The scales (belonging to the family Coccoidea) have been feasting on my Haworthias for the last year. They were treated with more eco-friendly solutions, as I posted earlier, without success. In fact, the more I tried, the more agressively they fought back, eventually killing few of my plants. This repeated treatment involved disturbing the roots too often as well. Using Imidacloprid was a last resort.
There are few reasons for using a systemic insecticide, and not a spray application: These are ornamental plants, providing neither fruits nor vegetable matter, so food contamination is not an issue. The potted plants can be taken to another location. The plants are small and do not need much insecticide. The solution is absorbed into all parts of the plants, eliminating the needs for full-cover spray application and leaving less material in the soil. Once the solution is absorbed, I can wait three months before next application, if necessary. The garden itself has enjoyed a completely organic treatment since I made it 20 years ago and there was never need for any chemical treatment. My solution to this dilemma is to take the potted plants away from the garden, where excess of the chemical leaching from the drainage hole will drain away from it.
Given that I followed the recommendations for adequate dosage, and I don't expose them to rain (giving the chemical time to be absorbed and not leached away by excess rain), (EDIT) how long is the best time to wait before it is safe to return the potted plants to the garden without the risk of insecticidal residue leaking from the pots into the garden? The potted plants were situated under fruit trees which I don't want to expose to the residues. Does the three-months waiting time apply to safe concentration levels in the ground?