Sublethal effects of nitenpyram on the development of silkworm.

The utilization of nitenpyram for aphid and whitefly control may induce environmental contamination and negative repercussions on non-target organisms. Formerly, we found that nitenpyram would pollute the peripheral and sub-peripheral areas of the adjacent mulberry orchard. Under acute toxicity conditions, nitenpyram induced oxidative damage in silkworms, affected biological metabolism, synthesis, immunity, and signal transduction. Considering the impact of nitenpyram mist drift on mulberry leaves, we investigated the effects of low concentrations of nitenpyram on silkworms. The results showed that silkworms exposed to 0.17 mg/L, 0.35 mg/L and 0.70 mg/L of nitenpyram (1/40 LC50, 1/20 LC50 and 1/10 LC50) showed obvious poisoning symptoms. The cocoon weight and cocoon shell weight decreased gradually with increases in the concentration, and these decreases prolonged the growth and development time of silkworms and induced the detoxification enzymes carboxylesterase (CarE) and glutathione-S-transferase (GST) to cope with the stress damage caused by nitenpyram. Exposure to low concentrations of nitenpyram downregulates genes involved in the drug metabolism-other enzymes and peroxisome pathway in silkworms. Additionally, through injection of miRNA mimics and inhibitors, we discovered that detoxifying enzyme pathway genes are influenced by bmo-miR-3382-3P, bmo-miR-3213-5P and bmo-miR-133, regulating the immune response of silkworms. This study provides an overall view of the toxicity and detoxification metabolism of nitenpyram in silkworm, and provides a reference for environmental assessment.

Toxicity of nitenpyram to silkworm (Bombyx mori L.) and its potential mechanisms.

Silkworm (Bombyx mori L.), as an economic insect, occupies a certain position in the development of China's economy. The neonicotinoid insecticide nitenpyram is commonly used in farmland to control planthoppers and aphids. In China, mulberry orchards are often planted adjacent to fields or commercial crops, and mist drifts occur during application, which may affect the production safety of Bombyx mori. In this study, a risk assessment of nitenpyram was carried out, and the results showed that there were risks in spraying nitenpyram around the periphery and subperipheries of mulberry fields. However, few studies have reported the mechanism underlying nitenpyram's toxic effect on silkworms. Here, we validated 25 differentially expressed (DE) miRNAs in the nitenpyram treatment group of silkworms, and the significantly enriched mTOR signaling pathway, oxidative phosphorylation and FoxO signaling pathway were verified. Among them, bmo-miR-2766-5P was up-regulated by 2.122-fold, and the expression of its regulated target gene 101,741,287 was up-regulated. After the injection of bmo-miR-2766-5P inhibitor, the Log2FC value of 101,741,287 was changed from 1.26 to -2.19. Bmo-miR-3326, bmo-miR-3378-5P and bmo-miR-2761-3P were down-regulated by 2.386-fold, 1.158-fold and 2.359-fold, respectively. After injecting miRNA mimics into silkworms, the Log2FC values of the target genes 100,302,609, 101,740,730 and 101,746,319 were changed from 1.24 to -11.94, -1.12 changed to 2.84 and 1.93 changed to -0.37, respectively. In addition, nitenpyram induced oxidative damage in silkworms, and the degree of DNA damage increased with the increase of concentration and time. Meanwhile Imd was significantly up-regulated in IMD-related pathways (38.7-fold, p < 0.01). The results indicated that nitenpyram could affect the growth and development process of silkworms, and these DE-miRNAs may have an important impact on the stress response of silkworms to nitenpyram.

Sublethal Effects of Neonicotinoid Insecticides on the Development, Body Weight and Economic Characteristics of Silkworm.

Silkworm Bombyx mori (L.) (Lepidoptera: Bombycidae) is a critical insect for silk producers, but the inappropriate application of insecticides negatively affects the physiology and behavior of silkworms. This study found that the effects of neonicotinoid insecticides applied using two spraying methods on the growth and development of silkworms were different: the median lethal concentration (LC50) values of two pesticides applied using the leaf-dipping method were 0.33 and 0.83 mg L-1 and those of two pesticides applied using the quantitative spraying method were 0.91 and 1.23 mg kg-1. The concentration of pesticides on the mulberry leaves did not decrease after their application using the quantitative spraying method, and a uniform spraying density was observed after the mulberry leaves were air-dried (no liquid) under realistic conditions. We then treated silkworms with the quantitative spraying method and leaf-dipping method. The treatment of silkworm larvae with imidacloprid and thiamethoxam at sublethal concentrations significantly prolonged the development time and significantly decreased the weight and pupation rate, as well as economic indicators of enamel layers and sputum production. Thiamethoxam treatment significantly increased the activities of carboxylesterase (CarE) and glutathione-S-transferase (GST). The activity of CarE and GST increased, decreased, and then increased, and the highest activity was detected on the 10th and 12th days. Thiamethoxam exposure significantly elevated the transcription levels of CarE-11, GSTe3 and GSTz2 and induced DNA damage in hemocytes. This study confirmed that the quantitative spray method is more stable than the leaf-dipping method. Moreover, imidacloprid and thiamethoxam treatment affected the economy and indexes of silkworms and induced changes in detoxification enzymes and DNA damage in silkworms. These results provide a basis for understanding the mechanism of the sublethal effects of insecticides on silkworms.