I haven't had this pest, but a dense mealybug infestation in fine-spined cactus can be dealt with by submerging the entire plant in soapy water for a day or two, then draining and allowing to dry. The insects drown. The soap is necessary to wet the waxy coat on most insects. This might work for burrowing larvae. I don't know how long larger grubs can survive underwater - any entomologists willing to chip in? Leo Martin Phoenix Arizona USA