Preliminary Research Report and Analysis of the Larch Sawfly

1 Distribution and Damage

The larch Itempast sawfly is mainly distributed in Heilongjiang and Jilin provinces. It is a dangerous pest of larch foliage, with a damage period from June to September, mainly attacking 10-30 year old larch. This insect spreads relatively quickly and causes severe damage. In 1985, the affected area in Hailin Forestry Bureau was 300 mu, and by 1986 the third-generation larvae affected an area of 150,000 mu, of which 67,000 mu were severely damaged, with the highest population density reaching 21,000 larvae per tree; the average population density was 6,026 larvae per tree. The peak period of Itempast sawfly larval damage coincides with the peak growing season of larch. In severely damaged stands, from a distance they look like a fire-scorched patch of withered yellow, and the growth and development of larch are severely inhibited.

2 Morphological Characteristics of the Itempast Sawfly

2.1 Adult: Female body length 8-9.5 mm, head width 2-2.5 mm, wingspan 18-18.5 mm; male body length 5.5-7 mm, head width 1.5-2 mm, wingspan 11.5-12 mm. The body is brown and glossy, compound eyes black, antennae filiform and brown, with small black spots on the head; males are darker in color than females. In females, the foreleg femur, midleg femur, hindleg femur and tibia, mesothorax, and lower half of the pleuron are all black; in males, the forelegs, midlegs, and hindlegs are all brownish-yellow, while the pronotum, mesonotum, metanotum, scutellum, and mesopleuron are black.

2.2 Egg: Elongated oval, length 1.3-1.5 mm, width (middle diameter) 0.5-0.55 mm. Newly laid eggs are milky white inside, turning grayish-white and watery transparent after 6-7 days, with a small black dot inside the egg, which is the compound eye of the unhatched larva within the egg.

2.3 Larva: Newly hatched first-instar larva body length 2.5-2.8 mm; head width 0.50-0.55 mm. Mature larva body length 10-18 mm, head width 1.8-2.2 mm. The body is milky white, the head is black, with 2 rows of black spots on the back; the first abdominal segment's dorsal plate at the posterior end is black; there are two rows of small black spots on the outer side of the abdominal proleg bases. It has a hypognathous chewing mouthpart, well-developed thoracic legs with black tarsal claws, and undeveloped abdominal prolegs without white tarsal claws.

2.4 Pupa: Naked pupa body length 6-9 mm, body white, head orange-yellow, compound eyes reddish-brown. The cocoon is elongated oval, dark brown; female cocoon length 8-12 mm, width 4-6 mm; male cocoon length 5-7 mm, width 3-5 mm. The cocoon has two layers: the outer layer is soft, silky, and grass-like, and the inner layer is tough and horny.

3 Biological Characteristics

3.1 Life history: This insect has 2-3 generations per year. The first-generation adult emergence period is mid-June, the second-generation adult emergence period is mid-July to late July, and the third-generation adult emergence period is mid-August to late August. In mid-September, mature larvae begin to descend from the trees and spin cocoons to overwinter. The larvae have 5 instars, with a cycle of about 15 days, an egg stage of about 9 days, and a pupal stage of about 8 days.

3.2 Life habits: This insect overwinters as a prepupa, pupates from late May to early June of the following year, and emerges in mid-June. Males emerge first, followed by females. At emergence, the adult bites a small hole at the top of the cocoon and crawls out. Newly emerged adults are relatively weak; they first crawl back and forth on the litter layer or grass, then make short-distance flights. Males are more active and have strong flight ability, generally flying repeatedly at about 0.5-1 m above the ground, and can fly up to about 2 m at most. After emergence, females crawl to the litter layer or to more conspicuous tips of understory plants, where males swarm around them in flight and make contact for mating. Mating lasts about 2 min. One female can mate multiple times with different males. Afterwards, she flies to the underside of old leaves at the tips of larch branches to lay eggs. When laying eggs, the female crawls on the underside of the leaf, holds the leaf, presses her abdomen against the leaf underside, and lays 1 egg along the leaf vein, then moves forward and lays another egg, or crawls to the underside of another leaf to continue laying. If she cannot finish laying in one day, she continues the next day. After finishing, she guards the spot (if other insects or females of the same species approach, she fights them desperately). A female lays a maximum of 77 eggs, a minimum of 44 eggs, with an average of 59 eggs. Adults have no death-feigning behavior. Adult lifespan is 4-5 days without supplemental nutrition, and up to about 10 days with supplemental nutrition.

The egg hatching rate is about 95%. Before hatching, the egg changes from milky white to grayish-white, transparent, with a glossy and plump surface. At hatching, the larva inside the egg can be seen. During hatching, the egg gradually becomes transparent from white, and the head changes from 1 dot to 2 dots; the larva moves within the egg along with the internal material. At hatching, the larva pushes through the eggshell with its head and crawls out. The newly hatched larva is white and transparent, with black compound eyes. The mouthparts are reddish-brown. After half an hour, the thoracic leg claws, head, and back darken in color. After 2 min, it begins feeding, first eating all the leaf mesophyll where the egg was laid, leaving the veins, then moving downward. They feed in groups; after the 2nd instar they can consume entire leaves, and from the 3rd instar they begin to disperse and cause damage. Larvae tend to feed on older leaves with weaker photosynthesis, and under special circumstances also feed on newly emerged leaves. Each larval instar lasts 2-3 days. When molting, the larva molts from the head suture to the tail. Newly molted larvae are white and transparent; after 1 min, the markings on the body darken in color, while the head and legs remain uniformly white and transparent. After 50 minutes, the head and legs darken in color. When disturbed, the larva raises its tail and swings it front, back, left, and right.