Aromatic wood moth

Cossus cossus L., commonly known as the poplar cocoon moth or red-harex insect, is found in northeastern, northern, and northwestern China. This pest primarily targets a variety of trees and plants, including walnuts, apples, pears, peaches, apricots, as well as poplars, willows, sorghum, birch, coriander, and hazel.

The damage caused by Cossus cossus is distinctive. The larvae bore into the xylem, creating shallow trenches or trough-like pits on the surface. At this stage, it's common to find more than 10 or even dozens of larvae feeding together. As they grow older, the larvae tend to disperse within the same section of the trunk, feeding on the inner pith and gradually forming thick, irregular tunnels. A characteristic feature of this species is the secretion of a fragrant liquid, which gives the insect its name.

Adults of Cossus cossus are dark brown with a slightly darker underbelly. They measure about 30 mm in length, with a wingspan ranging from 56 to 80 mm. Females are generally larger than males. Their antennae are serrated, and their compound eyes are dark brown. The forewings are grayish white with a grayish-brown leading edge and dense brown wavy patterns. A prominent, thick ripple runs from the trailing edge to the leading edge of the wing.

Eggs are initially pale white, turning dark brown just before hatching. They are nearly oval in shape, measuring approximately 1.5 mm in length and 1.0 mm in width. The egg surface has longitudinal ridges, with transverse lines between them.

Larvae start off flat and cylindrical, measuring around 3–4 mm long when first hatched, and growing up to 56–80 mm in length by the final instar. Their dorsal side is reddish or purple, shiny, while the ventral side is yellowish or light red. The head is dark purple with irregular fine lines, and there is a pair of large, purple-brown stripes on the pronotum.

The pupal cases (cocoons) are long and cylindrical, slightly curved, measuring 50–70 mm in length and 17–20 mm in width. These are made from soil particles collected by the larvae before they burrow into the ground. There’s also a secondary type of pupal case, which is more rounded and flattened, about 40 mm long, 30 mm wide, and 15 mm thick. It is formed by the last-instar larvae after they have exited the tunnel and secreted a sticky substance to seal the entrance. The texture of this structure is loose and porous.

The life cycle of Cossus cossus varies by region. In areas like Henan, Shaanxi, Shanxi, and Beijing, it completes one generation every two years, while in Xining, Qinghai, it may take three years. The larvae overwinter in the soil near the base of infested trees. In April to May, mature larvae begin to pupate, and adults emerge in June to July. Adults are nocturnal and are attracted to light. Eggs are laid at a depth of about 1.5 meters below the tree trunk, often in cracks or at the junction of roots and stems. Each female lays an average of 245 eggs in clusters, with each cluster containing 50–60 eggs. Some clusters may have fewer, while others can contain over 100 eggs.

After hatching, the larvae drill into the bark through wounds, cracks, or old entry holes, pushing out finely divided brown wood shavings. Initially, they feed beneath the bark, separating the xylem from the outer layer, causing visible trough-like damage. As the larvae grow, they move deeper into the trunk, feeding on the pith and creating irregular tunnels. By October, they retreat into these tunnels for winter. In the following year, they continue feeding and cause further damage. From late September to early October, the larvae become fully mature, exit the tunnels, and construct cocoons in the soil about 10 cm deep near the roots or a few meters away from the trunk. When disturbed, mature larvae can secrete a fragrant liquid, reinforcing the reason behind their common name.

To manage Cossus cossus infestations, several methods are recommended. First, during the adult mating season, applying clay-based coatings on the trunk can help prevent females from laying eggs. Second, during the 5–10 day larval feeding period, injecting 40% Dimethoate emulsion diluted 25–50 times into the infested holes can kill the larvae. After injection, sealing the hole with mud helps ensure effectiveness. Third, if larvae are found beneath the bark near the base of the tree, manually removing the bark to expose and kill the larvae is another effective approach.

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