Chronic Low-Dose Phoxim Exposure Impairs Silk Production in Bombyx mori L. (Lepidoptera: Bombycidae) by Disrupting Juvenile Hormone Signaling-Mediated Fibroin Synthesis

Abstract

Phoxim is a pesticide extensively applied in mulberry fields, and residues may persist on leaves even after the recommended pre-harvest interval. However, the potential risks of these residues to Bombyx mori L. (Lepidoptera: Bombycidae) have long been overlooked. The results demonstrated that chronic low-dose exposure from the second to fifth instars significantly impaired silkworm development and silk production. Specifically, larvae in the 0.316 μg/mL treatment group (1/2 LC₅₀) exhibited a significant reduction in body weight, while the cocoon shell ratio was significantly decreased in both the 0.079 μg/mL (1/8 LC₅₀) and 1/2 LC₅₀ groups. Cocoon deformities were observed in the 0.032 μg/mL (1/20 LC₅₀), 1/8 LC₅₀, and 1/2 LC₅₀ groups. Histopathological analysis revealed silk gland damage in the treatment groups, with severity increasing with higher phoxim concentrations. Biochemical analyses indicated elevated malondialdehyde (MDA) levels accompanied by increased superoxide dismutase (SOD) and peroxidase (POD) activities. Notably, phoxim exposure selectively reduced juvenile hormone (JH) titers without affecting ecdysone titers. JH-regulated genes including the receptors Met1 and Met2, and transcription factors Kr-h1 and Dimm were downregulated, accompanied by suppressed expression of the fibroin synthesis gene Fib-H. These results collectively indicate that chronic low-concentration phoxim exposure disrupts endocrine regulation, damages silk gland integrity, and ultimately reduces silk production in silkworm.

Keywords: chronic toxicity; juvenile hormone; phoxim; silk gland; silkworm.

Figure 1

Effects of chronic low-dose phoxim exposure on the growth of silkworms. (A) Body weight of fifth-instar larvae; (B) Cocoon shape in the control group; (C) Normal cocoons shape in the phoxim-treated groups; (D) Deformed cocoons in the phoxim-treated groups; (E) Cocoon shell ratio of all cocoons; (F) Cocoon shell ratio excluding deformed cocoons (e.g., thin shells). Significant difference vs. Control (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 2

Histopathological analysis of the PSG in fifth instar silkworm exposed to phoxim. Green arrows indicate protein aggregates; blue arrows indicate damage to the glandular membranes; red arrows indicate vacuolization in the gland lumen. Scale bar = 50 μm.

Figure 3

Oxidative responses in the PSG of silkworms exposed to phoxim. (A) Content of malondialdehyde (MDA); (B) Superoxide dismutase (SOD) activity; (C) Peroxidase (POD) activity. Significant difference vs. control (* p < 0.05, *** p < 0.001).

Figure 4

Effects of chronic low-dose phoxim exposure on the transcript levels of fibroin protein synthesis genes. (A) Relative expression level of Fib-H; (B) Relative expression level of Fib-L; (C) Relative expression level of P25. Significant difference vs. control (** p < 0.01, *** p < 0.001).

Figure 5

Effects of phoxim on hormone titers and related gene expression in the silk glands. (A) Juvenile hormone (JH) titer; (B) Ecdysone titer; (C) Relative expression level of JHAMT; (D) Relative expression level of FPPS. Significant difference vs. control (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 6

Effects of chronic low-dose phoxim exposure on the transcript levels of genes involved in the JH signaling pathway. (A) Relative expression level of Met1; (B) Relative expression level of Met2; (C) Relative expression level of Kr-h1; (D) Relative expression level of Dimm. Significant difference vs. control (* p < 0.05, ** p < 0.01, *** p < 0.001).