The evils of weevils

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QUESTION: Dan, I’ve had experience with stored product pests (SPP), but not specifically grain weevils. Now one of my clients says they have them. What next?
—Deliver us from weevils 

ANSWER: DUFW, I consulted with my colleague, Anna Berry, and we agree: The very first step you should take is to confirm that they are, in fact, weevils. Flour beetles (Tribolium castaneum), for example, are commonly mistaken for them.

Weevils (Curculionidae) are a specific group of SPP made up of several different species. They share common morphological characteristics, such as an easily distinguished “snout.” Unlike other common SPP, weevils develop inside a kernel of grain (corn, rice, wheat, bean or sorghum) or complex pasta, making monitoring and controlling for them a challenge.

Because weevils develop in the kernel, we don’t see them for quite a while. The egg, larval and pupal stages all are hidden within that kernel. With other SPP, we often see the damage caused by the larval stage, which lets us know there’s a problem. We’re not so lucky with weevils, and that means at least a generation has taken hold before we start to see the adults.

Adult weevils are unlikely to travel far if there’s plenty of food, so a bag of pasta (or rice, or bird seed, or any other susceptible product) can be infested by many generations, destroying the food within, before the adults finally leave the container to find new food.

Placing monitoring traps will not control a weevil population, but it will help to focus your inspection. Particularly in a large warehouse, where the number of potential sources can be daunting, this will help direct your efforts. In a retail environment, we often can see the source of infestation by inspecting the product and shelving for live insects; food that has been destroyed from the inside to the outside; and holes in the packaging.

As with all SPP, the absolute best control method for weevils is to remove the food source. Most life stages are physically in the food itself, and adults will be on top of the food (unless they are moving to find the next food source). Discarding infested products typically is the best solution.

Sanitation is essential, particularly in retail environments where weevils may thrive on forgotten debris under or in shelving. Vacuums can physically remove the infestation and clean the debris, an effective tool in small infestations.

Depending on the food’s intended use, further processing, and level of infestation, it may be possible to salvage infested products by freezing, heating or fumigation. Even if food cannot be recovered, these tend to be our best control techniques outside of sanitation. Particularly for large warehouse infestations, fumigation often is the only recourse.

Residual insecticides can play a role in controlling large infestations, because we can treat cracks and crevices to prevent adults from moving in these areas to infest other products.

Take a systematic approach through identification, monitoring, inspection and control. It may take patience, time and multiple solutions, but grain weevils can be controlled.

Email your questions about insect identification and pest management technologies to pmpeditor@northcoastmedia.net. Your questions most likely will be printed and answered in one of Pest Management Professional’s upcoming columns.