Bean weevils are truly appalling pests to contend with. You think you’ve got a glorious crop of beans to look forward to, and then you discover little dark holes all over your beans and seeds.
Let’s take a look at these insidious little critters so you can learn how to identify damage, and take action to prevent future infestations.
What are Bean Weevils?
Bean weevils are tiny beetles in the Curculionidae family. They’ve existed for about 170 million years now and have likely been messing with human food stores for about 200 thousand years.
The most common are the bean weevil (Acanthoscelides obtectus), broad bean weevil (Bruchus rufimanus), and the cowpea weevil (Callosobruchus maculatus). Broad bean weevils don’t infest stored beans.
They’re usually about 0.13 to 0.2 inch (3.5–5 mm) long.
These tiny creatures—also known as “seed beetles”—crawl into bean pods and lay their eggs directly into each bean. When the eggs hatch, the larvae munch away happily at the beans’ insides until there’s nothing left to devour.
At this point, they either make their way out of the beans to continue their weevil-y life cycle elsewhere or find themselves tucked away in your cupboard.
It’s possible that you’ve encountered these weevils while cooking.
For example, say you harvested and dried out a bean crop. Then, you rehydrated those beans to make a lovely chili or hummus, and you discovered hundreds of tiny, worm-like critters floating in the soak water.
Those critters were weevil larvae that had matured inside your dried beans and were killed by the cooking prep process.
Where do These Insects Come From?
Bean weevils thrive in warm, humid conditions and are more likely to appear in sub-tropical areas than colder, northern ones. As such, if you’re growing beans in the southeastern USA, California, Texas, Arizona, Ne
\w Mexico, the Caribbean, or various parts of Southeast Asia, you’ll likely have to contend with them at some point.
They spread through distribution of infested beans and seeds, as well as through human or animal contact. For example, if you’re tending your bean plants and some weevils hop onto your coveralls, they’ll travel with you to your friend’s place or the farmer’s market.
If a crop of infested beans is harvested, then the weevils may be transported to other areas, including other countries. Once the bean bags are cut open, the adult weevils will make their way over to the nearest seed or legume crop, and start another life cycle anew.
In terms of potentially affected species and cultivars, the types of beans that these weevils affect most include (but are not limited to):
- Cowpeas
- Chickpeas (aka garbanzo beans)
- Broad beans
- Fava beans
- Field peas
As such, if you’re cultivating any of these species on your property, be sure to inspect them regularly for any weevil damage.
For advice on how to grow healthy bean plants, check out our article.
What do Weevil Infestations Look Like?
Adult weevils are tiny little flying insects, which may be difficult to differentiate from gnats or blackflies. As such, your best bet for determining whether you have bean weevils is to check the pods. Look for tiny, discolored or darkened spots that imply entry or exit holes.
Should you find these spots on several pods, crack them open and inspect the beans within. If there are wrinkly little white grubs moving around inside, you’ve got weevils.
Similarly, if the beans or peas inside those pods also have dark spots, then the weevils have laid eggs inside them.
Another way to identify a bean weevil infestation is in harvested dry beans that you’ve put into storage. Check out the sacks or jars in which you’ve been storing your beans, and see if there are any white, grub-like larvae crawling around in there.
Alternatively, you may see the dark little adult weevils flying around like fruit flies trapped inside.
If you don’t want to release these critters so they can go on to infest another crop, you can bake the jar at 140°F for about half an hour. Alternatively, if you’re dealing with a bag or plastic baggie rather than a jar, you can toss that into the freezer for a few days.
Once the bean weevils inside have died, you can empty the contents into your compost heap without the risk that the weevils will live on to trouble you again in the future.
How to Deal with Bean Weevils
We have both good news and bad news here.
The good news is that bean weevils, while disgusting, won’t cause you any harm if accidentally ingested.
Furthermore, they won’t kill off your parent plants: just the bean pods that they’ve chosen to infest.
The bad news is that there’s really nothing you can do about them.
Bean weevils are notoriously resistant to just about every pesticide you can use in the garden, and the concentrations needed to fend them off would be harmful (if not lethal) to anyone who ate the treated beans.
As such, the only thing you can really do if you find that bean weevils have affected your plants is to pick off infested parts and destroy them.
Some scientific studies have shown limited success with reducing and controlling bean weevil populations in storage facilities, but those chemicals may also have detrimental effects on humans.
Since weevil eggs won’t harm anyone if ingested, the best thing you can do to salvage a potentially infested crop is to either freeze or bake the bean pods. Freezing them for 72 hours should be enough to kill the eggs inside the beans.
Similarly, you can heat-dry the beans for two to three hours at 120°F (49°C) to kill them off.
Preventing Future Infestations
Another way you can help keep bean weevils from affecting future crops is to ensure that you don’t plant infested beans or peas.
Inspect your harvest closely to see if there are any spots (i.e. entry or exit holes) in the beans, and freeze them for three or four days before drying them to ensure that any eggs die off before they’re put into storage.
Alternatively, if you’re growing a new crop from purchased beans or peas, be sure to buy your seeds from a reputable retailer. They will have gone to the trouble of ensuring that the products they’re selling are weevil-free. If the beans you plant don’t have weevil eggs or larvae in them, there won’t be any weevils frolicking around in your garden.
As your bean or pea plants develop, make sure that they’re fed well with fertilizer that’s rich in phosphorous, potassium, and calcium. The healthier your plants are, the better they’ll be able to survive bean weevil infestations. Check your plants regularly, and if you see any weevil activity, cut off and burn affected areas.
If we all do our part to keep bean weevils in check, we may be able to reduce their overall numbers in our communities while ensuring that everyone has enough tasty beans, peas, seeds, and nuts to eat.
Happy growing!
