The problem of using any insecticide for this problem is that the insects you are hoping to destroy are not on the outside surfaces of the plant, but are feeding/mining in the soft non-cellulose portions of the leaves, where the plant's vascular system is located. Surface insecticides will not kill the bugs, this is why treating the soil is recommended. Normally, this would not be a problem for most vegetative plants, but I suspect that plant uptake of insecticide (which is what is needed to kill the bugs) may be harmful to the person consuming the weed, since the insecticide will very likely also be within the cannabis buds. Use of organic insecticide (see wikipedia entry on leaf miners) might be a bit safer, but I'm not sure how long it will remain within the plant... will it still be there at harvest? I don't know.