Does the effect of a Metarhizium anisopliae isolate on Rhipicephalus microplus depend on the tick population evaluated?

Does the use of engorged adult ticks of Rhipicephalus microplus as substrate modifies the acaricidal behavior of Metarhizium anisopliae?

Entomopathogenic fungi (EPF) is one of the most promising alternatives to regulate tick populations. However, these fungi may lose acaricidal effectiveness over time, due to the storage period and/or successive cultivation on artificial media. It is known that using arthropod pests as a substrate for EPF could potentially alter their acaricidal behavior over time, however, studies using ticks for this purpose are scarce. Hence, the aims of this research were: (1) to isolate strains of Metarhizium anisopliae from paddocks of cattle farms, (2) to evaluate the effect of engorged adult ticks of Rhipicephalus microplus as a substrate on the tickicide behavior of Metarhizium anisopliae strains, and (3) to determine the lethal time of each M. anisopliae strain to kill 50% (LT50) and 99% (LT99) of engorged ticks. First, the natural acaricidal effect of 10 strains of M. anisopliae sensu lato isolated from soils of cattle farms on R. microplus was evaluated. Second, the influence of substrate (R. microplus, Galleria mellonella and Sabouraud dextrose agar [SDA]) on the acaricidal activity and virulence index (lethal time) of each strain during four generations was evaluated. Strains MaV69, MaV60 and MaV67 showed more than 90% mortality at day 20 post-treatment. The use of engorged adult ticks as substrate increased the virulence of five M. anisopliae s.l. strains. Larvae of G. mellonella and SDA as substrate did not modify the acaricidal behavior of the 10 strains evaluated. Seven of ten EPF decreased their LT50 and three decreased their LT99 as an index of their virulence enhancement.

Entomopathogenic Fungi as Alternatives to Chemical Acaricides: Challenges, Opportunities and Prospects for Sustainable Tick Control

Entomopathogenic fungi (EPFs) can infect and kill a diverse range of arthropods, including ticks (Acari: Ixodidae) that can transmit various diseases to animals and humans. Consequently, the use of EPFs as a biocontrol method for managing tick populations has been explored as an alternative to chemical acaricides, which may have harmful effects on the environment and non-target species. This review summarizes studies conducted on EPFs for tick control between 1998 and 2024, identifying 9 different EPF species that have been used against 15 different species of ticks. One of the most well-known and widely researched EPFs used against ticks is Metarhizium anisopliae, a fungus known for its ability to infect and kill various arthropods. When applied to tick-infested areas, M. anisopliae spores attach to the tick's cuticle, germinate, and penetrate through the cuticle, leading to the eventual death of the tick due to the fungal infection. Whilst a number of studies support the potential of this and other EPF species against ticks, this review suggests that limitations to their effective use may include factors such as heat, humidity, and ultraviolet light (UV-A and UV-B). This comprehensive review aims to provide an overview of the literature on the potential of EPFs in tick control, focusing on their mode of action, previous field successes/failures, advantages, potential applications, and prospects for future practical developments.

Field efficacy of Metarhizium anisopliae oil formulations against Rhipicephalus microplus ticks using a cattle spray race

Rhipicephalus microplus tick is the main ectoparasite of cattle in Brazil. The exhaustive use of chemical acaricides to control this tick has favored the selection of resistant tick populations. Entomopathogenic fungi, as Metarhizium anisopliae, has been described as a potential biocontroller of ticks. Therefore, the aim of this study was to evaluate the in vivo efficacy of two oil based formulations of M. anisopliae for the control of the cattle tick R. microplus under field conditions using a cattle spray race as a method of treatment. Initially, in vitro assays were carried out with an aqueous suspension of M. anisopliae, using mineral oil and/or silicon oil. A potential synergism between oils and fungus conidia for tick control was demonstrated. Additionally, the usefulness of silicon oil in order to reduce mineral oil concentration, while improving formulation efficacy was illustrated. Based on the in vitro results, two formulations were selected for use in the field trial: MaO1 (10⁷ conidia/mL plus 5% mineral oil) and MaO2 (10⁷ conidia/mL plus 2.5% mineral oil and 0.01% silicon oil). The adjuvants concentrations (mineral and silicon oils) were chosen since preliminary data indicate that higher concentrations caused significant mortality in adult ticks. For this, 30 naturally infested heifers were divided into three groups based on previous tick counts. The control group did not receive treatment. The selected formulations were applied on animals using a cattle spray race. Subsequently, tick load was evaluated weekly by counting. The MaO1 treatment significantly reduced the tick count only on day +21, reaching approximately 55% efficacy. On the other hand, MaO2 showed significantly lower tick counts on days +7, +14, and +21 after treatment, with weekly efficacy achieving 66%. The results showed a substantial reduction of tick infestation, up to day +28, using a novel formulation of M. anisopliae based in the mixture of two oils. Moreover, we have shown, for the first time, the feasibility of employing formulations of M. anisopliae for large-scale treatment methods, such as a cattle spray race, which in turn, may increase the use and adhesion to biological control tools among farmers.

From the laboratory to the field: efficacy of entomopathogenic nematodes to control the cattle tick

BACKGROUND

The control of ticks is challenged by the resistance of tick populations to chemical acaricides. In this study, we evaluated, under laboratory conditions, the efficacy of Heterorhabditis bacteriophora against Rhipicephalus (Boophilus) microplus engorged females with varying body weights (150, 200, 250, 300 or 350 mg per female) or from eight different geographical populations. We also determined the efficacy of H. bacteriophora for tick control under field conditions.

RESULTS

R. microplus engorged females with varying body weights exposed to 150 juveniles of H. bacteriophora resulted in a high control efficacy (97.5% to 98.4%). Tests with females from different geographical populations comprised eight tick strains treated with H. bacteriophora and their respective control groups. The biological parameters of females exposed to nematode treatments did not differ significantly and resulted in 89% to 99% of control efficacy. Trials conducted under field conditions were performed in field plots with Megathyrsus maximus grass. Treatment groups received eight cadavers of Tenebrio molitor fully colonized with H. bacteriophora at 1 week prior to the release of female ticks, whereas control groups were untreated. On the first day of the experiment, six engorged females were distributed in each plot. On day 42 and day 63, the apical portion of the grasses with R. microplus larvae were collected and quantified. The population of R. microplus larvae was reduced up to 73.1% in plots treated with H. bacteriophora at day 63 after treatment.

CONCLUSION

R. microplus engorged females with varying body weights or from different geographical populations were highly susceptible to H. bacteriophora. The field test demonstrated the efficacy of H. bacteriophora in reducing R. microplus larvae in infested pastures. © 2022 Society of Chemical Industry.

The role of complement in the tick cellular immune defense against the entomopathogenic fungus Metarhizium robertsii

Entomopathogenic fungi (EPF) have been widely explored for their potential in the biological control of insect pests and as an environmentally friendly alternative to acaricides for limiting tick infestation in the field. The arthropod cuticle is the main barrier against fungal infection, however, an understanding of internal defense mechanisms after EPF intrusion into the invertebrate hemocoel is still rather limited. Using an infection model of the European Lyme borreliosis vector Ixodes ricinus with the EPF Metarhizium robertsii, we demonstrated that ticks are capable of protecting themselves to a certain extent against mild fungal infections. However, tick mortality dramatically increases when the capability of tick hemocytes to phagocytose fungal conidia is impaired. Using RNAi-mediated silencing of tick thioester-containing proteins (TEPs), followed by in vitro and/or in vivo phagocytic assays, we found that C3-like complement components and α2-macroglobulin pan-protease inhibitors secreted to the hemolymph play pivotal roles in M. robertsii phagocytosis.

Entomopathogenic Fungi and Bacteria in a Veterinary Perspective

Simple Summary

Several fungal species are well suited to control arthropods, being able to cause epizootic infection among them and most of them infect their host by direct penetration through the arthropod’s tegument. Most of organisms are related to the biological control of crop pests, but, more recently, have been applied to combat some livestock ectoparasites. Among the entomopathogenic bacteria, Bacillus thuringiensis, innocuous for humans, animals, and plants and isolated from different environments, showed the most relevant activity against arthropods. Its entomopathogenic property is related to the production of highly biodegradable proteins. Entomopathogenic fungi and bacteria are usually employed against agricultural pests, and some studies have focused on their use to control animal arthropods. However, risks of infections in animals and humans are possible; thus, further studies about their activity are necessary.

Abstract

The present study aimed to review the papers dealing with the biological activity of fungi and bacteria against some mites and ticks of veterinary interest. In particular, the attention was turned to the research regarding acarid species, Dermanyssus gallinae and Psoroptes sp., which are the cause of severe threat in farm animals and, regarding ticks, also pets. Their impact on animal and human health has been stressed, examining the weaknesses and strengths of conventional treatments. Bacillus thuringiensis, Beauveria bassiana and Metarhizium anisopliae are the most widely employed agents. Their activities have been reviewed, considering the feasibility of an in-field application and the effectiveness of the administration alone or combined with conventional and alternative drugs is reported.

Entomopathogenic Fungi for Tick Control in Cattle Livestock From Mexico

Ticks are one of the main economic threats to the cattle industry worldwide affecting productivity, health and welfare. The need for alternative methods to control tick populations is prompted by the high prevalence of multiresistant tick strains to the main chemical acaricides and their ecological consequences. Biological control using entomopathogenic fungi (EPF) is one of the most promising alternative options. The objective of this paper is to review the use of EPF as an alternative control method against cattle ticks in Mexico. Metarhizium anisopliae sensu lato (s.l.) and Beauveria bassiana s.l. are the most studied EPF for the biological control of ticks in the laboratory and in the field, mainly against Rhipicephalus microplus; however, evaluations against other important cattle ticks such as Amblyomma mixtum and R. annulatus, are needed. A transdisciplinary approach is required to incorporate different types of tools, such as genomics, transcriptomics and proteomics in order to better understand the pathogenicity/virulence mechanism in EPF against ticks. Laboratory tests have demonstrated the EPF efficacy to control susceptible and resistant/multiresistant tick populations; whereas, field tests have shown satisfactory control efficiency of M. anisopliae s.l. against different stages of R. microplus when applied both on pasture and on cattle. Epidemiological aspects of ticks and environmental factors are considered as components that influence the acaricidal behavior of the EPF. Finally, considering all these aspects, some recommendations are proposed for the use of EPF in integrated control schemes for livestock ticks.