Parasitic castration and its physiological consequences: Insights from parasitism of the carrot weevil (Listronotus oregonensis) by the nematode Bradynema listronoti

Parasitic castration represents a strategy by which parasites suppress host reproduction by consuming gonads, disrupting the endocrine system, or indirectly manipulating host resource allocation. The nematode Bradynema listronoti Zeng parasitizes both sexes of the carrot weevil, Listronotus oregonensis (LeConte), but induces castration only in females. This study investigates the physiological and morphological impacts of B. listronoti parasitism on host developmental time, size, lipid metabolism, and supercooling point (SCP). It was hypothesized that castration would induce gigantism or enhance freeze tolerance in host females through reproductive resource reallocation, but not in males whose reproduction is unaffected by parasitism. Parasitism significantly prolonged host developmental time by 7% in both sexes, but it did not affect body weight in either males or females. Lipid analysis revealed that parasitized individuals had significantly lower total lipid reserves (-30%) and contained novel eicosanoids, suggesting shifts in lipid metabolism or incorporation of nematode-derived fatty acids. SCP were largely unaffected by parasitism, except in spring-collected weevils, where parasitism reduced SCP by ~19%. Sex effects were only detected in late-summer weevils, when males exhibited ~15% higher SCP than females. These results indicate that B. listronoti imposes physiological costs on its host, affecting development and lipid metabolism similarly in males and females, with limited context-dependent effects on cold tolerance. Overall, these findings highlight the complex effects of parasitic castration and their potential implications for host population dynamics and biological control strategies.