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

The current study investigated the thermotolerance of Metarhizium anisopliae s.l. conidia from the commercial products Metarril (R) SP Organic and Metarril (R) 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 (R) 80, 0.01 % (v/v)] or pure mineral oil. Conidia from Metarril (R) SP Organic suspended in water presented markedly delayed germination after heating to constant 40 degrees C (for 2, 4, or 6 h) compared to conidia suspended in mineral oil. Metarril (R) SP Organic suspended in oil and exposed to daily cycles of heat-stress (40 degrees C for 4 h and 25 degrees 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 (R) WP decreased from 96 % when conidia were cultured for 30 min prior to heat exposure (40 degrees 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 (R) is not registered for tick control, it may be useful for controlling R. sanguineus, especially if it is prepared in mineral oil.