Some information here about the best use of a systemic insecticide (imidacloprid) for pest control... this specifically directed at the use of a soluble product in containers.
The active ingredient in these products (available from various sources and for a long time part of Bayer Advanced Insect Control) is taken up by the plant and confers resistance to certain insects. If it is not taken up by the plant, there will be no effect (except perhaps temporarily in the root zone). You dilute the product in water and apply to the roots when you would normally water. Within a couple of weeks the plant should start poisoning any insects that try to eat it.
Key details:
1. This is not generally a great cure for florid infestations. It works much better in a preventive sense. If you do have a massive bug problem, you need to go in there with a spray bottle and do battle with the bugs first, using soapy water or 70% alcohol as appropriate, to remove and/or kill off the vast majority. Then wait a few days and repeat, before even considering the use of a systemic.
2. This product does not have an immediate effect. It is not taken up by the leaves and generally has no effect until it is taken up by the roots. That process may vary quite a bit in speed from plant to plant, from season to season, and so forth. Your plant needs to be properly awake (not dormant or headed that way) and it needs to have a decent root system. The more awake the plant, the more extensive the roots, the faster the effect. Otherwise you might not see an effect for weeks.
3. The product (or the carrier or both) may fall out of solution or separate when kept in storage, so you need to shake the bottle carefully before you measure anything out. Eyeball the stuff you put in the water. The stuff I use is milky and even at that point. It may settle out even after dilution so when in doubt shake regularly before use.
4. You need to measure and be consistent with the amount of product you use. Follow instructions on the label. I have a bottle of something about 1.5% imidacloprid and I use about 0.5 tbsp per liter. But do your own research, follow the instructions. Too much product may actually give worse results, too little product won't do the job. I recommend writing the final dilution (like the example I just gave) in permanent pen on the outside of the bottle so you will get it right in the future.
5. You need to properly saturate the soil. This is a real issue with succulents, especially the ones that like to grow on the dry side. Their soil may be bone dry at watering time, and that's fine, but bone dry soil is really slow to rehydrate. So water a bit over the whole surface, wait a few minutes, water some more, etc. (the bigger the pot, the more time and passes) until water comes out the bottom. As a general principle for watering succulents, this is good advice, but especially so when you're applying something that will be taken up by the roots.
6. Be aware this is a pesticide and handle it with respect, especially regarding disposal. I have a dedicated squirt bottle I use for systemic treatment and I rinse it carefully after use. I collect the flowthrough from treatment in a saucer and pour it down the drain. Our dog's flea collar contains some ridiculous amount of imidacloprid and it hasn't caused her any grief, but just treat any bioactive material with some caution.
7. Systemic insecticides may be bad for bees or other beneficial insect visitors. Try to avoid treating plants that are flowering or about to flower, if you like bees.
8. Imidacloprid (probably the most widely available product) is light sensitive. You will note it comes in a lightproof bottle. Once you've made your working stock diluted in water, you need to store that bottle in a dark place when you're not actually applying it. I use a low closet upstairs for that purpose but you can also use a lightproof container or put your bottle in a black plastic bag. I don't know what the half-life is in full sun but it's probably on the order of minutes, so it's worth paying attention to this detail. I do not keep any imidacloprid solution longer than a couple of hours (enough time to do the job).
9. There are various factors which may influence the uptake of the product, and therefore its utility in killing bugs. These relate to the water source you are using (is it well water? is it highly alkaline?) and the presence of friendly nutrients. I don't know how much any of this matters, but for my own personal use I acidify our alkaline groundwater to pH 6 and add very dilute nutrients before use. Presumably the conditions that promote healthy root activity would promote uptake of systemic insecticides, but I'm not aware of any specific evidence on the subject.
10. There is a limit on how long the product will continue to work after one application. That may be vastly different for plants in containers vs. plants in the ground. It may be vastly different for very fast growing plants. Generally I would imagine you might want to treat once or twice a year. For plants here that I know will require a preventive treatment, I apply a systemic at the start of spring and the start of fall, roughly.
I hope that helps.
