To found out a method for controlling with a biological agrochemical by which a control activity comparable to that of a chemical agrochemical can be expressed, to search for a means of solving other than an improvement in keeping quality and to provide a method for controlling by which control effects can remarkably be enhanced even by the biological agrochemical with a reduced viable cell count by making the best use of advantages which are living things impossible with chemical preparations.

There can be provided the method for controlling plant diseases and insect pests by which the control effects can remarkably be enhanced even by the biological agrochemical having bad keeping quality and the reduced viable cell count according to a method for controlling comprising mixing the biological agrochemical containing control bacteria with an aqueous medium containing a nutrient substance suitable for proliferation of the control bacteria, preparing a treating liquid, then allowing the treating liquid to stand for 3-48 h, proliferating the control bacteria and subsequently carrying out the treatment with the chemical. The control effects of the biological agrochemical containing the control bacteria and reducing the viable cell count from the production stage to the distribution and sites of use in spite of the bacteria having the control ability can remarkably be enhanced.