Introduction
Lambda-cyhalothrin is a pyrethroid insecticide which is a synthetic chemical analogues of pyrethrins and produced in the flowers of chrysanthemums (Chrysanthemum cinerariaefolium) (1). Pyrethroids may be classified into two large groups. Allethrin and permethrin are examples of type I pyrethroids that do not contain a cyano moiety. Type II pyrethroids, such as deltamethrin, fenvalerate, and cyhalothrin, have a cyano group in the α-position. The T-syndrome, which is brought on by type I pyrethroids, and the CS syndrome, which is brought on by type II pyrethroids, have both been identified as different toxic syndromes in mammals. Hyperexcitation, ataxia, convulsion, paralysis, and recurrent nerve firing are side effects of type I pyrethroids. Type II pyrethroid poisoning lacks repetitive nerve firing in sensory nerves and is characterized by hypersensitivity, excessive salivation, choreoathetosis, tremor, and paralysis. Additionally, Type II pyrethroids decrease resting chloride conductance, magnifying the effects of calcium or sodium.(2)
Lambda-cyhalothrin appears to be a type II pyrethroid with high activity against a variety of Lepidoptera, Hemiptera, Diptera, and Coleoptera species. Its chemical name is [-cyano-3-phenoxybenzyl 3-(2-chloro-3,3,3-trifluoroprop-1-enyl)-2,2-dimethylcyclopropanecarboxylate] and several products with brand name: Warrior, Scimitar, Karate, Demand, Icon, and Matador are available. Lambda-cyhalothrin is widely used in applications for human and animal health where it is effective in controlling a variety of insects and ectoparasites, such as cockroaches, flies, lice, mosquitoes, and ticks.(3)
Physiochemical Properties
The molecular formula of lambda-cyhalothrin is C23H19CIF3NO3, molecular weight is 449.8 and relative density is 1.3. It has a low vapour pressure (3.35e-09 mmHg) and Henry’s law constant has a low vapour pressure (3.35e-09 mmHg) and Henry’s law constant, which suggests that it is not easily volatilized into the atmosphere. This insecticide also has a high octanol-water partition coefficient (KoW), so it tends to partition into lipids.( Pubchem & 4)
Figure: 2D Structure of Lambda-cyhalothrin (Pubchem)
Figure: 3D Structure of Lambda-cyhalothrin (Pubchem)
Mechanism of Action
Pyrethroids are axonic toxins that disrupt nerve fibers by attaching to a protein that controls the voltage-gated sodium channel. This gate typically opens to stimulate the neuron and closes to block the nerve signal. Ions are allowed to enter the channels and produce excitation in the axon. When the channels are left open, nerve cells discharge repeatedly, eventually paralyzing the victim. Lambda-cyhalothrin can disrupt chloride and calcium channels, which are crucial for proper nerve function, in addition to interfering with sodium channel activity in the central nervous system. Pyrethroids are rapidly absorbed by biological membranes and tissues because of their lipophilic nature. Lambda-cyhalothrin specifically penetrates the insect cuticle and disrupts nerve conduction within minutes, causing the insect to stop eating, lose control of its muscles, become paralyzed, and eventually die. (4)
Instability of lamda-cyhalothrin
While recently developed synthetic pyrethroids have enhanced photostability, naturally occurring pyrethrins remain unstable in light. The degradation of lambda-cyhalothrin after 20 minutes of exposure to UV radiation (18 W, 254 nm) was nearly complete, with losses of more than 95% of the initial levels. The photoproduct de-carboxy cyhalothrin (P4) of lambda-cyhalothrin is produced through the decarboxylation pathway. When the ester link in lambda-cyhalothrin breaks, 3-(2-chloro-3,3,3-trifluoroprop-1-en-1-yl)-2,2-dimethyl cyclopropane carboxylic acid (P1) and (3-phenoxy phenyl) acetonitrile are produced (P2).(5)
When the pH is below 8, lambda-cyhalothrin is stable, but in alkaline conditions, it hydrolyzes due to the hydroxyl ion's nucleophilic attack. A cyanohydrin derivative is created, and when it breaks down, it releases HCN and the related aldehyde.(5)
Ecotoxicity of Lambda-Cyhalothrin
On Fish and Shellfish
To prevent insects, rice fields frequently receive applications of lambda-cyhalothrin. Various fishes and shellfishes are extremely harmful to it. The reported LC50 (96 hr) for bluegill sunfish, rainbow trout, Daphnia magna, and mysid shrimp is 210 ng/L, 240 ng/L, 360 ng/L, and 4.9 ng/L, respectively. Shrimp and zebrafish demonstrated considerable toxicity to lambda-cyhalothrin. The 96-hour LC50 for shrimp was 20–70 ng/L and for zebrafish, it was 0.98–7.55 g/L. Gamma-cyhalothrin and lambda-cyhalothrin both had identical 96-hour LC50 values for zebrafish (1.93 g/L for gamma and 1.94 g/L for lambda)(6)
The genotoxic effects on C. i. interruptus erythrocytes were demonstrated by the results of in vivo fish exposure to lambda-cyhalothrin. After 24 hours of exposure, the maximum response was produced. Over the next 23 days, there was a progressive decrease in the frequency of micronuclei after this point. (7)
Aneurism, epithelial hyperplasia, epithelial necrosis, desquamation, epithelial lifting, oedema, shortening of secondary lamellae, and lamellar fusion were found in gill tissues subjected to lambda-cyhalothrin in a histological test of Cirrhinus mrigala. After exposure, fish kidney tissues showed signs of tubular epithelium necrosis, hazy swelling of epithelial cells in the renal tubules, narrowing of the tubular lumen, constriction of the glomerulus, and expansion of space within the Bowman's capsule. Fish exposed to lambda-cyhalothrin developed hepatocellular enlargement, hazy degeneration, congestion, karyolysis, karyohexis, sinusoidal dilatation, and localized necrosis as hepatic lesions. Eosinophils infiltrated the lamina propria and atrophy of epithelial cells were two features of the intestinal lesions.(8)
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