Susceptibility and Immune Defence Mechanisms of Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) against Entomopathogenic Fungal Infections

Abstract

Insects infected with entomopathogenic fungi, experience physiological changes that influence their growth and immune defence. The potential of nine isolates of entomopathogenic fungi was evaluated after determining percent germination and relative conidial hydrophobicity. However, nutritional indices were evaluated after immersing eighth-instar Rhynchophorus ferrugineus larvae into each isolate suspension (1 × 10⁷ conidia/mL). The results showed that isolates B6884 and M9374 had 44.51% and 39.02% higher conidial hydrophobicity compared with isolate I03011 (least virulent). The results of nutritional index assays revealed a significant reduction in growth indices after infection with different isolates. Compared with control, B6884 and M9374 greatly decreased larval growth by reducing the efficacy of conversion of ingested food (36%-47%) and Efficacy of conversion of digested food (50%-63%). Furthermore, only isolate B6884 induced 100% mortality within 12 days. Compared with control, isolate I03011, possessing the lowest conidial hydrophobicity, only reduced 0.29% of the efficacy of conversion of ingested food (ECI) and 0.48% of the efficacy of conversion of digested food (ECD). Similarly, transcriptomic analysis of genes related to the Red palm weevil (RPW) immune response, including pathogen recognition receptors (C-type lectin and endo-beta-1,4-glucanse), signal modulator (Serine protease-like protein), signal transductors (Calmodulin-like protein and EF-hand domain containing protein) and effectors (C-type lysozyme, Cathepsin L., Defensin-like protein, Serine carboxypeptidase, and Thaumatin-like protein), was significantly increased in larval samples infected with B6884 and M9374. These results suggest that for an isolate to be virulent, conidial hydrophobicity and germination should also be considered during pathogen selection, as these factors could significantly impact host growth and immune defence mechanisms.

Keywords: feeding performance; immune defence; immune-related genes; red palm weevil; virulence.

Figure 1

Cumulative corrected percent mortality of Rhynchophorus ferrugineus larvae infected with different concentrations of the tested isolates of entomopathogenic fungi. Means ± SE values followed by different letter(s) are significantly different. (Fisher’s least significant difference (LSD) test, α = 0.05).

Figure 2

Relative fold-expression of pathogen recognition receptors (PRRs) of Rhynchophorus ferrugineus larvae in response to fungal infections using quantitative real-time PCR. Means ± SE values followed by different letter(s) are significantly different. (Fisher’s Least Significant Difference (LSD) test, α = 0.05).

Figure 3

Relative fold-expression of signal modulators of Rhynchophorus ferrugineus larvae in response to fungal infections using quantitative real-time PCR. Means ± SE values followed by different letter(s) are significantly different. (Fisher’s Least Significant Difference (LSD) test, α = 0.05).

Figure 4

Relative fold-expression of signal transductors of Rhynchophorus ferrugineus larvae in response to fungal infections using quantitative real-time PCR. Means ± SE values followed by different letter(s) are significantly different. (Fisher’s Least Significant Difference (LSD) test, α = 0.05).

Figure 5

Relative fold-expression of effectors of Rhynchophorus ferrugineus larvae in response to fungal infections using quantitative real-time PCR. Means ± SE values followed by different letter(s) are significantly different. (Fisher’s Least Significant Difference (LSD) test, α = 0.05).