First report on the enzymatic and immune response of Metarhizium majus bag formulated conidia against Spodoptera frugiperda: An ecofriendly microbial insecticide

Entomopathogenic fungi from microbial sources are a powerful tool for combating insecticide resistance in insect pests. The purpose of the current study was to isolate, identify, and evaluate bag-formulated entomopathogenic fungal conidial virulence against insect pests. We further investigated the enzymatic responses induced by the entomopathogenic fungi as well as the effect on a non-target species. Entomopathogenic fungi were isolated from the Palamalai Hills, India, using the insect bait method, and the Metarhizium majus (MK418990.1) entomopathogen was identified using biotechnological techniques (genomic DNA isolation and 18S rDNA amplification). Bag-formulated fungal conidial efficacy (2.5 × 10³, 2.5 × 10⁴, 2.5 × 10⁵, 2.5 × 10⁶, and 2.5 × 10⁷ conidia/ml) was evaluated against third instar larvae of Spodoptera frugiperda at 3, 6, 9, and 12 days of treatment, and acid and alkaline phosphatases, catalase, and superoxide dismutase enzymatic responses were evaluated at 3 days post-treatment. After 12 days of treatment, non-target assays on the earthworm Eudrilus eugeniae were performed using an artificial soil assay. Results of the bag formulated fungal conidial treatment showed that S. frugiperda had high susceptibility rates at higher concentrations (2.5 × 10⁷ conidia/ml) of M. majus. Lower concentration of 2.5 × 10³ conidia/ml caused 68.6% mortality, while 2.5 × 10⁷ conidia/ml caused 100% mortality at 9 days post treatment. Investigation into enzymatic responses revealed that at 3 days post M. majus conidia exposure (2.5 × 10³ conidia/ml), insect enzyme levels had significantly changed, with acid and alkaline phosphatases, and catalase enzymes significantly reduced and superoxide dismutase enzymes significantly raised relative to the control. After 12 days of treatment, no sublethal effects of M. majus conidia were observed on E. eugeniae, with no observed damage to gut tissues including lumen and epithelial cells, the nucleus, setae, coelom, mitochondria, and muscles. This study offers support for the use of fungal conidia in the target-specific control of insect pests.

Improving Contagion and Horizontal Transmission of Entomopathogenic Fungi by the White-Spotted Longicorn Beetle, Anoplophora malasiaca, with Help of Contact Sex Pheromone

Simple Summary

The white-spotted longicorn beetle, Anoplophora malasiaca, is one of the most destructive pests of many fruits and street trees. Effective controls are needed because the effect of marketed insecticides is limited. Entomopathogenic fungi offer a solution, combination with the beetles’ contact sex pheromone. The surface of the female body is covered with contact sex pheromone, which we extracted. Males held onto a glass model coated with female extract for 5 h, but males held onto one without extract for <0.3 h. Males that held onto coated model, attached to fabric impregnated with an entomopathogenic fungus, Beauveria brongniartii, picked up significant fungi. The fungi were then transferred to females during mating. Our results indicate that a combination of contact pheromone with a pathogen could improve entomopathogenic infection of both male and female beetles.

Abstract

The white-spotted longicorn beetle, Anoplophora malasiaca, is one of the most destructive pests of horticultural crops and street trees. Effective controls are needed because the effect of marketed insecticides is limited. Entomopathogenic fungi offer a solution, and improving the rate of infection would be a breakthrough in this beetle’s control. The combination of pathogenic fungi and the beetle’s contact sex pheromone was suggested. The surface of the female body is covered with contact sex pheromone, which elicit male mating behavior. To develop a method for the practical control of this beetle, we evaluated the arrestant activity of female extract containing contact pheromone coated on a black glass model. Males presented with a coated model held on for 5 h (mean) during an 8-h experiment. In contrast, males presented with a control model held on for <0.3 h. Males that held onto coated models attached to fabric impregnated with conidia of the fungus Beauveria brongniartii picked up much conidia, which they then passed on to females during mating.

Activities of Select Enzymes Involved in Insecticide Resistance in Spinosad-Resistant and -Susceptible Strains of Musca domestica (Diptera: Muscidae)

A Musca domestica L. strain collected from Pakistan has recently been shown to be resistant to spinosad; however, there is scarce information about the mechanism of resistance. For this reason, we explored whether a metabolic-based mechanism was responsible by analyzing the activities of the metabolic detoxifying enzymes, carboxylesterases, glutathione S-transferases, and mixed-function oxidases, in both a spinosad-selected (Spin-SEL) strain of M. domestica and a susceptible counterpart (Lab-susceptible). The results revealed that both strains were statistically at par in terms of enzyme activities. The activity of carboxylesterases in the Lab-susceptible strain was 78.17 ± 3.06 in comparison to 79.16 ± 3.31 nmol min-1 mg-1 in the Spin-SEL strain. The activity of mixed-function oxidases was 51.58 ± 4.20 in the Lab-susceptible strain, whereas 54.33 ± 4.08 pmol min-1 mg-1 was recorded in the Spin-SEL strain. The activity of glutathione S-transferases was 86.50 ± 4.59 (Lab-susceptible) and 90.33 ± 2.81 nmol min-1 mg-1 (Spin-SEL). These results revealed that the studied enzymes might not be responsible for spinosad resistance in the studied strain of M. domestica. Therefore, studies should be extended to find out other possible mechanisms of spinosad resistance.

Distribution of Toxin Genes and Enterotoxins in Bacillus thuringiensis Isolated from Microbial Insecticide Products

Bacillus thuringiensis microbial insecticide products have been applied worldwide. Although a few cases of B. thuringiensis foodborne illness have been reported, little is known about the toxigenic properties of B. thuringiensis isolates. The aims of this study were to estimate the pathogenic potential of B. thuringiensis selected from microbial insecticide products, based on its possession of toxin genes and production of enterotoxins. Fifty-two B. thuringiensis strains selected from four kinds of microbial insecticide products were analyzed. PCR assay for detection of toxin genes and immunoassay for detection of enterotoxins were performed. The hemolysin BL complex as a major enterotoxin was produced by 17 (32.7%), whereas the nonhemolytic enterotoxin complex was detected in 1 (1.9%) of 52 B. thuringiensis strains. However, cytK, entFM, and ces genes were not detected in any of the tested B. thuringiensis strains. The potential risk of food poisoning by B. thuringiensis along with concerns over B. thuringiensis microbial insecticide products has gained attention recently. Thus, microbial insecticide products based on B. thuringiensis should be carefully controlled.

Isolation, characterization, and insecticidal activity of an endophyte of drunken horse grass, Achnatherum inebrians

Abstract Endophytic microorganisms reside within plant tissues and have often been found to promote plant growth. In this study, endophytic microorganisms were isolated from the roots, stems, leaves, and seeds of healthy drunken horse grass, Achnatherum inebrians (Hance) Keng (Poales: Poaceae), through the use of a grinding separation method and identified by a dual approach of morphological and physiological observation and 16S rRNA gene-based (for bacteria) and internal transcribed sequence-based (for fungi) molecular identification. The endophytes were then inoculated into liquid media for fermentation, and their crude extracts were employed for insecticidal activity tests using slide disc immersion and nebulization methods. A total of 89 bacteria species, which were classified into eight genera, Bacillus, Pseudomonas, Actinomyces, Corynebacterium, Acinetobacter, Sphingomonas, Paenibacillus, and Phyllobacterium, and two fungi, Claviceps and Chaetomium, were isolated. Of these species, isolates Streptomyces albus (Rossi-Doria) Waksman and Henrici (Actinomycetales: Streptomycetaceae) (GA) and Claviceps purpurea (Fr.) Tul. (Hypocreales: Clavicipitaceae) (PF-2) were shown to produce mortality rates of more than 90% in the cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), after first and second screenings. The isolates PF-2 and GA associated with A. inebrians had significant insecticidal activities towards A. gossypii Glover (Hemiptera: Aphididae) and may provide a new biological resource for exploring a new microbial insecticide.