In my friend's large garden, mealybug is the main pest. They spread quickly, often killing whole plants if left unattended. Sometimes they even attack trees.
The garden and many of its affected plants are rather large, so the process can't require tedious, leaf by leaf removal or application, or anything like that.
My friend has several edible plants and is conscious of the local environment. Therefore, we prefer answers that are as eco-friendly as possible.
The climate is arid tropical, except for periods of heavy rains once or twice a year. We are in southern India.