Evaluation of the Entomopathogenic Fungi Metarhizium anisopliae, Beauveria bassiana and Isaria sp. for the Management of Aphis craccivora (Hemiptera: Aphididdae)

Abstract

Cowpea, Vigna unguiculata ((L.) Walp; Fabales: Fabaceae), is an important indigenous vegetable and grain legume in the tropics where it represents a major diet component. Cowpea aphid, Aphis craccivora (Koch; Hemiptera: Aphididdae) is a major pest causing up to 100% yield losses. Aiming at establishing alternative approach to synthetic insecticides, we evaluated the pathogenicity of 23 fungal isolates including Metarhizium anisopliae ((Metschn.) Sorokin; Hypocreales: Clavicipitaceae), Beauveria bassiana ((Bals.) Vuill.; Hypocreales: Cordycipitaceae), and Isaria sp. (Hypocreales: Cordycipitaceae) against adult A. craccivora in the laboratory. Adult apterous aphids were sprayed with conidial suspensions titred at 1 × 108 conidia ml-1 for pathogenicity tests while 1 × 104, 1 × 105, 1 × 106, 1 × 107 and 1 × 108 conidia ml-1 were used in dose response bioassays. All the fungal isolates were found pathogenic to A. craccivora, causing mortality of between 34.5 and 90%. The lethal 50% mortality time (LT50) values varied between 3.3 and 6.3 d, with the best isolates being ICIPE 62, ICIPE 41 and ICIPE 644. The lethal concentration mortality (LC50) values were 2.3 × 106, 1.3 × 108 and 1.3 × 109 for ICIPE 62, ICIPE 41, and ICIPE 644, respectively. M. anisopliae isolate ICIPE 62 produced more conidia on aphid cadavers (4.5 × 107) than ICIPE 41 (2.7 × 107) and ICIPE 644 (2.1 × 107) 6 d post-treatment. Relative potency comparison showed that ICIPE 62 was more potent than the other two isolates. In the screenhouse, conidia of ICIPE 62 significantly reduced A. craccivora population compared to control but there was no significant difference between emulsifiable and aqueous formulations. Small-holder leafy vegetable producers could gain more profits using fungal-based biopesticides in Aphid-IPM strategies, leading to reduction of pre-harvest intervals after their application compared to synthetic insecticides.

Efficacy of aqueous and oil formulations of a specific Metarhizium anisopliae isolate against Aphis craccivora Koch, 1854 (Hemiptera: Aphididae) under field conditions

“…isolate ICIPE 62 in the management of A. craccivora under field conditions. The efficacy of this isolate in inducing high mortality on A. craccivora and other aphid species has been also demonstrated under laboratory conditions and screenhouse in previous studies (Bayissa et al, ; Mweke et al, ). Application of both M. anisopliae ICIPE 62 formulations did not result in significant decline in aphid densities in wet season compared to untreated control.…”

“…Fungal isolate ICIPE 62 ( Metarhizium anisopliae s.l. ) used in this study was obtained from icipe's Arthropod Germplasm Centre and was recently demonstrated to be pathogenic against A. craccivora under laboratory conditions (Mweke et al, ).…”

“…The EPF M. anisopliae sensu lato ( s.l. ) isolate ICIPE 62 was previously demonstrated to be highly pathogenic to A. craccivora under laboratory conditions (Mweke et al, ). Formulation of EPF and application methods significantly influence their efficacy in pest management under field conditions (Inglis et al, ).…”

“…The EPF M. anisopliae sensu lato (s.l.) isolate ICIPE 62 was previously demonstrated to be highly pathogenic to A. craccivora under laboratory conditions (Mweke et al, 2018).…”

“…Additionally, distribution of infective spores has been shown to be influenced by formulation; for example, conidia formulated in oil are usually evenly distributed on the insect cuticle and leaf surface while aqueous spore formulations remain as drops on the surface after application and thereby enhancing the efficacy of EPF-based biopesticides (Inglis, Ivie, Duke, & Goettel, 2000;Inyang et al, 2000;Wraight, Filotas, & Sanderson, 2016). Previous studies have reported pathogenicity of M. anosopliae isolates against A. craccivora under laboratory studies (Ekesi, Akpa, Onu, & Ogunlana, 2000;Mweke et al, 2018;Sahayaraj & Borgio, 2010;Saranya, Ushakumari, Jacob, & Philip, 2010). However, these isolates were not evaluated under field conditions neither of them was commercialized for control of A. craccivora.…”