The effect of two agricultural techniques on populations of the canefield rat (Rattus sordidus) in sugarcane crops of north Queensland

Rattus sordidus (the canefield rat) is a major pest in the sugarcane- growing areas of north Queensland; however, little is known about factors affecting populations. This study investigated the effect of a minimum- tillage practice (green-cane harvest and trash-blanketing) compared with the conventional practice (pre-harvest burn and intensive cultivation) on the dynamics of populations of R. sordidus in sugarcane crops of the Herbert River District, north Queensland, from November 1987 to September 1988 (from planting to harvest of crops). Populations of R. sordidus became established earlier in trash-blanketed crops, but breeding was reduced as a result of the suppression of 'summer grasses', identified as a favoured food source of R. sordidus during the breeding period (January-May). Cover of summer grasses was high in conventional areas, and these areas supported populations characterized by a female-biased sex ratio, a high incidence of pregnancy, females in the most productive age-class, and low turnover rates, relative to populations in trash-blanketed areas. Damage to sugarcane increased once breeding had ceased and the diet of R. sordidus had changed from seed and non-cane vegetation to sugarcane. Damage was similar in trash-blanketed and conventional areas, probably as a result of dispersal of individuals from conventional areas into trash-blanketed areas following the breeding period. Implications for population and damage levels of R. sordidus on a regional scale are discussed.