Postharvest tips: preventing "wormy" chestnuts

Chestnut weevils emerge from shells soon after harvest, eating tunnels through the nut kernel.

The following article about chestnut weevil is excerpted and condensed from the Chestnut Grower newsletter, produced quarterly by the Center for Agroforestry in cooperation with the Chestnut Growers of America (CGA). CGA represents chestnut growers, researchers and others throughout the United States and Canada involved in the chestnut industry. The organization advocates the delivery of high quality chestnuts to the marketplace and the family’s table. A membership directory, information forum and access to the quarterly newsletter The Chestnut Grower are included with membership; visit http://www.wcga.net for moreinformation.

"My chestnuts are wormy!" is a common complaint from producers and consumers. The "worm" causing the problem is the larva of either the small chestnut weevil (Curculio sayi Gyllenhal) or the large chestnut weevil (Curculio caryatrypes Boheman). Both species are native to North America where they commonly infested American chestnuts before the chestnut blight epidemic. When the blight wiped out the American chestnut trees it also wiped out the chestnut weevils - almost. Now that chestnuts, mostly Chinese, have been widely planted the chestnut weevils have made a comeback. Many chestnut plantings in eastern North America are now plagued with these pests. The good news is that acceptable control is possible.

The bad news is that without control, they can render the entire crop unmarketable. Weevil grubs emerge from chestnuts shortly after harvest, but the problem begins before the chestnuts ripen. Adult weevils lay tiny eggs inside the shell, while the nuts are still on the tree. After hatching, the grubs eat tunnels through the nut kernel. Sometime after nut drop, they eat their way out through the shell, leaving pencil lead-sized exit holes in the shell. The squirming, cream-colored grubs, about 7-14 mm (1/4- to 1/2 in) long, are repulsive but harmless. Adult chestnut weevils are hard-shelled beetles which vary in color from mottled light tan to dark brown, almost black. A prominent snout (proboscis) is as long as or longer than the body. The small chestnut weevil female is 5-11 mm (1/6 to 1/3 in) long; the large chestnut weevil female is 7- 14 mm (1/4- to 1/2 in) long. Females have a much longer snout than males. Other than the difference in size, both the large and small weevils look similar. The small chestnut weevil is more common than the large, but both species are reportedly distributed wherever chestnuts occur. When both species are found together, the large chestnut weevil seems to predominate. But new weevil infestations are almost always the small species. Apparently, small chestnut weevils can fly several miles or more to find previously uninfested trees. In our orchards, thus far, we have found only the small chestnut weevil.

Life Cycles of the Insects Small chestnut weevils (C. sayi) emerge from the ground as adults during May and June. After emergence they remain on the ground for several days before flying. They can be found in chestnut trees during flowering, apparently feeding on catkins. After flowering they disappear and it is not known where they harbor for the two to three months between their emergence and their appearance in the trees at egg-laying time. Mating and then egg laying begin about three weeks before nut ripening, i.e., from mid-August to the beginning of September. Egg laying occurs throughout the nut ripening period. Adult females deposit eggs into the nut, usually on the round side through tiny holes that look like pin-pricks. Three to five pear-shaped eggs are laid in each nut. Each adult female lays up to 50 eggs. Eggs hatch in about 8 days and the larvae tunnel through the kernel. Larval development is usually complete in two to three weeks, but development time is very temperature dependent. Warm temperatures promote fast development while cold temperatures may delay emergence for two months or more. Larvae generally exit after the nut has fallen to the ground.

Properly scored chestnut shells pop open upon cooking, revealing attractive golden nutmeats. The nutmeat should be easy to pull from the shell after cooking.

Full-grown larvae enter the soil, burrowing 5-20 cm (2 to 8 in) below the surface to hibernate inside earthen cells that they construct. They pass the first winter as larvae and most pupate and become adults by the next autumn. Then they pass the second winter as adults in the soil, and emerge the following May or June. Some larvae pupate the second autumn and pass the third winter as adults. In summary, small chestnut weevils usually have a two-year life cycle, but a few individuals delay to a three-year life cycle. This tactic ensures survival, and complicates control.

The extent of crop damage depends on weevil population and crop size. Weevil infestation can range from 0 to 100% of the crop. While the main damage is caused by feeding larvae, the adults also feed on nuts, and this provides a point of entry for fungus and yeast organisms. This damage is often not noticeable until after storage.

Large chestnut weevils (C. caryatrypes) have a life cycle and habits similar to the small species except for the following differences. Adults emerge from the soil in late July to early September, just before egg-laying. Mating and egg laying begin when the kernel begins to form, from mid to late August, which is a week or two before the small chestnut weevil. Females usually lay no more than three eggs per kernel. Because of their earlier egg laying, a few larvae complete development and exit the shells before the chestnuts fall off the tree. Large chestnut weevils usually have a one-year life cycle, with a few individuals delaying until two years.

Control

Three control methods can reduce weevil infestation: sanitation, hot water treatment, and insecticide applications. Sanitation and hot water treatment target larvae, insecticide application targets adults.

Sanitation: Sanitation involves collecting fallen nuts before the grubs emerge. If the fallen chestnuts have exit holes, the grubs have already emerged and it’s too late for sanitation to work well. Collected nuts should be stored in containers from which emerging larvae cannot reach the soil, e.g., plastic buckets. Be careful; weevils can chew through plastic or paper bags. Make sure the weevils are destroyed don’t throw them outside! If sanitation is practiced consistently, it will prevent weevil populations from increasing. It is most effective when weevil populations are relatively low, (less than 5 percent infestation), and the orchard is isolated from other chestnut trees. For sanitation to work, the chestnuts should be picked up within three days after they drop. Sanitation won’t work if a significant part of the crop is taken by chipmunks, squirrels, or other varmints that cache their food, since the cached chestnuts will allow weevil larvae to escape to the ground. After weevils have emerged, leaving obvious exit holes, the infested nuts can be destroyed. (Don’t just throw them outside, some grubs may have not yet emerged!) Uninfested nuts can be eaten.

Properly scored chestnut shells pop open upon cooking, revealing attractive golden nutmeats. The nutmeat should be easy to pull from the shell after cooking.

Hot water: Rather than waiting for them to emerge, larvae or eggs can be killed inside the kernels by soaking the chestnuts in water at exactly 49oC (120oF) for about 20 minutes. The temperature is critical, the time of soaking chestnuts continued from page 80 less so. After soaking, the chestnuts should be allowed to cool and surface-dry before storage. This treatment kills the larvae but does not damage the kernel. If the chestnuts are promptly harvested and hot-water treated, many of the infested chestnuts will contain only unhatched eggs or very small larvae. These small infestations are not noticeable and can be eaten. On the other hand, if exit holes are present in the chestnut shells at treatment time, the weevils have already emerged. All chestnuts which have any chance of being infested should be hot water treated before being sold. Buyers have zero tolerance for living grubs crawling out of the chestnuts they bought, but seem to tolerate dead grubs in a few nuts. If one chestnut in a thousand is infested, this means that every 25 lb lot of chestnuts probably harbors several grubs; this is unacceptable. But most people don’t mind discarding one in a thousand bad nuts.

Properly scored chestnut shells pop open upon cooking, revealing attractive golden nutmeats. The nutmeat should be easy to pull from the shell after cooking.

Insecticides: When 10% or more of the crop becomes infested, insecticide sprays are necessary to produce marketable chestnuts. Insecticides have proven effective only when applied to adults during the mating and early egg-laying period. For small chestnut weevils this is mid- August to early September. Presently, the only insecticide labeled for chestnut weevil control is carbaryl (Sevin). Follow label instructions for application rates. Spraying should begin when adult weevils begin arriving in the tree crowns. Weevils tend to seek out early-ripening trees first. So, it is best to look for weevils on early trees, and spray whenever they are present. Spreading a large sheet on the ground and shaking branches is a good way to monitor weevil presence. (Weevils have a habit of dropping to the ground whenever they are disturbed.) Spraying should be done when the weevils are active, i.e., on warm, calm days. The interval between spray applications should range from three to seven days depending on weather and the presence of weevils. Two to four spray applications per season should be sufficient to provide adequate control.

Greg Miller,
Empire Chesnut Company,
Carrollton, Ohio