Heat-stressed Metarhizium anisopliae: viability (in vitro) and virulence (in vivo) assessments against the tick Rhipicephalus sanguineus

Abstract

The current study investigated the thermotolerance of Metarhizium anisopliae s.l. conidia from the commercial products Metarril® SP Organic and Metarril® WP. The efficacy of these M. anisopliae formulations against the tick Rhipicephalus sanguineus s.l. was studied in laboratory under optimum or heat-stress conditions. The products were prepared in water [Tween® 80, 0.01 % (v/v)] or pure mineral oil. Conidia from Metarril® SP Organic suspended in water presented markedly delayed germination after heating to constant 40 °C (for 2, 4, or 6 h) compared to conidia suspended in mineral oil. Metarril® SP Organic suspended in oil and exposed to daily cycles of heat-stress (40 °C for 4 h and 25 °C for 19 h for 5 consecutive days) presented relative germination of conidia ranging from 92.8 to 87.2 % from day 1 to day 5, respectively. Conversely, germination of conidia prepared in water ranged from 79.3 to 39.1 % from day 1 to day 5, respectively. Culturability of Metarril® WP decreased from 96 % when conidia were cultured for 30 min prior to heat exposure (40 °C for 4 h) to 9 % when conidia were cultured for 8 h. Tick percent control was distinctly higher when engorged females were treated with oil suspensions rather than water suspensions, even when treated ticks were exposed to heat-stress regimen. Oil-based applications protected fungal conidia against heat-stress. Although Metarril® is not registered for tick control, it may be useful for controlling R. sanguineus, especially if it is prepared in mineral oil.

Increased tolerance of Beauveria bassiana and Metarhizium anisopliae conidia to high temperature provided by oil-based formulations

“…In the present study, we examined the deleterious effects of water temperature and exposure time in water on formulated and unformulated conidia of B. bassiana and M. anisopliae and then on the ability of treated conidia to infect an insect host. Although the negative effect of high temperatures on formulated conidia has been previously described only in shelf-life studies (Alves et al, 2002;Hedgecock et al, 1995;McClatchie et al, 1994;Starthers et al, 1993) and for conidia diluted in mineral oil (Barreto et al, 2016;Alves et al, 2016), we are the first to report on the partial protection provided by emulsifiable oil to conidia against high temperatures in oil-in-water suspensions, with reductions in conidia death and with conidia maintaining infectivity.…”

“…The negative effect of temperature on conidia viability has received particular attention. However, the effect of temperature on conidia formulations has been evaluated primarily in shelf-life studies (Alves et al, 2002;Hedgecock et al, 1995;Moore et al, 1995;Starthers et al, 1993;McClatchie et al, 1994) or with conidia diluted in mineral oil (Barreto et al, 2016;Alves et al, 2016), and little is known about the harmful effects of water temperature and exposure time in tank mixtures immediately before use.…”

Ultrastructural and Cytotoxic Effects of Metarhizium robertsii Infection on Rhipicephalus microplus Hemocytes

“…Here, M. robertsii ARSEF 2575 was virulent to R. microplus engorged females after fungal inoculation. Several factors are involved in the virulence of a fungal isolate to arthropods, especially ticks that are much less susceptible than insects (Rot et al, 2013; Erler and Ates, 2015; Mohammadyani et al, 2016; Alves et al, 2017; Fischhoff et al, 2017). Distinct fungal virulence to different populations of the same host (Perinotto et al, 2012), host species, life stage of the host (i.e., some species and stages are more susceptible than others), and the quantity of fungal propagules applied to the arthropod pest (Alden et al, 2001; Ment et al, 2012) are some of these factors.…”

Mode of Infection of Metarhizium spp. Fungus and Their Potential as Biological Control Agents

“…The effect of oil formulation, oil-in-water emulsion formulation, and water formulation of M. anisopliae conidia has been tested and compared for different hosts. Oil formulated conidia caused higher mortality in hosts compared to water formulated conidia [ 3 , 8 , 25 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 ]. Besides, oil formulated conidia were found to provide thermotolerant protection to the conidia and remain effective even under low humidity conditions.…”

“…The efficacy of M. anisopliae in killing the nymphs and adults of many genus and species of ticks has been demonstrated in many studies for Rhipicephalus microplus , Rhipicephalus sanguineus , Haemaphysalis qinghaiensis , Hyalomma excavatum , and Ixodes ricinus [ 25 , 28 , 76 , 78 , 101 , 102 , 103 , 104 , 105 ]. Although studies have shown that the conidia of M. anisopliae reduced the percentage of egg production and hatchability, the precise mechanism of how M. anisopliae caused a decline in egg production and hatchability is still unknown [ 105 , 106 ].…”