Bean Weevils

Bean weevils are not really weevils, but they sure can mess up your bean crop.
 
Bean weevils are seed-eating beetles. Also known as seed beetles, most of them are oval with small heads. They can be brown, tan, solid, spotted, striped, or have several other patterns.

Bean weevil, 5mm long (Tobias 67) CC BY-SA 4.0

What they do not have, despite the name, is the long snout associated with weevils.
 
There are more than 1500 species of bean weevil, and many of them eat more than just beans. Some of the more common bean weevil species and their favorite foods include the following:

• Bean weevils (Acanthoscelides obtectus)—beans, buckwheat, corn, peas
• Broad bean weevils (Bruchus rufimanus)—fava beans, lima beans
• Cowpea weevils (Callosobruchus maculatus)—adzuki beans, cowpeas, mung beans

Female cowpea weevil (Simon Hinkley & Ken Walker, Museum Victoria) CC BY 3.0 au

These pests are very tiny and may spend most of their lives inside a single seed. When I say tiny, some species of bean weevil are so small that you could line up 17 of them across the top of an American dime! Of course, this makes them easy to miss.
 
How do you know if bean weevils are in your garden?
 
Two clues indicate the presence of bean weevils: leaf-notching and holes in seeds.

Notched leaf margins due to insect feeding

Beans, peas, and pods damaged by broad bean weevil (Prozentzwanzig) CC BY-SA 3.0

Adult bean weevils feed on pollen and leaves. They seem to prefer leaf edges. As they feed, they leave curved notches along the margins, similar to the damage caused by Fuller rose beetles.
 
Female bean weevils lay their eggs on seeds. If you look closely, bean weevil eggs are easily mistaken for tiny sesame seeds.

Bean weevil eggs on cowpea and adzuki (limbatus) CC BY 2.0

When those eggs hatch, larvae burrow into the seed and begin feeding. After eating their fill, they are ready to pupate. To guarantee they can escape after pupating, they create a larger exit hole before returning to the relative safety of their food-lined apartment.
 
How to manage bean weevils
 
Bean weevils are hard to control. Insecticides are not recommended.
 
The best way to keep bean weevils out of your garden is to not bring them in in the first place. This means investing in certified disease- and pest-free seed stock, not grocery store beans.

Parasitic wasps prey on bean weevils, so keeping your landscape healthy for them makes your job easier. Those garden helpers love plants with umbrella-shaped flowers, like carrots and parsley. And avoid using unnecessary chemicals.
 
And if bean weevils do appear, be sure to remove plant debris after harvesting. This makes it harder for overwintering adults to hide.
 
To be on the safe side, it’s a good idea to freeze harvested dried beans for a couple of days, just in case. And then store your bean crop in airtight containers.
 
Finally, adult bean weevils have a funny trick – when startled, they play dead and fall to the ground. Don’t be fooled! As soon as you walk away, they get right back to what they were doing.