures about 1/13” to 1/8” in length. It has a long head with a distinct snout and ridged wing-covers. Unlike the rice weevil, the granary weevil is flightless, but it is more cold tolerant.

Similar to the rice weevil, the granary weevil has a minimum development cycle of 28 days. This can lead to rapid population growth if left unchecked.

Granary weevils feed on whole and broken kernels of grain. Like rice weevils, they deposit their eggs inside the kernels, where the larvae grow and eventually exit, causing damage to the grain.

Signs of a granary weevil infestation are similar to those of rice weevils: live adult weevils and grain with a "shot hole" appearance.

Control Tips for Both Rice and Granary Weevils

  • To control weevils, it is essential to clean and treat grain bins before storing grain. We recommend Gravista® Insecticide, a combination product comprised of an insect growth regulator, an adulticide, and a synergist.
  • For longer-term treatments (six months or more), inspect and treat incoming grain with Gravista® Insecticide and monitor the grain throughout the storage period. We also recommend dry-formulated Gravista®-D Insecticide for on-farm storage or in areas where water is scarce.
  • For shorter storage periods (six months or less), and for situations when live bugs have already been spotted, Centynal Synergized Insecticide can be used as an alternative to costly fumigation.

With proper prevention and control methods, weevils can be managed effectively. Understanding their characteristics, life cycles, and signs of infestation is the first step in keeping post-harvest stored grain free from insect damage.

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