Assessment of entomopathogenic nematodes and fungi against Eubulus cf. elongatus Hustache (Coleoptera: Curculionidae), a destructive cassava pest in Brazil

ABSTRACT

Eubulus cf. elongatus was recently recorded as one of the most destructive pests of cassava in the Brazilian Cerrado. In this study, we evaluate the susceptibility of E. cf. elongatus to entomopathogenic fungi and nematodes in laboratory. Adults and larvae of E. cf. elongatus were first collected in the field to assess the natural occurrence of fungi and nematodes. Seven nematode species within Heterorhabditis and Steinernema genera were tested in direct applications onto the larvae or indirectly through soil inoculation (2000 infective juveniles per group of 10 larvae). Bioassays with 16 strains of entomopathogenic fungi (beauveria and metarhizium species) were conducted by immersion (10⁸ conidia per mL), direct spray or by exposing adults to soil sprayed with conidial suspensions (6 × 10⁴ conidia per cm²). In all bioassays insects were kept at 24–25 °C and 75-90% RH for 16 days. Nematodes were not found naturally occurring in E. cf. elongatus larvae collected in field. Of the adults collected, 1.76% and 0.04% were infected with B. bassiana and Metarhizium sp., respectively. In the laboratory trials, the highest mortality of larvae was caused by H. bacteriophora (95.0% for application on larvae and 60% for soil application). Entomopathogenic fungi caused low mortality rates in adults of E. cf. elongatus, being less than 11% in direct and soil applications and reaching a maximum of 16.07% when insects were treated by immersion. Our results show the potential of nematodes, such as H. bacteriophora, in controlling the larvae of E. cf. elongatus in soil.

KEYWORDS:

• Soil pest
• cassava root borer
• manihot esculenta
• Heterorhabditis bacteriophora
• Beauveria
• Metarhizium

Acknowledgments

We thank José Carlos Gonçalves dos Santos, Fabio Honorato da Cunha, Orlando Vieira, and Mateus Rodrigues Neves for their support in the field collections and the Brazilian Agricultural Research Corporation (Embrapa), Banco do Brasil Foundation (FBB), Federal District Research Support Foundation (FAPDF), and National Council for Scientific and Technological Development (CNPq) for financial support. EAV, RBL, and MRF are supported by CNPq fellowships.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was financially supported by Brazilian Agricultural Research Corporation (Embrapa) (grant: 20.18.01.012.00.02.007), Banco do Brasil Foundation (FBB) (grant: 15288), Federal District Research Support Foundation (FAPDF) (grant: 00193-00000080/2019-65), and National Council for Scientific and Technological Development (CNPq) (grant: 305925/2020-2, 309070/2020-1, and 13952/2018-3).