Reduced Fecundity in Free-Ranging Norway Rats After Baiting With a Liquid Fertility Control Bait

Norway rats (Rattus norvegicus) cause extensive crop loss, infrastructure damage, and are vectors for zoonotic diseases. Due to reduced efficacy, environmental and animal welfare concerns related to traditional pest management tools, such as rodenticides, it is imperative to find new methods for controlling commensal rodents. Fertility control is emerging as a safe, humane, and effective method of long-term population management. Vinylcyclohexene diepoxide (VCD) and triptolide are 2 compounds that are known to target and inhibit ovarian function. Furthermore, triptolide debilitates spermatogenesis and sperm motility. We prepared liquid bait containing no active ingredients (control) or containing VCD (0.098%) and triptolide (0.0012%, treatment) and offered ad libitum for 56 days to male and female Norway rats placed in open, indoor arenas and allowed to breed for 4 rounds (a total of 138 days). The first 3 breeding rounds of treatment-matched matings produced fewer pups in the treatment rats compared to control rats (P < 0.001). In the fourth breeding round, control rats were cross-bred with treatment rats. There were no differences in pup numbers between these cross-breeding groups, but the litter sizes in both groups were smaller than those seen in the control female/control male matings. In addition to reduced pup numbers, treatment males and females exhibited decreased reproductive organ weights without any effect on adrenal, kidney, spleen, and liver weights compared to control rats. Use of a liquid contraceptive bait containing VCD and triptolide may be a suitable alternative to the traditional pest management tools used to control wild rat pest populations.