# Evaluation of Insecticidal Activity of Macrolide and Neonicotinoid Insecticides Against Zeugodacus tau (Walker) and Their Residue Dissipation Dynamics in Luffa cylindrica

## TL;DR

This study evaluates how well different insecticides work against a fruit fly pest and how pesticide residues break down on sponge gourd crops.

## Contribution

The study provides new insights into the efficacy and residue dynamics of macrolide and neonicotinoid insecticides against Zeugodacus tau on Luffa cylindrica.

## Key findings

- Macrolide insecticides showed high efficacy against both adult and larval stages of Z. tau with low residue risks.
- Pesticide residues were mainly found on the peel of sponge gourd, with minimal presence in the flesh.
- Neonicotinoids showed lower efficacy and sometimes left trace residues in fruit flesh due to their systemic nature.

## Abstract

Sponge gourd (Luffa cylindrica) is a widely cultivated crop in the Cucurbitaceae family, commonly affected by Zeugodacus tau in southern China. Chemical control remains the main method for managing Z. tau; however, improper pesticide use can lead to pest resistance and high pesticide residues, posing food safety risks. Our study assessed the insecticidal activity of seven key insecticides from two major classes against different developmental stages of Z. tau and monitored the degradation of these pesticides on sponge gourd. Results showed that pesticide residues were mainly concentrated on the peel, with much lower levels in the flesh. Macrolide insecticides had significant effects on both adults and larvae, degrading quickly and posing low residue risks. However, their efficacy against larvae that had already penetrated the flesh was limited. These findings offer valuable guidance for farmers to select effective, low-residue pesticides and optimize pest control strategies for fruit flies.

Zeugodacus tau (Walker) is a major invasive pest that targets cucurbitaceous plants and other economically significant crops, causing substantial yield and quality losses. The pest’s method of infesting the fruit, however, limits the efficacy of conventional chemical control strategies. This study comprehensively evaluates the insecticidal efficacy of seven insecticides from two key classes—macrolides and neonicotinoids—against various developmental stages of Z. tau. Additionally, it investigates the distribution and degradation patterns of pesticide residues on sponge gourd. Laboratory toxicity tests showed notable differences in sensitivity among the three pest stages. Larvae were the most sensitive to insecticides, followed by adults, with eggs being relatively resistant. Four macrolide insecticides showed high activity against both adults and larvae, while three neonicotinoids displayed low activity against eggs, larvae, and adults of Z. tau. Field residue trials using common recommended dosages demonstrated rapid degradation of pesticide residues on sponge gourd, with residue levels dropping below the national maximum residue limits (MRLs) within 2 to 24 h after application. Further results indicated that pesticide residues accumulated mainly on the peel, with low or undetectable levels in the flesh. Among the macrolide insecticides, residues were nearly undetectable, whereas neonicotinoids, due to their systemic nature, sometimes showed trace residues in the fruit flesh. Oviposition hole measurements revealed an average depth of 0.57 ± 0.29 cm, just beneath the skin, with the hole’s structure effectively protecting the eggs laid by females. Based on the comprehensive analysis of insecticide toxicity and residue degradation, control strategies should primarily target adults. Macrolide insecticides, which have low residue risks and high efficacy against both adults and larvae, are recommended as the preferred option for management. This study, considering the unique damage characteristics of Z. tau, offers valuable insights into chemical control strategies, focusing on insecticide sensitivity at different life stages, residue distribution, and degradation, providing important references for the scientific use of pesticides and the optimization of pest management approaches.