The Rice Weevil
The Rice Weevil (sitophilus oryzae )
August 2023 Edition By: John Moore
The rice weevil is one of the most serious stored grain pests worldwide causing millions of dollars in product loss and damage to valuable seed. This pest of whole grain originated in India. It now benefits from worldwide distribution and can be found anywhere whole seed or grain is stored or processed. It is even known to infest whole-seed products like corn when it is still in the field pre-harvest. It is more common in the southern United States but through commerce is regularly found in the northern tier along with the Granary Weevil. They attack wheat, corn, oats, rye, barley, sorghum, buckwheat, dried beans, cashew nuts, wild bird seed, and cereal products, especially macaroni, and other hard-packed products like pet kibble. The adult rice weevil is an excellent flyer and is attracted to lights. When disturbed, adults pull in their legs, fall to the ground, and feign death. The larval rice weevil must complete its development inside a seed kernel or a man-made equivalent, like pet kibble. Larval rice weevils have been known to develop in hard-caked flour. The adult female eats a cavity into a seed and then deposits a single egg in the cavity, sealing in the egg with secretions from her ovipositor. The larva develops within the seed, hollowing it out while feeding. The larva then pupates within the hollow husk of the grain kernel.
The adult female rice weevil lays an average of 4 eggs per day and may live for 4 months to over a year in some cases. The full life cycle may take only 26 to 32 days during hot summer months but requires a much longer period during cooler weather. The eggs hatch in about 3 days. The larvae feed inside the grain kernel for an average of 18 days, and the pupal stage lasts an average of 6 days. The new adult will remain in the seed for 3 to 4 days while it hardens and matures.
Because much of their life cycle is very cryptic (inside the seed), detection is difficult and you are often unaware of their presence until the adults build up to large numbers and become very visible, showing up in light traps and windowsills as they are dispersing throughout the facility. The best chance of early detection is through regular inspection of high-risk commodities and a robust pheromone monitoring program which is essential to any facility handling high-risk products. Pheromone monitoring programs give you the best chance of detecting a problem while it is still small enough to be controlled by discarding a small amount of infested product. Beyond early detection, control options are very limited; discard all infested products or fumigate the product and sometimes the entire facility.