How to keep weevils from eating your chestnuts

Tyrant Farms' articles are created by real people with real experience. Our articles are free and supported by readers like you, which is why there are ads on our site. Please consider buying (or gifting) our books about raising ducks and raising geese. Also, when you buy through links on our site, we may earn an affiliate commission. Learn more

We planted chestnut trees in our yard in 2010 as a gift to “future us.” For the record, future us is very thankful for the gift from “past us.”

We now produce so many chestnuts each year that our family can’t possibly use them all. Thus, we sell our excess to Chef Adam Cooke at Topsoil Restaurant in Travelers Rest, SC. (Chef Cooke is a James Beard Award semi-finalist, and rumor has it Topsoil is being considered for a Michelin star; ergo it’s a phenomenal restaurant you should visit if you live in the area!)

Chef Cooke, the proud new owner of 30 pounds of fresh, organically grown local chestnuts.

As I’m writing this article in mid-September, chestnut season has just started. We just sold over 30 pounds of chestnuts to Topsoil and still probably have an additional 50 pounds of chestnuts still to come.

Why am I telling you this? Because when selling chestnuts to a top restaurant, your chestnuts better be in perfect shape, as ours thankfully are. And what’s the most common cause of ruined chestnuts? Weevils.

What are weevils?

Weevils are a type of beetle characterized by their elongated snouts (rostrum), which they use to bore holes in their food. Female weevils also have a special ovipositor at the end of their snout (female snouts are longer than male snouts of their species) for inserting eggs into their preferred host food source, which may be fruits, nuts, grains, wood, etc, depending on the weevil species.

Once weevil eggs hatch, the larvae (which look similar to fly maggots) eat their food source before burrowing into the ground to overwinter. The following year, they pupate and emerge from the soil as adult beetles, and the cycle repeats.

What species of weevils infect chestnuts in the US

There are two weevil species native to the US whose larvae may eat your chestnuts (adults will eat foliage and flowers, not nuts):

Lesser chestnut weevil (Curculio sayi), which are the more common type here in the southeast, and
Large (or greater) chestnut weevil (C. caryatrypes).

How do weevils get into chestnuts?

There is conflicting information about exactly how lesser weevils get their eggs into chestnuts. For instance, the paper Phenology and Monitoring of the Lesser Chestnut Weevil (Curculio sayi) published in the journal Insects, states:

Adult female C. sayi lay their eggs in chestnut burs, where the larvae burrow through the nut, consuming nut meat as they progress through instars.

A Kentucky University Department of Entomology extension agency guide states:

Eggs are usually laid in the downy inner lining of the brown shell covering the nut.

A closer look at the inside of a chestnut bur shortly after the nut has fallen out. FYI: unlike me, you should wear gloves when handling the spiky burs.

However, forestry scientists at Michigan State University state:

Adult female weevils use their specialized mouthparts at the end of their long snout to chew a hole through the bur and shell of the chestnut into the kernel. This leaves small black bruises on the shell of the nut. After feeding, the females pivot, extend their ovipositor into the hole in the kernel then lay several eggs just inside the shell of the nut. These eggs hatch in one to two weeks and the white or cream-colored larvae feed on the kernel for approximately five weeks. Mature larvae chew circular exit holes through the shell of the nut then drop to the ground and burrow into soil.

Regardless of whether female lesser weevils lay their eggs inside the nut or the larvae hatch outside the nut and have to chew their way inside, there is agreement that egg laying happens right around the time that chestnuts are ripening and falling to the ground. Once the eggs hatch, the larvae will start eating the nut meat and it won’t take them long to ruin the nut for human consumption.

If you don’t do anything to prevent weevils from eating your chestnuts, you may eventually experience up to 100% infestation of your nuts once weevil populations are established. Many commercial growers are finding this out the hard way.

Chestnuts ready for the pre-heated oven. — A batch of chestnuts from our trees being processed to make chestnut flour. As you can see, there is zero weevil damage in any of our nuts.

How do we keep weevils out of our chestnuts without pesticides?

Now that you know a bit more about the basic phenology of chestnut weevils, you might be wondering how we keep them out of our chestnuts. We don’t use any pesticides, synthetic or organic/OMRI-listed, on our chestnut trees. Given the incredible quantity and diversity of lifeforms our trees harbor, doing so would be a travesty. Instead, here’s how we keep our chestnuts from being ruined by weevils:

We pick up all nuts from the ground at least once per day.
We immediately put the nuts into ziplock bags with a paper towel, which absorbs moisture and acts like a desiccant. We immediately put our bags of chestnuts in the fridge until we plan to use them. For long-term storage, whole chestnuts are either frozen or made into chestnut flour.
Every day when we’re harvesting chestnuts, we also remove any of the spiky burs from around our trees. Since they may contain weevil eggs, they’re either immediately discarded or burned. (They’re the only living thing on our property that doesn’t get composted!)

How do these practices prevent chestnut weevils? By harvesting the nuts and removing the burs daily, we prevent chestnut weevil larvae from emerging and overwintering in the soil. Thus, no future generations of chestnut weevils are being bred around our trees.

By immediately putting our chestnuts in the fridge, we also prevent any weevil eggs that may be present from hatching, and any just-emerged larvae will go into torpor rather than eating the nut meat. Thus, 100% of our chestnuts are in perfect condition when we’re ready to use them!

Answers to questions you may have:

How far can weevils fly to find new chestnut trees?

Even if you do everything right on your property, you still may have to deal with weevils flying in from elsewhere. Adult weevils are attracted to chestnut trees’ highly fragrant flowering catkins which bloom in spring.

In a recent study, male lesser weevils were observed flying to new chestnut trees over 2.5 kilometers (1.55 miles) away and females flew more than 3 km (1.86 miles) to new trees.

Weevil candy. The smell of these chestnut catkins flowering in late spring draws in numerous insects, including weevils.

Are there any organic pesticides you can use against chestnut weevils?

Yes. One interesting organic control approach that shows a lot of promise is using entomopathogenic (insect killing) nematodes as a soil drench around chestnut trees (to kill overwintering larvae) combined with applications of entomopathogenic fungi on the trunk of the tree as the weevils emerge. You can read more about that approach here.

Are there other ways to kill weevils on harvested chestnuts?

A few other options various hobby growers, farmers, and university extensions recommend to kill weevils immediately post-harvest include:

Water curing – Submerging chestnuts in water for 7-9 days, which initiates a partial lactic fermentation process and kills weevils. Nuts are then dried for a few hours at room temp, then refrigerated until used or sold.
Hot water submersion – Chestnuts are placed in 120°F (49°C) water for 15-20 minutes. Nuts are then dried for a few hours at room temp, then refrigerated until used or sold. (*You can read more about water curing and hot water submersion here.)
Heating – University of Kentucky Extension recommends heating them to 140˚F (60°C) for 30 minutes, presumably in an oven, prior to cold storage.
Dehydrating – Cut chestnuts in half (use a large chef’s knife) then dehydrate them at about 130°F (54°C) for 48-72 hours or until nuts are completely dry. At that point, the hard shells and papery inner coating (testa) completely separate from the nuts, making the nuts easy to extract. The dried nuts can then be stored in air-tight bags at room temperature until use.

We hope this article helps other backyard, hobby, and small-scale chestnut growers maximize their yields and reduce pest pressure without utilizing pesticides!

KIGI,