The author once discovered a large number of dead plants in a young Tea garden, while at the same time, the number of holes in the lower old leaves of surrounding mature tea trees was increasing day by day, which appeared to be caused by insect feeding. However, after searching for several days without finding any trace of the culprit, it was suspected that the pest might be nocturnal. Indeed, a gathering of insects feeding at night was observed. The initial judgment was that these were beetles. Simultaneously, larvae were found in the soil around the roots of the dead plants, which, upon observation, were identified as different from the common copper-green beetle larvae. After rearing and observing them, it was confirmed that the pest affecting the Tea Garden was the larva of the Chinese Adoretus beetle (Adoretus sinicus Burmeister).

Distribution and Damage

The Chinese Adoretus beetle (Adoretus sinicus Burmeister) belongs to the family Scarabaeidae, subfamily Rutelinae, and is distributed in provinces such as Shandong, Zhejiang, Anhui, Jiangsu, Jiangxi, Hunan, Fujian, Guangdong, Guangxi, and Taiwan in China. Both the adult and larval stages of this beetle can damage various crops including tea, mulberry, cherry, elm, walnut, peanut, and many other types of forest trees, fruit trees, and agricultural crops. The host plants of the adult Chinese Adoretus beetle include nearly 60 families and over 500 species. This pest is commonly seen causing damage in tea gardens in Fujian, with some local gardens experiencing severe infestations.

Morphological Characteristics

1. Adult The adult Chinese Adoretus beetle (Fig. 1) is 9.0-11.2 mm in length, with a head width of 4.4-5.0 mm. It has a long elliptical shape that is slightly flattened, chestnut brown or tea brown in color, covered with long needle-like gray scales. Its antennae consist of 10 segments, with the club portion having 3 segments. The pronotum is short and broad, with rounded edges, and the posterior angles are round or obtuse. The scutellum is nearly triangular, and there are four indistinct ridges on the elytra, with faintly visible white hairy spots. The outer edge of the tibia of the hind leg has two tooth-like protrusions. The sides of the abdomen are longitudinally ridged, and the underside is chestnut brown, densely covered with scales. The elytral margin of the female folds sharply inward toward the rear, becoming narrowly pointed. The average body length and width of male adults are smaller than those of females.

Fig. 1: Adult Chinese Adoretus beetle

2. Egg The egg of the Chinese Adoretus beetle is milky white, initially oval-shaped, measuring 1.5-1.8 mm in length and 1.0-1.2 mm in width, gradually expanding into a nearly spherical shape before hatching. Just before hatching, one can see the mandibles of a first-instar larva within the egg.

3. Larva The larva of the Chinese Adoretus beetle (Fig. 2) is grub-like and goes through three instars. It is 15-19 mm in length, plump with many wrinkles, and milky white in color. The head is light brown, slightly narrower than the prothorax, with dark brown mouthparts that curve into a horseshoe shape when touched. There are three pairs of thoracic legs. The abdomen consists of nine segments, with sparse hook-like spines arranged on the ventral plate of the anal segment, with the tips of the spines extending beyond the midpoint of the plate.

Fig. 2: Larva of the Chinese Adoretus beetle

4. Pupa The pupa of the Chinese Adoretus beetle (Fig. 3) is 10-12 mm in length, blunt at the front and pointed at the back. Initially white, it gradually turns pale yellow and then yellow-brown just before eclosion, with minute bristles on its surface.

Fig. 3: Pupa of the Chinese Adoretus beetle

Life Cycle Characteristics

The Chinese Adoretus beetle generally produces two generations per year. The larvae typically overwinter in the soil, with an overwintering period lasting over 200 days. From mid to late October, the larvae overwinter in the topsoil layer at a depth of 5-10 cm, often with up-and-down migration tunnels in the soil. As the ground temperature drops, the overwintering larvae move deeper, reaching a maximum depth of 15 cm. The overwintering larvae usually start to become active in mid-to-late March of the following year, and in late April, they construct a smooth-walled, relatively hard, ellipsoid soil chamber in the topsoil to pupate, with more pupae occurring at a depth of about 6 cm. The pupal periods of both generations are similar, lasting 5-11 days. The overwintering pupae begin to emerge from the soil in early May, with the peak emergence of overwintering adults occurring from late May to mid-June. After eclosion, adults usually remain in the soil for 2-3 days before emerging to feed, with the highest number emerging around 21:00. Adults mate 4-5 days after emerging, with mating most frequent around 19:00. Females start laying eggs 10-15 days after mating, scattering the eggs in the topsoil layer at a depth of 5-10 cm. Eggs are laid in early June, with an egg period of 6-13 days, averaging 9 days. The larval period of the first generation lasts 45-55 days, and the adults of the first generation start to emerge in early August, with the peak emergence occurring in late August. Egg laying begins in early September, with an egg period of 8-22 days, averaging 13 days.

The adults are lively and have strong flying abilities. When startled, they immediately fly away. They exhibit feigning death but do not have phototaxis, and have a habit of being active at night. During the day, they hide under grass and in the topsoil, starting to climb onto the leaves at dusk and feeding on tea leaves, creating irregular holes between the Leaf veins (Fig. 4). The feces drop onto the lower leaves or the ground. Weak young tea plants suffer severe damage, and the damaged leaves fall prematurely. Typically, the peak periods of adult damage occur from mid-May to early June and from mid-to-late August. After hatching, the larvae feed on the fine roots in the topsoil and also on the root neck cortex. When the damage is slight, the injured root neck cortex can heal or produce root nodules (Fig. 5). However, when the damage is severe, it can cause a loss or rotting of the root neck cortex (Fig. 6), leading to dead plants (Fig. 7).

Fig. 4: Damage caused by the adult Chinese Adoretus beetle