Reproductive responses of rice field rats(Rattus argentiventer)following treatment with the contraceptive hormones,quinestrol and levonorgestrol

The ricefield rat,Rattus argentiventer,is a significant pest of rice in Southeast Asia.Fertility control methods have the potential to provide safe and effective alternatives to control methods that often include indiscriminate use of rodenticides or electric barriers.The aim of this laboratory study was to assess uptake of bait coated with different concentrations of the contraceptive hormones,quinestrol(E)and levonorgestrel(P),delivered alone and in combination(i.e.EP-1)and determine the short-term effects on reproductive parameters of adult male and female R.argentiventer.In Experiment 1,2 concentrations of E,P,and EP-1(10,20 ppm)were fed to groups of wild-caught rats for 7 days.In females,both E and EP-1 induced uterine edema.In males,EP-1 reduced epididymis and seminal vesicle weights and lowered sperm motility.However,these responses were inconsistent due to low bait acceptance,especially with increasing concentrations.In Experiment 2,EP-1(0,20,50,100 ppm)was administered by oral gavage daily for 7 days to male R.argentiventer.There were significant reductions in epididymal and seminal vesicle weights for all oral doses of EP-1,in sperm counts for the 50 ppm dose,and in sperm motility for the 20 and 50 ppm doses compared to the control group.To select the optimum dose of EP-1,we must address the poor acceptance of contraceptive-coated baits by ricefield rats.Further research is required to improve the palatability of EP-1 and to test its uptake underfield conditions.

The stadium effect:rodent damage patterns in rice fields explored using giving-up densities

Rodents are globally important pre-harvest pests of rice.In Southeast Asia,rodent damage to growing rice crops is commonly concentrated towards the center of rice fields,away from the field edge,resulting in a clear pat­tern known as the“stadium effect.”To further understand this behavior of rodent pests and to develop recom­mendations for future research and management,we examined the relation between giving-up densities(GUDs)and damage patterns.In Tanay,Luzon,Philippines,GUD trays containing pieces of coconut in a matrix of sand were placed at 4 different distances from the field edge to quantify the perceived risk of predation in a rice field pest,Rattus tanezumi.GUDs were recorded during a dry and wet season crop at the reproductive and ripening stages of rice.In addition,assessments of active burrows,tracking tile activity and rodent damage to the rice crop,were conducted in the dry season.GUDs were significantly lower in the center of the rice fields than on the field edges,suggesting that rodent damage to rice is greater in the middle of rice fields due to a lower per­ceived predation risk.Furthermore,this perception of predation risk(or fear)increases towards the field edge and was greatest on the rice bund,where there was no vegetation cover.We discuss the implications for rodent management and rodent damage assessments in rice fields.This is the first documented use of GUDs in a rice agro-ecosystem in Asia;thus we identify the challenges and lessons learned through this process.