The chestnut weevil, scientifically known as *Curculio davidi* Fairmaire, is also referred to by several common names such as chestnut elephant, chestnut weevil, fruit weevil, and more. This pest belongs to the order Coleoptera, not Lepidoptera, and is closely related to other beetles. It is commonly found in chestnut-producing regions across China and primarily infests plants in the genus *Castanea*, including chestnuts, hazelnuts, and even alfalfa. The larvae are particularly destructive, causing damage rates of over 80% in chestnut crops, significantly affecting both storage safety and market value.
What makes this pest dangerous? The adult weevils feed on young leaves, shoots, and fruits, while the larvae burrow into the inner cotyledons of chestnuts, filling them with frass (insect excrement), which renders the nuts unsuitable for consumption or sale. This damage not only reduces yield but also affects the quality of the harvested chestnuts.
Regarding its physical characteristics, adult chestnut weevils have a dark brown coloration. Females measure between 7–9 mm in length, while males are slightly smaller, around 7–8 mm. The head length of females ranges from 9–12 mm, and males have a head length of 4–5 mm. On the prothoracic plate, there are two semi-circular white stripes near the posterior edge, along with white spots at the base of the elytra. The elytra feature one white stripe near the base, approximately at the 1/3 and 2/5 positions. These markings are composed of white scales, and the wings have ten vertical grooves. The body is covered in fine white scales.
The eggs are oval-shaped, about 1.5 mm long, smooth, and transparent when laid, turning milky white when they hatch. The larvae grow up to 8.5–12 mm in length, with a pale yellow to milky color and a brown head. They lack legs and often curve slightly in a "C" shape. Their bodies are covered with transverse wrinkles and sparse short hairs. The pupae are milky white to grayish, with a gray-black area near the head and a stretched tube-like structure beneath the abdomen.
In terms of life habits, the chestnut weevil completes one generation per year in southern China and takes two years to complete a cycle in the northern regions of the Yangtze River. Both mature larvae dig into the soil to form chambers for wintering, typically at a depth of around 10 cm. In Jiangsu, the larvae emerge from the soil between June and July, and adults become active from July to August. They feed on nectar first, then move on to chestnuts and *Castanea henryi*. Spawning occurs mainly between late August and the end of September, lasting about 10–15 days. Eggs are mostly laid on the shoulders of the fruit or near the setting areas. Adults are active during the day, feeding on trees and laying eggs at night. They exhibit a pseudo-dead behavior and are not strongly attracted to light.
To manage this pest, several control methods are recommended:
- Improve garden conditions: Clearing weeds and unwanted trees around chestnut fields, and plowing the soil in autumn and winter can help destroy soil chambers and reduce larval populations.
- Select resistant varieties: Planting chestnut varieties that are large, hard-shelled, and thick-skinned can help reduce damage caused by the weevil.
- Timely harvesting: Harvesting chestnuts promptly after ripening and removing all infested fruits helps prevent larvae from entering the soil for wintering.
- Proper drying and storage: Using cement floors or hard ground for threshing, drying, and piling chestnuts can prevent larvae from overwintering in the soil.
- Pesticide application: Spraying 50% phoxim EC diluted 500–600 times at 1–1.5 kg per square meter can effectively kill larvae. If the ground is solid, applying granules or spraying around the soil can also be effective.
- Hot water treatment: Soaking chestnuts in 50–55°C water for 10–15 minutes achieves a 90% insecticidal effect without harming seed germination, provided temperature and time are strictly controlled.
- Fumigation: Using methyl bromide or carbon disulfide in sealed environments can completely eliminate larvae. Dosages vary depending on the fumigant used, and treatment times range from 20 to 48 hours.
- Adult control: Sprinkling 5% phoxim granules or spraying 50% phoxim EC 1000 times before or during the adult emergence period can effectively reduce their numbers. Pesticides like dichlorvos, chlorantrifos, and deltamethrin can also be used to target adults and prevent egg-laying.
By combining these strategies, farmers can significantly reduce the impact of the chestnut weevil and protect their crops from serious economic losses.
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