Toxicology of Silkworm

Insect toxicology is a cross-discipline formed in recent decades, intertwined with entomology, biochemistry, environmental chemistry, pesticide science, molecular biology, genetics and other disciplines. It is the theoretical basis for researching the creation of new pesticides and the application of pest control. . However, due to the widespread application of pesticides in agriculture and industrial environmental pollution, economic resource insects such as silkworms and bees, as well as natural enemy insects, are also threatened by poisons. It is very sensitive to pesticides and environmental chemicals. According to Japanese reports, ant silkworms can sense very low concentrations of poisons and can be used as biological test materials to monitor the presence of poisons in the environment or food. The production of sericulture is mostly intercropping between agriculture and mulberry. In agriculture, pesticide poisoning of the neighboring mulberry trees by spraying pesticides to prevent and control pests is more common. In addition, due to industrial pollution, environmental chemicals have also increased the poisoning of silkworms. In order to prevent the poisoning of silkworms, silkworm toxicology draws on the research results and development trends of insect toxicology to understand the poisoning nature, poisoning process and poisoning mechanism of silkworms; Utilization is also an issue that should be studied in the toxicology of silkworms. In the future, insect toxicology will also make further research progress in detoxification and detoxification mechanisms based on further studies on poisoning mechanisms, pest resistance and management.

1. Pesticide poisoning and poisoning mechanism of silkworm

There are many types of pesticides, and new products are constantly coming out. Most of the poisoning caused by silkworms are insecticide pesticides, most of which are neurotoxic agents, mainly organophosphorus insecticides, carbamate insecticides, organic nitrogen insecticides, and pyrethroid insecticides And plant pesticides. According to the mode of action and the way into the silkworm body, it can be divided into 5 categories: stomach poison, contact killer, fumigant, systemic and antifeedant. Many pesticides often have two or three effects on silkworms. Pesticides can cause poisoning by polluting mulberry leaves, or by contacting silkworm bodies or entering the silkworm bodies through the air valve to cause poisoning. Depending on the type, dosage and time of the pesticide, it manifests as acute and chronic symptoms. Acute poisoning silkworms show sudden poisoning and death. Chronic poisoning does not show symptoms of poisoning for a while, but because the poison accumulates in the silkworms, it often occurs without cocooning in the later stage. Silkworms, deformed cocoons may induce viral diseases. In general, the symptoms of silkworm poisoning go through the incubation period, excitement period, convulsive period, paralysis period and death. Due to the different types and concentrations of pesticides, the symptoms of silkworm poisoning are also different. Most pesticide poisoned silkworms exhibit excitatory characteristics, but there are also Exceptions do not exhibit excitatory characteristics, such as bisultap or rotenone. The difference in symptoms of silkworm poisoning is related to the poisoning mechanism of different insecticides.

Poisoning and mechanism of organophosphorus and carbamate pesticides

Organophosphorus insecticides are currently widely used and have a large variety of insecticides, such as trichlorfon, dichlorvos, pyridoxine, phoxim, methamidophos, etc. This type of pesticide is more effective in acidic solutions. Stable, but decomposes rapidly when encountering strong alkaline substances. Carbamate pesticides are new types of pesticides developed after organophosphorus pesticides, such as carbaryl. The poisoning symptoms of the two types of agents to silkworms are roughly similar. Taking trichlorfon as an example, silkworms are acutely poisoned with a short incubation period. They quickly stop eating mulberry and crawl around, then breast enlargement, cramping, vomiting, and sometimes polluting the whole body, discharging irregular feces or reddish liquid. Finally, paralyzed lying down on the silkworm seat. When dying, the stomach and feet twitch, the front half of the body is swollen and swollen, and the back half of the body shrinks, especially in the rear part, and there is prolapse of the anus. Dichlorvos has a knockdown effect. The silkworm can be poisoned to death within minutes. However, carbamate insecticides are less toxic to silkworms, and the poisoning symptoms can disappear once the medicine is stopped.

Organophosphorus and carbamate insecticides are a kind of neurotoxic agent, their action site is the nerve synapse, the toxicological mechanism is mainly to inhibit the activity of acetylcholinesterase (AchE), so that the neurotransmitter acetylcholine (Ach) cannot Caused by decomposition. Acetylcholinesterase hydrolyzes the active site of acetylcholine by two parts, namely the lipomobility site and the anion site. Organophosphorus and carbamate are inhibitors of the lipomobility site and can form phosphorylase derivatives or carbamylation with AchE. Enzyme derivative, after AchE is inhibited and inactivated, it cannot decompose the neurotransmitter acetylcholine (Ach) at the nerve synapse. Ach accumulates rapidly in the synapse, and the nerve is in a state of excessive excitement. Organophosphate insecticide poisoning often nerves The conduction is finally interrupted and the body dies, but carbamate insecticides are a kind of reversible inhibitors, and carbamylation enzymes can be restored. According to experiments, acetylcholinesterase (AchE) exists not only in the central nervous system of insects, but also in other tissues and organs, and has nothing to do with the distribution of nerves. Therefore, it is speculated that organophosphorus and carbamates kill from a toxicological point of view. In addition to the recognized AchE that inhibits cholinergic synapses, insecticides can also inhibit AchE in other parts of the body. However, the extent of the inhibition of AchE in other parts on the death of insects has not been reported yet.

Poisoning and mechanism of organic nitrogen pesticides

Organic nitrogen pesticides are also nerve agents, but bisultap and chlordimeform have completely different symptoms of poisoning to silkworms. The bisultap poisoned silkworm showed symptoms of paralysis and paralysis. Chlordimeform showed symptoms of excitement and vomited messy silk.

Bisultap is an analogue of Bataan, which can be converted into Negrotoxin in the living body. Its action site is also at the nerve synapse, but the poisoning mechanism is that the Negrotoxin occupies the acetylcholine receptor on the post-synaptic membrane of the nerve and causes the anterior membrane to secrete. The acetylcholine cannot bind to the receptor, thereby blocking nerve conduction, so the body is paralyzed, paralyzed and died.

Chlordimeform can definitely act on the nervous system. According to mammalian experiments, it is an inhibitor of monoamine oxidase (MAO). Monoamine oxidase has a function similar to acetylcholinesterase. It can inactivate the amine neurotransmitter dopamine or norepinephrine. However, due to the limited knowledge about the presence or absence of monoamines in insects, as well as the location, metabolism and function of the insects, the toxicity mechanism is still unclear.

Poisoning and mechanism of pyrethroid insecticides

Pyrethroid insecticides were first used for household sanitation and insecticides. After the 1970s, they were improved to become light-stable compounds and were widely used in agricultural insecticides. This type of pesticide has a strong contact effect on silkworms, and also has a gastric toxic effect. After being poisoned, silkworms are usually very excited and have dyskinesias. The head, chest and tail are bent to the back. The injured body of large silkworms sometimes bends like a spiral, and finally curls to death.

Pyrethroid insecticides are another type of nerve toxins that act on nerve axons. Electrophysiologists have used experiments to prove that the main target site of pyrethroid in insects is the sodium ion channel on the nerve membrane. In the process of normal nerve conduction, the ability of nerve cells to transmit excitement or impulse is mainly due to the uneven distribution of Na ⁺ and K ⁺ inside and outside the cell membrane , and the result of selective changes in the permeability of the nerve cell membrane to these ions. Esters can bind in the lipoprotein bilayer part of the sodium ion channel and disturb the normal opening and closing of the channel, resulting in changes in the permeability of the membrane, interfering with the normal ion current movement, and causing abnormal depolarization. Repeated response to depolarization stimuli strengthens the activity of nerves and muscles and is in a state of abnormal excitement. In the end, the insects are poisoned to death.

Poisoning and mechanism of plant insecticides

Approximately 2,400 species of plants have been found to have the biological activity of controlling pests, showing toxic, repellent, antifeedant, anti-oviposition and insect growth regulation effects on insects. In recent years, this field is actively promoting biological activity, molecular structure, Research on the mechanism of action, and directly or indirectly develop and utilize them. Examples of direct development and utilization include nicotine preparations, rotenone preparations, and stilbene preparations, but the commercial production scale is relatively small.

Nicotine is present in tobacco plants, and will evaporate in large quantities during the flowering period. It is inhaled or adhered to nicotine near mulberry leaves, which can cause silkworm poisoning when it is touched or eaten by silkworms. Nicotine is a neurotoxic agent with the same target as bisultap, and it also acts on acetylcholine receptors. In vertebrates, small doses of nicotine excite the receptors, but large doses inhibit the receptors. The nicotine poisoning of silkworm has a short excitatory period and a long paralysis period.

Another type of poisoning mechanism for silkworms is the preparation of rotenone, which is not a neurotoxic agent, but a metabolic agent. It mainly acts on the respiratory enzyme system and inhibits the activity of glutamate dehydrogenase. After the silkworm is poisoned, the breathing is hindered, the back tube beats weakly, the whole body is weak and weak, and it dies slowly.

Detoxification and resistance mechanisms of insects

The fact that insects in nature still survived after eating natural poisons, and the different responses of different types of insects to a certain toxic compound show that there is indeed a mechanism for removing or resisting toxic compounds in insects. According to current research, the detoxification process of removing foreign compounds is generally divided into two categories.

1. Degradation through oxidation, reduction and hydrolysis reactions, of which oxidation is the most obvious, and insect microsomal multifunctional oxidase system plays an important role;
2. Hydrophilic degradation products and highly water-soluble endogenous metabolites undergo various binding reactions under the action of enzymes, and their products are more easily eliminated by the body’s excretion mechanism. The enzymes involved in the reaction in the above process, such as multifunctional oxidase and glutathione transferase that act as binding enzymes, are called detoxification enzymes. The detoxification reaction ability of insects depends on the activity level of the detoxification enzyme.

There are many studies on insect detoxification mechanisms in the insect-resistant houseflies. There are also some studies on the detoxification mechanism of silkworms, such as the successful development and application of drugs that selectively kill flies in silkworms. However, how insects can achieve detoxification levels in the face of various deadly pesticides is a big problem to be explored. After sericulture production is poisoned, silkworm farmers have used tea, rice water and some drugs to rescue the silkworms, which fully reflects people’s hopes. Desire for detoxification.

In recent decades, many species of insects have acquired resistance, and their resistance has been inherited. The resistance mechanism has been shown to involve the increase of the activity of several enzymes, including the target enzymes acetylcholinesterase and detoxification enzymes. The main mechanism of resistance to pyrethroid insecticides is the variation in the structure of neural membrane Na ⁺ channels. In addition, in addition to biochemical mechanisms, penetration mechanisms and behavioral avoidance types can also cause resistance. Can silkworms prevent insecticide poisoning by trying resistance genetics?

2. Silkworm poisoning by environmental chemicals

Environmental chemical poisoning of silkworms mainly refers to poisoning caused by industrial pollution, including factory waste gas poisoning and heavy metal element poisoning. Most of the harms are caused by eating mulberry leaves to cause silkworm poisoning, and some directly cause silkworm poisoning.

Fluoride poisoning and its mechanism

Fluoride mainly comes from waste gas discharged from factories producing bricks and tiles, phosphate fertilizer, glass, metallurgy and petrochemical industries. Generally, when mulberry trees are exposed to hydrogen fluoride (HF) gas above 30 µg/kg, mulberry leaves will be harmed. HF penetrates into mulberry leaves through stomata, and accumulates in leaf tips and leaf edges, showing light brown to reddish brown burnt spots and death. When it spreads to the leaf surface, chlorosis and yellowing occurs, and fluoride can accumulate in mulberry leaves. Generally, when the fluorine content (dry content) of mulberry leaves reaches 35-50 mg/kg, it is harmful to silkworms, small silkworms are poisoned, and appetite decreases. It is rusty color, poor population development, large silkworms and sleeping silkworms are poisoned, internode membranes are raised like bamboo joints or black ring spots appear, the lesions are easy to break and flow out light yellow blood, and the corpses are mostly dark brown and not easy to rot.

Fluoride is a kind of tissue protoplasm toxicant. It first acts on the digestive system and passes through the intestinal wall with water-soluble fluoride. It accumulates in the respiratory, blood circulation, body wall, nerves and other tissue systems. It inhibits the activity of many enzymes and hinders tissues. Respiration and energy metabolism lead to obstacles to the physiological functions of various tissues. Regarding the formation mechanism of skin dark spots, it is believed that blood cells and dermal cells are deposited on the epidermis after necrosis, combined with the accumulation of dark brown granular materials to form dark spots.

Different silkworm species have different resistance to fluoride. Although the resistance mechanism is not yet clear, experiments have shown that fluoride resistance can be selected and inherited. According to recent data, my country has selected quite fluoride-tolerant silkworm species, which can tolerate 60%. -100 mg/kg of fluoride-containing mulberry leaves were raised without being poisoned. It is said that genetic material with a major gene for fluoride resistance has been found. In addition, there are also reports of defluoridation tests on silkworm fluoride poisoning by adding medicaments.

Sulfide poisoning

The sulfides that cause silkworm poisoning are mainly SO₂ and H₂ S, which mainly come from factory waste gas and coal burning waste gas. The direct harm of the two poisonous gases to silkworms is more serious than the poisoning caused by polluting mulberry leaves and nibbling them.

SO₂ is widely distributed in the atmosphere, which is also the cause of acid rain. Under normal circumstances, it will not cause harm to silkworms and mulberry trees. However, if the pollution source discharge exceeds the standard, nearby mulberry trees will be harmed. Although mulberry leaf tissue can absorb low concentrations of SO₂ It is converted into sulfuric acid, but when high concentration of SO₂ invades, it will show obvious symptoms of damage. The chloroplasts are demagnetized, the leaves appear degreening and turn yellow or white, there are oily brown spots between the veins, and the sulfur content of mulberry leaves is greater than 0.63. % Is harmful to silkworms. Silkworms suffer from developmental disorders and eventually die with softening symptoms. If the silkworm house is heated by coal and the air flow is not smooth, gas poisoning will occur, and the SO₂ produced directly affects and destroys the respiratory enzyme system of silkworms.

Hydrogen sulfide has the smell of preserved eggs. In the environment of sericulture, when the human sense of smell clearly perceives this odor, the silkworm will be poisoned, the silkworm will be sluggish, not eating mulberry or loss of appetite, the body will be softened, and continuous contact for a long time Hydrogen sulfide, the silkworm poisoning symptoms cannot be recovered, and they die with softening symptoms. The corpse becomes black and rotting. If the silkworm is injured before and after the tuft, the silkworm body is weak and unable to form a cocoon. The main cause of toxicity is that hydrogen sulfide affects and destroys the respiratory system of silkworms. Hydrogen sulfide is similar to SO₂ damage to mulberry , but to a lesser degree.

Poisoning of other compounds

Chloride (CL₂ and HCL smoke) and iodide (containing I₂  24-292 mg/kg) polluted mulberry leaves can cause poisoning if eaten, and nitrogen oxides (NO₂ etc.) generally do not cause silkworm poisoning.

Poisoning of heavy metals

The pollution of heavy metal elements mostly comes from metal smelters and chemical plants, and the discharged waste gas containing zinc (Zn), cadmium (Cd), mercury (Hg), copper (Cu), lead (Pb), etc. pollutes the atmosphere and water bodies. And soil, mulberry trees can cause poisoning of silkworms after being eaten up by excessive absorption.

The maximum allowable amount of heavy metal elements in the feed alone: ​​arsenic (As) within 2.5 mg/kg, cadmium (Cd) at 5 mg/kg, zinc (Zn) at about 10 mg/kg, copper (Cu), lead ( The safe concentration of Pb) is within 100 mg/kg. In the case of the coexistence of various heavy metal elements, the safe concentration of each element in the feed: As = 1.25 mg/kg, Cd=2.5 mg/kg, Zn = 100 mg/kg, Cu = 50 mg/kg, Pb = 50 mg/kg. Above the safe concentration, the silkworms show obvious symptoms of chronic poisoning, the silkworms are slow to develop, vomiting fluid, diarrhea, small body, and die slowly. It is generally believed that heavy metal elements are a kind of tissue protoplasm toxicant.