Fertility control of rodents could be used as an additional approach for the sustainable management of pest populations in agricultural production systems. One potential technique is the use of chemosterilants which induce long-term infertility in female animals. The aim of my research was to assess the effects of an industrial chemical, 4-vinylcyclohexene diepoxide (VCD), on reproduction of female and male laboratory rats (Rattus norvegicus). In addition, the uptake of bait types and basic formulations with potential to deliver a fertility control agent to the ricefield rat, Rattus argentiventer, a target species for fertility control in Southeast Asia, was undertaken.
In female rats, the effects on ovarian follicle populations were assessed after a period of oral treatment (500 mg VCD/kg/day for 10 days), which was then repeated either once or twice at 14 and 21 day intervals. The effect of three periods of VCD treatment on fertility was examined by mating with fertile, untreated males for 4 breeding cycles. In male rats, the effects of a single 5-day period of oral administration (500 mg VCD/kg/day) on reproductive tissues of juvenile and adult rats were assessed at different times after treatment (days 16 to 90). The effect of VCD treatment on the fertility of adult males was determined by mating with fertile, untreated females for 5 breeding cycles. In the bait-uptake study, laboratory evaluations of bait acceptance by ricefield rats were conducted using different physical forms of bait, vegetable oils, food additives and types of cereal. Based on these laboratory results, the most accepted bait formula plus Rhodamine B (RB) as a bait marker was used under field enclosure conditions to determine the proportion of animals consuming bait and the rate of bait consumption per individual.
In females, oral administration of VCD (500 mg/kg) for all treatment schemes induced significant depletion of primordial and primary follicles in the ovaries. Three periods of VCD treatment significantly enhanced ovarian follicle depletion and induced delayed but irreversible infertility. In male rats, a single oral VCD treatment for 15 days did not induce any effects on either reproductive tissues or fertility suggesting an absence of effect on the reproductive status of male rats.
Under laboratory conditions, bait in kibble form, containing cereal-base (1O% broken rice, 30% wheat and 30% rice flours), 5% coconut oil, 20% intralipid (a fatty emulsion), and 5% sugar was shown to be most preferred by ricefield rats. Under field enclosure conditions, this bait formula was highly accepted with approximately 90% of ricefield rats consuming bait, and at a high rate per individual (approximately 9.5% body weight).
This research has demonstrated that VCD has potential as a candidate for fertility control of rats, and that a cereal-based bait formula derived in this study could potentially be used as a carrier for oral delivery of fertility control agents in ricefield rats. In the future, fertility control could be used as an additional integrated tool for improved management of pest rodents and thus could significantly contribute to food security and poverty alleviation programs in developing countries
