The chestnut weevil, scientifically known as *Curculio davidi Fairmaire*, is also referred to as the chestnut elephant, fruit weevil, or chestnut bug. It belongs to the order Coleoptera and is closely related to beetles. This pest is commonly found in chestnut-growing regions across China and primarily targets plants from the genus *Castanea*, including chestnuts, hazelnuts, and even alfalfa. The larvae cause extensive damage by feeding inside the nuts, resulting in a damage rate of over 80% in some areas. As a serious agricultural pest, it significantly impacts the quality, storage, and commercial value of chestnuts.
What are the main threats? The chestnut weevil mainly infests chestnuts and hibiscus, but can also affect other nut-bearing plants. Adult weevils feed on young leaves, shoots, and developing fruits, while the larvae consume the inner cotyledons of the nuts, often filling them with fecal matter. This damage not only reduces yield but also makes the nuts unsuitable for sale or consumption.
What do they look like?
Adults: Female weevils measure between 7–9 mm in length, while males are slightly smaller, ranging from 7–8 mm. Their bodies are dark brown, with distinctive white markings. On the posterior edge of the prothoracic plate, there are two semi-circular white stripes, which are connected to white spots at the base of the elytra. Additionally, there is a single white stripe near the base of the elytra, extending about one-third to two-fifths along the wing. These features are made up of white scales. The wings have 10 vertical grooves, and the body is covered in fine white scales.
Eggs: Oval-shaped, approximately 1.5 mm long, smooth, transparent when laid, and turn milky white after hatching.
Larvae: Mature larvae range from 8.5–12 mm in length, pale yellow to milky white, with a brown head. They lack legs and often appear slightly "C"-shaped. The body has numerous transverse wrinkles and sparse short hairs.
Pupae: Approximately 7.5–11.5 mm long, milky white to grayish, with a dark gray-black area near the head. The head tube extends below the abdomen.
Life Cycle: In southern China, the chestnut weevil completes one generation per year, while in northern regions, it may take two years. Both mature larvae create soil chambers to overwinter, typically burrowing about 10 cm deep. In Jiangsu, the pupal stage begins in June to July, with adults emerging between July and August. They first feed on nectar before moving on to chestnuts and *Castanea henryi* seeds. Adults lay eggs on fruit shoulders and fruit-setting areas from late August to September, with a spawning period lasting 10–15 days. They are active during the day, feeding on chestnut trees, and lay eggs on leaves at night. They exhibit pseudo-dormancy and are not strongly attracted to light.
Control Methods:
1. Improve garden conditions: Remove weeds and unwanted trees around chestnut fields. Plow the garden in autumn and winter to destroy soil chambers and kill larvae, helping to reduce infestations.
2. Select resistant varieties: In heavily infested areas, choose chestnut varieties that are large, hard-shelled, and thick-walled to minimize damage.
3. Harvest promptly: Pick chestnuts as soon as they ripen and remove all fallen nuts to prevent larvae from entering the soil for winter survival. This is one of the most effective ways to reduce future infestations.
4. Use proper drying and storage areas: Dry and store chestnuts on concrete or hard surfaces to prevent larvae from overwintering in soft ground.
5. Apply insecticides: Before threshing and drying, spray 50% phoxim EC diluted 500–600 times, applying 1–1.5 kg/m². For solid or cemented ground, apply 5% phoxim granules around the area to kill larvae.
6. Hot water treatment: Soak chestnuts in 50–55°C water for 10–15 minutes, achieving up to 90% effectiveness in killing larvae. This method does not harm germination if done correctly.
7. Fumigation: In controlled environments, use methyl bromide or carbon disulfide to fumigate chestnuts. Methyl bromide at 2.5–3.5 g/m³ for 24–48 hours, or carbon disulfide at 30 ml/m³ for 20 hours, can eliminate all larvae without affecting seed viability.
8. Spray adult weevils: When adult populations are high, apply 5% phoxim granules (10 kg/acre) or spray 50% phoxim EC diluted 1000 times. During the adult stage, applications of 80% dichlorvos, 50% chlorantrifos, or 2.5% deltamethrin can be used every 10 days for 2–3 sprays to control adult numbers and prevent egg-laying.
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