I was thinking… Does it make sense to apply a systemic [spray/chemical] to your bonsais in the winter time [Jan/Feb] to head off the infestation of Spider mites that usual happens in April/May time ?
I typically wait for first signs of them then apply a granular systemic. Others may have other approaches.
If you know you have trouble with spider mites at a certain time of year, treating a tree ahead of that time can help. As for the precise timing, the instructions on the pesticide label will say how long it takes for the treatment to become effective.
Like @Cjlopez4 I wait to see signs of infestation before treating. I also overhead water my conifers which can help keep the population down.
I tend to take the preventative approach, spraying my conifers, junipers in particular on a regular basis with water. Prefer to use insecticidal soap two or three times throughout the year. Only turn to specific insecticides if it is a major outbreak.