How to Control Walnut Slug Moth Pests and Diseases

  1. Distribution and damage symptoms. They are a group of omnivorous pests, commonly including the yellow slug moth, the brown slug moth, the green slug moth, and the flat slug moth, all distributed throughout the country. The larvae feed on leaves, affecting tree vigor and yield, and they are important pests of walnut leaves. Young larvae feed on the lower epidermis and mesophyll of the leaf surface, leaving only the epidermis as net-like transparent patches, so the leaves appear mesh-like. After the larvae grow larger, they disperse to cause damage, chewing the leaves into notches and leaving only the main veins and petioles, or even eating them entirely.

  2. Morphological characteristics and living habits

  (1) Yellow slug moth, also called yanglazi, stinging caterpillar, or mao bajiao. The adult is 13-17 mm long with a wingspan of 29-36 mm, yellow, with filiform brown antennae. The mature larva is yellow-green, 18-25 mm long and about 8 mm wide, with two dumbbell-shaped purple-brown large markings on the back and branched spines on the body, with venomous hairs on the spines. The egg is flat and round, pale yellow, 1.4 mm long. The pupa is oval, about 12 mm long, hard in texture, gray-white, with dark brown longitudinal stripes.

  In the Northeast and North China, it mostly has 1 generation per year, while in Henan, Shaanxi, and Sichuan it can have 2 generations per year. It overwinters as mature larvae in hard cocoons on branch forks and branches. Adults are phototactic, hiding by day and emerging at night. After eclosion they mate immediately and soon lay eggs, mostly on the underside of leaves near the tip.

  (2) Brown slug moth. The adult is about 18 mm long, gray-brown. The forewings are brown, with two dark brown curved lines, and the area between the two lines is lighter in color. Between the outer transverse line and the anal angle there is a copper-purple triangular spot. The egg is flat, oval, and yellow. The larva is 35 mm long and green. The cocoon is broadly oval and gray-brown. This insect has 1-2 generations per year and overwinters as mature larvae in cocoons in the soil.

  (3) Green slug moth. The adult is 16 mm long with a wingspan of about 36 mm, mostly green, with dark brown compound eyes. The female antennae are filiform, and the male antennae are short feathery. There is a light brown longitudinal stripe in the center of the back. The egg is flat-oval, about 2 mm long, milky white when first laid, gradually becoming yellow-green, mostly in clusters of 10 arranged like fish scales. The mature larva is 24-27 mm long, slightly cylindrical. The pupa is about 13 mm long, oval, and yellow-brown.

  In the Northeast and North China it has 1 generation per year, while south of the Yangtze River it can have 2-3 generations per year. It overwinters as old larvae in cocoons in the soil at the root collar of the trunk. After the larvae mature, they spin cocoons on the trunk or in shallow loose soil and grass under the tree to overwinter. Adults are phototactic, hiding by day and emerging at night. Eggs are mostly laid on both sides of the main vein on the underside of leaves. Newly hatched larvae first eat the egg shell and then feed on the mesophyll.

  (4) Flat slug moth, also known as the black-dotted slug moth. The adult is 16 mm long with a wingspan of 28-39 mm. The female antennae are filiform, and the body is gray-brown. The egg is flat-oval, about 1.5 mm long, and yellow-white. The mature larva is 25-28 mm long, long-oval, yellow-green, with a red spot on each side of the fourth segment, bearing stinging hairs. The pupa is flat-oval, about 13 mm long, and yellow-brown.

  In the Northeast it has 1 generation per year, in the middle and lower reaches of the Yangtze River it has 2 generations, and in the south it can have 3 generations. All overwinter as old larvae in cocoons under the tree. After adult eclosion, eggs are laid on the leaf surface. After hatching, the larvae feed after one molt, first eating the egg shell and then gnawing the mesophyll. The larvae feed day and night, with a total of 8 instars, and after maturing they enter the soil to make cocoons and pupate.

  3. Control methods

  ① Eliminate overwintering cocoons. Overwintering cocoons can be eliminated in combination with management practices such as digging the tree for fertilization in autumn and winter pruning.

  ② Black light trapping and killing. During the adult occurrence period of the overwintering generation from mid-June to mid-July, set up black lights in the field to trap and kill adults.

  ③ Manual control. In early July, when young larvae gather on the underside of leaves, the leaves can be cut off promptly and the young larvae destroyed together; in mid-to-late August, when mature larvae are looking for suitable places on the branches and trunk bark to spin cocoons, concentrate manpower to capture and kill the mature larvae.

  ④ Biological control. Shanghai green wasp is the dominant natural enemy population of the yellow slug moth. It can be protected and used to eliminate the mature larvae of the yellow slug moth inside overwintering cocoons.

  ⑤ Chemical control. When larval damage is severe, spray 25% phosmet emulsifiable concentrate at 600 times dilution or 2.5% deltamethrin emulsion at 5000 times dilution during the larval occurrence period.