# Combination of the Parasitoid Spalangia endius Walker and Chemical Pesticides for the Control of Zeugodacus cucurbitae (Coquillett)

## TL;DR

This study finds that combining the parasitoid Spalangia endius with abamectin pesticide improves control of melon fly pupae when applied in a specific sequence.

## Contribution

The study identifies abamectin as a compatible pesticide for use with S. endius and determines optimal application timing for enhanced pest control.

## Key findings

- Abamectin is the least toxic insecticide to S. endius and can be used in combination with it.
- Spraying abamectin before melon fly pupation followed by releasing S. endius increases host mortality.
- Prior exposure to abamectin reduces S. endius avoidance of treated host pupae.

## Abstract

The melon fly (Zeugodacus cucurbitae) is a pest that prefers to bore into and feed on cucurbitaceous crops and that pupates in the soil. The parasitoid Spalangia endius can burrow into the soil to attack melon fly pupae, but it only kills about 33% of them. Since insecticides are essential for controlling melon fly populations, we assessed how five commonly used insecticides affect the toxicity, survival, reproduction, choice-making behaviour, and parasitism ability of S. endius. Our goal was to determine how to combine insecticides with S. endius to more effectively control melon fly pupae. Through a series of laboratory and semi-field trials, we confirmed that abamectin is suitable for combined use with S. endius. However, we must be careful about the concentration and timing of abamectin application. The best approach is to spray abamectin on the soil surface before the melon flies pupate and then release the parasitoids.

Spalangia endius Walker, a pupal parasitoid of the alien invasive pest Zeugodacus cucurbitae (Coquillett), causes 33% host mortality. This study assessed whether combining S. endius with insecticides (abamectin, thiamethoxam, nitenpyram, emamectin benzoate, or beta-cypermethrin)—all effective against Z. cucurbitae—could enhance control efficacy. Among these, abamectin was the least toxic to adult S. endius. Surface contact treatments with 12 and 15 mg a.i./kg of abamectin did not significantly increase S. endius mortality. However, mixing 12 mg a.i./kg of abamectin into a honey solution to encourage ingestion decreased the survival, parasitism, and fecundity of S. endius. In olfactometer assays, S. endius adults avoided abamectin-treated host pupae, though prior exposure to abamectin mitigated this avoidance. The timing of abamectin soil application relative to host pupation and S. endius release affects host mortality. The most effective timing is spraying abamectin before host pupation (to expose Z. cucurbitae larvae) and then releasing S. endius. Field trials confirmed that combining abamectin (12 mg a.i./kg) with S. endius increased host mortality more than either treatment alone. In conclusion, abamectin (12 mg a.i./kg) is a suitable insecticide for combination with S. endius to control Z. cucurbitae. The application sequence should be spraying abamectin before hosts pupate and, only after that, releasing the parasitoid.