Chestnut (Lachnus tropicalis Van der Goot) is also known as Chestnut Scale, Black Chestnut Scale, and sometimes referred to as the chestnut mealybug. This pest is widely distributed and not only attacks chestnut trees but also affects other plants such as white lice and causes various types of damage.
The harmful features of this insect include adults and nymphs that congregate on shoots, twigs, and leaves to feed on sap. This feeding activity can hinder shoot growth and fruit development, leading to weakened trees and making it one of the most significant pests in chestnut cultivation.
Adults are wingless females with a body length of 3 to 5 mm, dark black in color, and covered with dense hair on their backs. Their abdomens are swollen and spherical. Winged and parthenogenetic forms are slightly smaller, also black, but with a paler belly. The wings are narrow and elongated, with two distinct types: one has transparent wings with black veins, while the other has dark wings with black veins, an oblique middle section on the forewings, two posterior horns, and a small transparent spot near the leading edge of the wing.
Eggs are long-elliptical, about 1.5 mm in length, initially dark brown and later turning black and shiny. They are laid in dense clusters on the shaded base of branches and at the base of thick branches.
Nymphs resemble the wingless female but are smaller, lighter in color, typically yellow-brown, and become darker after molting. Their bodies are more elongated and nearly oval. They have well-developed beaks and wing buds on their thorax.
The life cycle of Lachnus tropicalis is quite rapid, with over ten generations per year. Overwintering eggs survive in chestnut buds and branch cracks. In late March to early April, these eggs hatch into "dry mothers" that feed intensively on the original branches, producing mature parthenogenetic females and offspring. By the end of April to mid-May, reproduction peaks, marking the most severe period of infestation. During this time, they secrete honeydew, which pollutes the leaves. By mid to late May, winged forms begin to develop, some of which migrate to summer hosts for further breeding. From January to October, the chestnut trees remain the primary host, where parthenogenetic females continue to reproduce. These insects often cluster on the stems of chestnut seedlings. In November, sex mothers emerge, giving rise to both females and males. After mating, they lay eggs for the next winter. The ideal conditions for reproduction are an average temperature of around 23°C and relative humidity of approximately 70%. Under these conditions, one generation takes about 7 to 9 days. However, when temperatures exceed 25°C and humidity goes above 80%, population numbers begin to decline. Severe storms can cause significant mortality.
Control methods include eliminating overwintering eggs by spraying 50-60 times oil emulsion before bud break in winter or early spring. Alternatively, manually scraping off the eggs can be effective. For chemical control, apply 10% imidacloprid diluted 2000 times, 80% dichlorvos EC at 1000–1500 times, or 2.5% deltamethrin EC or 20% cypermethrin EC at 4000–5000 times after the overwintering eggs have hatched. Young trees can be treated with dimethoate diluted 5 times and then wrapped in plastic film, which is effective but does not harm natural predators.
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