Compensation of rodent pests after removal: control of two rat species in an irrigated farming system in the Red River Delta, Vietnam

Rodent pests have a strong capacity to recover rapidly from imposed reductions in abundance, but it is unclear how populations compensate to removal when farmers apply rodent control. The response of two rat species to rodent control was monitored using regular live-trapping in an irrigated lowland mixed rice agroecosystem. Rice field rats, Rattus argentiventer (52% of rodent captures), and lesser rice field rats, Rattus losea (29%), were removed using trap-barrier systems (TBS) at two sites while rice crops were present. The TBS was a plastic fence that enclosed a small field planted 3 weeks prior to the surrounding fields and set with multiple-capture cage-traps to capture rats that were attracted to the early-planted crop. Demographic responses of rats were compared to two untreated sites. There was a reduction in abundance of rodents on treated sites relative to untreated sites during the summer rice crop (by 45% and 28% for R. argentiventer and R. losea, respectively) an increase in abundance during the winter season (31% and 69%), and a mixed response during the spring rice crop (39% decrease and 41% increase). There was an increase in the proportion of juveniles captured on treated sites relative to untreated sites post-treatment (148% and 158%) and the body mass was lower on treated sites post-treatment (males: 13% and 41%; females: 22% and 22%). Older, larger animals were removed by the TBS and the rodent populations were compensating through high recruitment of young and high immigration into treated sites. No clear patterns were observed in the occurrence of adult breeding females for either R. argentiventer or R. losea. Rodent management, should therefore, occur over large areas (> 100 ha) to reduce the chance of reinvasion.