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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?

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The answer to this question depends on a number of factors. I am going on published fact sheets and research but how long Imidacloprid lasts in the environment is subject to new research frequently. The latest news is that it seems when this product is used as a flea collar that is marketed as fipronil for dogs and they go for a swim in a river

that it degrades to compounds that are more persistent in the environment, and more toxic to most insects, than fipronil itself.

That would be an unexpected consequence so a cautious approach is recommended. As of November 2020 a fact sheet for the product indicates

Imidacloprid can last for months or years in soil. The residues become more tightly bound to the soil with time.

Imidacloprid is broken down rapidly by water and sunlight. The pH and temperature of water affect the speed of the imidacloprid breakdown process.

Imidacloprid may leach from soil into groundwater under some conditions. Imidacloprid is broken down into a number of other chemicals depending on which bonds in the molecule are broken.

Other factors which affect the breakdown of the product are quoted from the reference:

  • As the organic carbon levels and laminar silicate clay content in the soil increases, the potential for imidacloprid to leach would decrease (Cox et al., 1997, 1998).
  • Organic fertilizers, such as chicken and cow manure, increased the pesticide adsorption to the organic matter and also increased its half-life.
  • Plants readily absorb imidacloprid through the roots.
  • In the absence of light, the longest half-life of imidacloprid was 229 days in field studies and 997 days in laboratory studies (Miles Inc., 1993).

You asked how long before you can return the plants to the garden. Using the "abundance of caution" approach I recommend one year to completely allow the treatment to degrade.

Do not compost any clippings or soil from the plants that have been treated.

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