Predator-prey theory suggests that pest damage that occurs over a period of time is related to the initial pest density during this period and the functional and numerical responses of the pest to the resource. Estimating such relationship is therefore fundamental to determining the threshold population density (DT), above which the economic benefits of control exceed the economic costs of control. Although DT has been a theoretical basis for economically sound management of invertebrate pest damage, few studies of vertebrate pest damage have estimated such relationships and none had previously estimated DT. A manipulative experiment was conducted to parameterize a model of DT for managing yield loss (YL) due to damage to wheat caused by mice (Mus domesticus L.) from the milky stage to harvest. DT was determined through estimating the relationship between YL and initial density of mice at the milky stage (DI). This relationship subsumed the functional and numerical response of mice to crops from the milky stage to harvest. The experimental crops were enclosed by mouse proofed pens. The experiment was conducted under condition typical of wheat fields to emulate natural habitat conditions for mice and crops. The estimated relationship was asymptotic exponential: YL increased almost linearly with DI until apparent competition between mice occurred at densities in excess of about 500 mice ha-1; and apparent competition limited further increases in YL at densities in excess of 2,133 mice ha-1. DT varies depending on the effectiveness of the control method in reducing DI and the cost of control as a percentage of the farm-gate value of wheat. For example, DT was 89 mice ha-1 for triggering aerial zinc phosphide baiting if this method was effective in reducing DI by 41.15% and given the cost of baiting was 7.54% of farm-gate value of wheat. It is recommended that mouse densities be monitored well before the milky stage so that the density a week before this stage can be forecast by the trend of monitored densities. If the forecast DI exceeds DT, management should be implemented at least a week before the milky stage (allowing a week for control such as baiting to take effect). However, if the forecast DI is much higher than DT so that even after control the reduced density would still be higher than DT, then additional control may be applied earlier so that the control applied a week before the milky stage would reduce the density below
