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Many endosymbionts of insects have been shown to manipulate and alter their hosts' reproduction with implications for agriculture, disease transmission, and ecological systems. Less studied are the microbiota of classical biological control agents and the implications of inadvertent endosymbionts in laboratory colonies for field establishment and effects on target pests or nontarget organisms. While native-range field populations of agents may have a low incidence of vertically transmitted endosymbionts, quarantine and laboratory rearing of inbred populations may increase this low prevalence to fixation in relatively few generations. Fixation of detrimental endosymbionts in founding biological control agent populations prior to release may have far-reaching effects. Significant female-biased sex-ratio distortion was found within laboratory populations of the weevil Ceratapion basicorne (Illiger), a classical biological control agent that was recently approved for use against yellow starthistle (Centaurea solstitialis L.). This sex-ratio distortion was observed to be vertically inherited and reversible through antibiotic treatment of the host insect. Molecular diagnostics identified a Rickettsia sp. as the only bacterial endosymbiont present in breeding lines with distorted sex ratios and implicated this as the first reported Rickettsia associated with sex-ratio distortion within the superfamily Curculionoidea., (© The Author(s) 2024. Published by Oxford University Press on behalf of Entomological Society of America.)