Chestnut (Lachnus tropicalis Van der Goot) is also known as Chestnut Scale or Black Chestnut Scale. This pest is widely distributed and not only affects chestnut trees but also damages other plants such as white lice and causes paralysis in some cases.
The harmful features of this pest include adults and nymphs clustering on shoots, twigs, and leaves to feed on sap. This feeding activity interferes with shoot growth and fruit development, often leading to weakened trees and making it one of the most significant pests in chestnut cultivation.
Adults are wingless females, measuring 3 to 5 mm in length, black in color, with dense hair on their backs. Their abdomens are swollen and spherical. Winged forms and parthenogenetic females are slightly smaller, also black but with a paler belly. The wings are narrow and elongated, with two types: one with transparent wings and black veins, and the other with dark wings, black veins, oblique middle wings on the forewings, two posterior horns, and a transparent spot near the vertex of the leading edge.
Eggs are long and elliptical, approximately 1.5 mm in length. Initially dark brown, they turn black and shiny over time. These eggs are usually found in clusters on the shaded base of branches and at the base of thick branches.
Nymphs resemble the wingless female but are smaller and lighter in color, typically yellow-brown. As they develop, they become darker and more oval in shape. They have more developed beaks on their chests and show signs of wing buds.
The life cycle of this pest can produce over ten generations per year. Overwintering eggs survive in the buds and cracks of chestnut branches. In late March to early April, these eggs hatch into dry mothers, which feed on the original branches. As they mature, they give rise to parthenogenetic females and offspring. From late April to mid-May, reproduction peaks, marking the most severe period of infestation. During this time, they secrete honeydew that pollutes the leaves. By mid to late May, winged forms begin to develop, and some migrate to summer hosts for further breeding. Throughout the year, from January to October, chestnut trees host the reproduction of these parthenogenetic females, often clustering on the stems of chestnut seedlings. In November, sex mothers emerge, and after mating, they lay eggs for the next winter. The ideal conditions for Li Dayi reproduction are an average temperature of around 23°C over 10 days and a relative humidity of about 70%. One generation typically completes in 7 to 9 days. However, temperatures above 25°C and humidity over 80% may reduce population numbers. Severe storms can cause significant mortality.
Control methods include eliminating overwintering eggs by spraying a 50-60 times diluted oil emulsion before bud break in winter or early spring, or manually brushing off the eggs. For chemical control, it's recommended to apply 10% imidacloprid at 2000 times dilution, 80% dichlorvos EC at 1000 to 1500 times, or 2.5% deltamethrin EC or 20% cypermethrin EC at 4000 to 5000 times dilution. Young trees can be treated with 5 times dimethoate solution and then wrapped in plastic film, which is effective without harming natural enemies.
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