Abstract
Simple SummaryThe spider mite is a destructive pest of various crops during warm and dry conditions in tropical countries, including Thailand. The pest is difficult to control despite synthetic acaricide use. In the field, insect populations gradually develop resistance to synthetic pesticides over long-term use. The use of entomopathogenic fungi is more human- and environmentally friendly. We searched and identified a potential fungal isolate from a culture collection, focusing on the genus Metarhizium. Metarhizium sp. BCC 4849 not only controls the spider mite but also plays a significant role as a natural regulator of other insect pests. Here, we investigated its infection process on the mite, optimized a conidial formulation for extended shelf life, and conducted toxicological assays in animals to assess its biosafety in humans. The fungal genome has been sequenced. The genomic data indicated that oxidoreduction proteins; zinc-, heme-, and iron-binding proteins; and transmembrane transporters are abundant in the genome.Five isolates of Metarhizium sp. were evaluated for their pathogenicity against the spider mite (Tetranychus truncatus Ehara) (Acari: Tetranychidae) and Metarhizium sp. BCC 4849 resulted in the highest mortality (82%) on the 5th day post-inoculation (DPI). Subsequent insect bioassay data indicated similar high virulence against five other insects: African red mites (Eutetranychus africanus Tucker) (Acari: Tetranychidae), bean aphid (Aphis craccivora Koch) (Hemiptera: Aphididae), cassava mealybug (Phenacoccus manihoti Matile-Ferrero) (Hemiptera: Pseudococcidae), sweet potato weevil (Cylas formicarius Fabricius) (Coleoptera: Brentidae), and oriental fruit fly (Bactrocera dorsalis Hendel) (Diptera: Tephritidae), at mortalities of 92–99%, on 3rd–6th DPI, and in laboratory conditions. The pathogenicity assay against E. africanus in hemp plants under greenhouse conditions indicated 85–100% insect mortality on 10th DPI using the fungus alone or in combination with synthetic acaricide. Genome sequencing of Metarhizium sp. BCC 4849 revealed the high abundance of proteins associated with zinc-, heme-, and iron-binding; oxidation-reduction; and transmembrane transport, implicating its versatile mode of interaction with the environment and adaptation to various ion homeostasis. The light and scanning electron microscopy indicated that at 24 h post inoculation (PI), adhesion and appressorial formation occurred, notably near the setae. Most infected mites had stopped moving and started dying by 48–72 h PI. Elongated hyphal bodies and oval blastospores were detected in the legs. At 96–120 h PI or longer, dense mycelia and conidial mass had colonized the interior and exterior of dead mites, primarily at the bottom than the upper part. The shelf-life study also indicated that conidial formulation combined with an oxygen-moisture absorber markedly enhanced the viability and germination after storage at 35 °C for four months. The fungus was tested as safe for humans and animals, according to our toxicological assays.
Highlights
Species in the genus Metarhizium are well-known biopesticides for controlling a wide range of insect pests [1]
The isolate had high virulence against the nymphs of bean aphid and cassava mealybug; the larvae of sweet potato weevils; and the fruit fly adults at 96, 92, 92, and 99% insect mortality, respectively, on 3th–6th day post-inoculation (DPI) depending on the developmental stages and sizes of target pests (Table 2)
This study demonstrated that the entomopathogen Metarhizium sp
Summary
Species in the genus Metarhizium are well-known biopesticides for controlling a wide range of insect pests [1]. These pests included phytophagous mites, citrus rust mite, twospotted spider mite, and bulb mite [2]. Entomopathogenic fungi can play a significant role as natural regulators of the insect and mite populations. They can be applied alone or as a combination with synthetic pesticides/acaricides. The effect of the fungus alone and in combination with acaricide fenpyroximate against African red mite, in hemp plants (Canabis sativa L.) (Rosales: Canabaceae) under greenhouse conditions, was evaluated. Biosafety assessments in animals for the human safety evaluation, including acute oral toxicity, acute dermal toxicity, acute dermal irritation, acute eye irritation, and acute pulmonary toxicity/pathogenicity, were tested
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