Pathogenicity of Steinernema carpocapsae ALL Entomopathogenic Nematodes and Their Symbiotic Bacteria as a Biological Control Agent on Red Palm Weevil

Abstract

Insect-specific pathogens present a sustainable alternative to pesticides for managing the red palm weevil (RPW). This study assessed the efficacy of Steinernema carpocapsae ALL nematodes and their symbiotic bacteria against the third-instar larvae and adults of RPW under laboratory conditions. The symbiotic bacteria were isolated, morphologically characterized, and genetically identified. The results indicated that the mortality rates of RPW larvae treated with S. carpocapsae exceeded 50% in all treatments at 120 h, reaching 93.33% at a concentration of 250 IJs/mL. The morphology of isolated symbiotic bacterium from S. carpocapsae on NBTA medium exhibited a light green color with a glossy surface, a raised center, and a mucilaginous texture. A novel strain of symbiotic bacterium was identified and named as LZ-G7. The bacteria toxicity on RPW adults showed a notable mortality rate of 66.67% at 48 h after feeding with concentration of 10 × 10⁷ CFU/mL. The mortality rate of the third-instar larvae of RPW reached 83.33% after feeding with 0.30 × 10⁸ CFU/g at 96 h and 93.33% after injection into blood cavity with 8 × 10⁶ CFU at 48 h. These results suggest that S. carpocapsae and a novel symbiotic bacterium strain exhibit strong virulence against RPW and have the potential to serve as effective biological control agents in integrated pest management strategies.

Keywords: Rhynchophorus ferrugineus; Steinernema carpocapsae ALL; bio-control; entomopathogenic nematodes; symbiotic bacterium.

Figure 1

Mortality percentage of third-instar larvae of RPW after treatment with S. carpocapsae. The letters (a, b, c, etc.) indicate statistically significant differences (p < 0.05) among treatments on the same day.

Figure 2

Morphological characteristics of symbiotic bacteria LZ-G7: (A) single colony obtained by culturing the hemolymph of infected G. mellonella by S. carpocapsae on NBTA medium for 48 h, (B) morphology of the colony of the initially isolated symbiotic bacteria cultured on the LB medium plate for 48 h, (C) morphology of the initial colony for isolated symbiotic bacteria cultured on NBTA medium plate for 48 h, and (D) isolated symbiotic cells from NBTA medium.

Figure 3

Agarose gel electrophoresis results of PCR amplification products of 16S rRNA of LZ-G7 strain (1: 2000 bp maker; 2, 3: PCR amplification products; 5, 6: DNA recovery products; 8, 9: conversion connection product).

Figure 4

Phylogenetic tree based on 16S rRNA sequences homology.

Figure 5

Mortality percentage of RPW adults after feeding with different concentration of LZ-G7. The letters (a, b, c, etc.) indicate statistically significant differences (p < 0.05) among treatments on the same day.

Figure 6

Mortality of RPW third-instar larvae after feeding with different concentrations of LZ-G7 bacteria solution. The letters (a, b, c, etc.) indicate statistically significant differences (p < 0.05) among treatments on the same day.

Figure 7

Mortality of the third-instar larvae of RPW after injected with LZ-G7 bacteria solution. The letters (a, b, c, etc.) indicate statistically significant differences (p < 0.05) among treatments on the same day.