Common Disease Prevention and Control Methods and Technical Key Points for Silkworms

I. Pebrine. Commonly known as flower intestine, it is a devastating infectious disease. 1. Cause: Infection by the spores of Nosema bombycis. Transmission routes: first, transovarial transmission, that is, infected eggs laid by diseased female moths pass the infection to offspring; second, oral infection, that is, silkworms contract the disease after eating spores of Nosema bombycis. 2. Symptoms: Uneven colony development, thin and small silkworm bodies, poor appetite, many become non-molting silkworms, rise late or molt poorly, and sometimes there are dark brown spots of varying sizes like pepper powder on both sides of the ventral surface. Tearing open the outer skin of diseased 5th-instar silkworms reveals that the silk glands show milky white lesions (protrusions). Severely diseased ones mostly do not spin silk and die in the mounting frame. Those infected through transovarial transmission mostly die before the 2nd or 3rd instar, while those infected at the 3rd instar die only at the 5th instar. 3. Prevention and control: ① Seed production farms should strictly carry out microscopic examination of moths to produce disease-free silkworm eggs and eliminate transovarial transmission. ② @#@250@#@ days before rearing, thoroughly disinfect the rearing room and rearing tools. During the rearing period, do a good job of silkworm body disinfection and disease prevention to reduce oral infection. ③ Strengthen feeding management: when collecting newly hatched silkworms, do not collect the earliest and latest hatched ones, implement slightly high-temperature rearing, and eliminate late-molting silkworms at the first and second molts.

II. White mouth disease. This includes midgut-type polyhedrosis and empty-headed soft disease. It can occur in silkworms of all instars, especially in the high-temperature seasons of the 3rd to 6th rearing batches, and more easily causes outbreaks in 5th-instar silkworms. It is the most common and most serious infectious disease. 1. Causes: ① Silkworms eat mulberry leaves carrying the virus. ② It is induced by adverse environmental conditions. For example, high temperature and stuffiness, poor ventilation, old hard and wilted leaf quality, and starvation of silkworms can easily cause disease. Silkworms develop disease 4 to 7 days after eating a disease-causing amount of the virus. After infection, within 48 hours the virus is excreted with silkworm feces and transmitted to healthy silkworms, causing mixed rearing infection in the rearing bed. If a few silkworms are infected during the 2nd to 3rd instars, a large number will develop disease at the 5th instar. If infected during the 4th to 5th instars and exposed to adverse conditions such as high temperature, silkworm disease will break out in the late 5th instar. 2. Symptoms: ① Midgut-type polyhedrosis: Young silkworms eat little and move sluggishly. Large silkworms become white in body color, thin and small, and excrete white soft feces; tearing open the silkworm body reveals that the posterior part of the midgut or the entire midgut is milky white and wrinkled and protruding. ② Empty-headed soft disease: The external symptoms are similar to midgut-type polyhedrosis, but the diseased silkworm's thorax is more swollen and transparent, resembling false maturity, and it excretes loose feces or brownish sewage. The midgut has no milky white wrinkled protrusions, and there is mostly no accumulated mulberry leaf in the intestine, which is yellowish brown and slightly transparent. 3. Prevention and control: Before the silkworms begin heavy feeding, focus on disinfection, disease prevention, and satiation feeding; after the large molt, focus on satiation feeding and preventing heat and stuffiness, and white mouth disease can be controlled. ① Strictly carry out disinfection before rearing, during rearing, and after rearing is completed, and promptly remove diseased, weak, and late silkworms to prevent mixed rearing infection. ② Select mulberry leaves and rear strong young silkworms, letting the silkworms eat their fill and eat well. In dry weather, add water to mulberry leaves before feeding to strengthen the silkworms' constitution. ③ Strive to keep the rearing room cool, ventilated, and airy, and achieve sparse rearing, low rearing, reduced density, and dispersed careful rearing. ④ Feed fully before molting, and promptly provide mulberry leaves to newly molted silkworms.

III. Body cavity-type polyhedrosis. 1. Causes: ① Oral infection. The pathogen is a filtrable virus, which often attaches to the egg surface or mulberry leaves and is eaten by silkworms, causing infection. Young silkworms and newly molted silkworms have weak disease resistance and are prone to infection through eating. ② Wound infection. After the silkworm body skin is wounded, the virus invades the silkworm body through the wound and causes disease. Large silkworms have more opportunities for wounds, so the incidence rate is relatively high. ③ Induction by adverse environmental conditions. After silkworms are infected with the virus, in humid and heavy environments with large temperature changes, or when eating old or tender mulberry leaves, or under conditions such as excessive dense rearing, the silkworm body becomes weak and is more likely to develop disease. 2. Symptoms: Young silkworms have green bodies; silkworms about to molt have tense and shiny skin; newly molted silkworms have loose and shrunken skin and flesh. Large silkworms have milky white body color, are manic and restless, often crawl toward the edges of the rearing bed, have swollen segments and often become high-segment silkworms, and their skin easily breaks and flows out milky white pus before death. This disease is acute, and death occurs 4 to 6 days after infection. 3. Prevention and control: ① Thoroughly disinfect the rearing room and rearing tools. When diseased silkworms are found, promptly pick them out and throw them into a lime basin. ② Before clearing feces at the start of each instar, use fresh lime powder to disinfect the silkworm body and rearing bed; when diseased silkworms are found, scatter it once a day. ③ During the large silkworm stage, appropriately reduce rearing density to reduce mutual scratching among silkworms. When separating rearing beds, be meticulous and gentle to avoid damaging the silkworm body. ④ Avoid feeding tender mulberry leaves and strengthen the silkworms' constitution. Temperature and humidity should be adjusted evenly and must not fluctuate sharply, and good moisture removal work should be carried out.