Cadmium exposure through the food chain reduces the parasitic fitness of Chouioia cunea to Hyphantria cunea pupae: An ecotoxicological risk to pest control.

The toxicity of heavy metals on various trophic levels along the food chain has been extensively investigated, but no studies have focused on parasitic natural enemy insects. Herein, we constructed a food chain consisting of soil-Fraxinus mandshurica seedlings-Hyphantria cunea pupae-Chouioia cunea to analyze the effects of Cd exposure through food chain on the fitness of parasitic natural enemy insects and its corresponding mechanism. The results showed that the transfer of Cd between F. mandshurica leaves and H. cunea pupae and between H. cunea pupae and C. cunea was a bio-minimization effect. After parasitizing Cd-accumulated pupa, the number of offspring larvae, and the number, individual size (body weight, body length, abdomen length) and life span of offspring adults decreased significantly, while the duration of embryo development extended significantly. The contents of malondialdehyde and H2O2 in Cd-exposed offspring wasps increased significantly, accompanied by a significantly decrease in antioxidant capacity. The cellular immunity parameters significantly decreased in Cd-accumulated pupae, including the number of hemocytes, melanization activity and the expression level of cellular immunity genes (e.g. Hemolin-1 and PPO1). The humoral immunity disorder was found in the Cd-accumulated pupae, as evidenced by that the expression level of immune recognition gene (PGRP-SA), signal transduction genes (IMD, Dorsal, and Tube), as well as all antimicrobial peptide genes (e.g. Lysozym and Attacin) decreased significantly. Cd exposure decreased the content of glucose, trehalose, amino acid, and free fatty acid in H. cunea pupae. The expression of Hk2 in glycolysis pathway and the expression of Idh2, Idh3, Cs, and OGDH in TCA cycle pathway were significantly down-regulated in Cd-accumulated pupae. Taken together, exposure to Cd through the food chain causes oxidative damage on the offspring wasps and disrupts energy metabolism of the host insect, ultimately reducing the parasitic fitness of C. cunea to H. cunea pupae.

Defense response of Fraxinus mandshurica seedlings to Hyphantria cunea larvae under Cd stress: A contradiction between attraction and resistance.

Heavy metal pollution, as a common and serious environmental problem worldwide, has been regarded as an abiotic stimulus that can affect plant insect resistance and pest occurrence. This study evaluated the defense response of Fraxinus mandshurica seedlings to Hyphantria cunea larvae under Cd stress, with consideration given to chemical defense, physical defense, and elemental defense. Our results showed that the H. cunea larvae had a strong preference for Cd-treated F. mandshurica seedlings, but there was a significant reduction in body weight and survival rate in larvae that fed on leaves of Cd-treated seedlings. Under Cd treatment, the increase in attractant metabolites (e.g., styrene, dibutyl phthalate, and d-limonene) and the decrease in repellent metabolites (e.g., aromadendrene, heptadecane, and camphene) in leaf volatiles were responsible for the high attractant activity to H. cunea larvae. Based on leaf physicochemical properties, tissue structure, and phenolic acid content, an overall reduction in physical defense, chemical defense and their combination in F. mandshurica seedlings exposed to Cd stress was identified by Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) analysis. Elemental defense occurred in Cd-treated F. mandshurica seedlings, as evidenced by the high concentration of Cd in leaves and H. cunea larvae under Cd treatment. Taken together, these findings demonstrate that under Cd stress, elemental defense replaces the dominant role of basic defense in F. mandshurica seedlings and accounts for the enhanced ability to defend against H. cunea larvae.