A promising biological agent to control the carrot weevil.

The carrot weevil, Listronotus oregonensis, is an important pest of carrots causing yield losses of up to 50%. Among the natural enemies that can control its populations, the parasitic nematode, Bradynema listronoti, has been identified in an infested carrot field in Quebec. Population dynamics of both species under field conditions was followed over a period of 16 years. The mechanisms and outcomes of this parasitic relationship were analyzed, at both physiological and molecular levels. The infection rate of carrot weevil adults by B. listronoti under field conditions was high (>40%) and relatively constant over the 16 years of survey. Under controlled conditions, all weevil instars exposed to an inoculum of B. listronoti were found susceptible, larvae being more vulnerable (57 ± 8% infected) compared to pupae (4 ± 3 % infected) and adults (7 ± 8% infected). The fertility of infected females was greatly affected (60-fold reduction) due to an inhibition of the maturation of the reproductive system. Transcriptomic analyses revealed that this parasitic castration may have been triggered by the inhibition of reproductive hormones production. Although sterile, the females still attempted to oviposit and, doing so, deposited nematodes at the oviposition site which increases the transmission of the parasite. Because B. listronoti has a low impact on carrot weevil mortality, it is more likely to be spread on a wider area by weevil movements. The B. listronoti – L. oregonensis interaction represents a case of parasitic castration with a very interesting potential for biological control against this pest.