Harnessing environmental microbiological interventions with micro- and macroorganisms for assessing cattle tick management

Abstract

Rhipicephalus microplus, commonly known as the cattle tick, is responsible for causing severe economic losses in livestock production in several countries. The utilization of entomopathogens in infested pastures may represent a sustainable and eco-friendly alternative for tick control. This study evaluated the effectiveness of combining entomopathogenic fungi (EPF, Metarhizium spp.) and entomopathogenic nematodes (EPN, Heterorhabditis bacteriophora) for controlling R. microplus. Laboratory assays tested sub-doses of M. robertsii IP 146 conidia in combination with H. bacteriophora HP88 infective juveniles against engorged females of R. microplus, whereas field trials assessed a granular formulation containing M. robertsii microsclerotia applied with H. bacteriophora infective juveniles in infested field plots to target the tick's non-parasitic phase during rainy and dry seasons. In laboratory experiments, the co-application of fungal sub-doses and nematodes demonstrated synergistic effects, significantly enhancing tick control. Field applications during the rainy season achieved tick population reductions of 54.09% (M. robertsii), 38.11% (H. bacteriophora), and 46.72% (combination). During the dry season, only the fungal formulation significantly reduced tick populations, with 26.27% efficacy. These findings underscore the potential of EPF and EPN, either singly or in combination, as complementary tools to traditional chemical methods for sustainable cattle tick management.

Keywords: Biological control; Entomopathogenic fungi; Entomopathogenic nematodes; Field trial; Granular formulation; Microsclerotia.

Fig. 1

The area containing 1 m² plots of Megathyrsus maximus BRS Quênia used in the field test at Embrapa Arroz e Feijão, Santo Antônio de Goiás—Goiás, Brazil. Location: 16°30′50.1" S and 49°16′33.8" W. A) test performed in the rainy season and B) in the dry season.

Fig. 2

The fungus Metarhizium robertsii IP 146 and the nematode Heterorhabditis bacteriophora HP 88 applied to field plots under challenging environmental conditions show promise as biological agents for controlling the non-parasitic phase of the tick Rhipicephalus microplus in cattle pastures: 1) Production of fungal granules or nematode suspension; 2) Application to field plots; 3) Deposition of engorged females to treated plots; 4) Larval hatching; 5) Larvae collection, counts and analysis. The images in a and b were sourced from free stock media websites (pixabay.com and designi.com.br, respectively), while the additional schematics were created using BioRender.com.

Fig. 3

Fungal development of different groups of granular formulations of Metarhizium robertsii IP 146 in water-agar medium or non-sterilized soil. A) Mean conidial production (± SE) of different granular formulations sporulated on water-agar medium and suspended in 10 mL 0.01% Tween 80; B) Colony forming units per gram of soil (± standard error) of different granular formulations sporulated on non-sterilized soil. Bars (± SE) followed by different letters differ significantly from each other (p < 0.05).

Fig. 4

Impact of treatment performed in the rainy season with entomopathogenic fungus (EPF) and nematode (EPN) applied separately or in combination in the population of tick larvae recovered from pastures. A) Segplots represent the mean (± SE, n = 8) for each evaluation week. Means followed by different letters within the same week differ significantly from each other (p < 0.05) based on the Tukey HSD test. B) Segplots represent the mean (± SE, n = 64) of ticks recovered for each treatment over all weeks of evaluation. Means followed by different letters differ significantly (p < 0.05) based on the Tukey HSD test. No significant differences between means are indicated by “ns”.

Fig. 5

Impact of treatment performed in the dry season with entomopathogenic fungus (EPF) and nematode (EPN) applied separately or in combination in the population of tick larvae recovered from pastures. A) Segplots represent the mean (± SE, n = 8) for each evaluation week. Means within the same week did not differ significantly (p ≥ 0.05) based on the Tukey HSD test. B) Segplots represent the mean (± SE, n = 72) of ticks recovered for each treatment over all weeks of evaluation. Means followed by different letters differ significantly (p < 0.05) based on the Tukey HSD test. No significant differences between means are indicated by “ns”.

Fig. 6

Fungal persistence in the soil from field plots treated with granular formulation of Metarhizium robertsii IP 146 (EPF) or combined with the nematode Heterorhabditis bacteriophora HP88 (EPN) in the two seasons (dry or rainy). Box plot of colony forming units (CFU) of M. robertsii IP 146 per gram of soil obtained after fungal application under field conditions. The boxes show the median, 25th, and 75th percentiles, while the error bars show the 10^th and 90^th percentiles and the means (n = 8) are the black diamonds.

Fig. 7

Persistence of nematodes in the soil from treated field plots. Persistence was evaluated by assessing the percentage of larvae of Tenebrio molitor infected by H. bacteriophora HP88 (n = 10) exposed to the soil from plots (n = 8) treated with infective juveniles of H. bacteriophora HP88 (EPN) or its combination with granular formulation of M. robertsii IP 146 (EPF) in the dry season. Means (black diamonds) followed by p < 0.05 on the same assessment day differ significantly. The boxes show the median, 25th, and 75th percentiles, while the error bars show the 10th and 90th percentiles.