Stage-specific susceptibility of Tetranychus urticae and Tetranychus merganser to Beauveria bassiana and Metarhizium anisopliae and their effects on reproductive traits.

Tetranychus urticae and Tetranychus merganser are economically important phytophagous mites for which sustainable control alternatives are required. Entomopathogenic fungi, such as Metarhizium anisopliae and Beauveria bassiana, have been proposed as biological control agents. However, information on stage-specific susceptibility and reproductive effects in these species remains limited. This study evaluated the susceptibility of eggs, larvae, nymphs and adult females of T. urticae and T. merganser to isolates of M. anisopliae and B. bassiana under laboratory conditions, and assessed their effects on oviposition and offspring sex ratio. Eggs and larvae of both mite species showed low susceptibility, with no significant differences between mite species or fungal isolate; mortality never exceeding 20%, indicating limited efficacy against early developmental stages of mites. At the nymphal stage, significant differences were detected only in T. urticae, where isolate Ma129 (M. anisopliae) caused higher mortality than B. bassiana isolate Bb88, although overall mortality remained below 20%. In contrast, adult females were the most susceptible stage, exhibiting approximately 70% mortality, with no significant differences between mite species or fungal isolates. Fecundity differed significantly between mite species; T. urticae females produced more eggs than T. merganser. However, within each mite species, fungal isolates did not significantly affect oviposition compared with the control. Female longevity was the principal determinant of total fecundity, with an estimated increase of 6.98 ± 0.17 eggs per additional day of survival, confirming the strong relationship between lifespan and reproductive output. Offspring sex ratio was female-biased (> 70%) and not influenced by fungal isolate or mite species, although it varied significantly over time. These results demonstrate clear stage-dependent susceptibility, with adult females representing the most vulnerable stage, while early developmental stages exhibited substantial tolerance. Under the conditions tested, the fungal isolates did not induce detectable sublethal effects on reproductive output or progeny sex ratio, highlighting the importance of targeting life-stage when incorporating entomopathogenic fungi into mite management strategies.