You've done the hard work. The infested plants are isolated and have been treated. But now what? You've got leftover mixture in your sprayer. Can you store it to use for the next treatment? And if not, is there a safe way to dispose of it?
Plan Ahead
Firstly, the obvious advice (which isn't much help AFTER the fact), but the best solution is to prevent having leftover pesticide in the first place. Make up a LOT less than you think you'll need!
I say a lot less, because you'll be surprised how far a few sprays of pesticide go once mixed up. Especially if you've put the mixture in one of those extra-fine mist sprayers (like the Mister 360 or Egmont 360). Typically, coverage is only recommended to the point of run-off, so a little usually goes a long way.
By the way, the point of run off means when the mixture just starts to drip off the leaves.
As a general rule, I like to make up no more than 250mls at a time (that's about a cup of water). If your directions are based on an amount of mls per litre of water, divide that amount by 4 and use that instead. So instead of 10mls per 1 litre water for example, you'd use 2.5mls per 250mls (1 cup) of water.
A 5ml pipette is always worth having on hand for these smaller measurements, but if you already use one for your fertiliser, don't cross-contaminate and use it for your pesticide also!
And yes, it DOES matter that you get amounts exact. Typically the amount of pesticide will kill pests without harming the plant so you don't want to make it too strong. Going under the recommended amount may end up being less effective at saying bye bye to the bad bugs, but also require more repeat treatments and could lead to pests becoming resistant.
Always check the label. Pesticides will often have a maximum number of applications per 6 or 12 months to avoid pests becoming resistant. Groventive for example is 4 treatments in 6 months. BioNeem is 4 applications per year. That's why it's not recommended to use Groventive as a regular preventative (or if you do, beware of that maximum).
How long can you store pesticides once mixed up?
Unlike how long you can store fertiliser, pesticide should always be mixed up and used fresh. It depends on the pesticide, but in general most will start to break down, separate and and be less effective within 24 hours.
Exactly how long depends on a few factors, including the water used, the temperature it's stored at, the pH of the mixture (the more alkaline the faster it breaks down), any fungi or bacteria that get into the mixture, exposure to sunlight and more.
Keep in mind the more eco-friendly the pesticide, the shorter the life after mixing and applying. This reduces harm to the environment. Some will have stabilising agents or preservatives added if bought in a ready-to-use form but if you've mixed from a concentrate, that typically has a shorter life once mixed.
Once applied, never leave pesticides in a spray bottle
Unless you bought the pesticide in a spray bottle in a ready-to-use premixed formula, never store pesticides in the spray bottle used. That's because pesticides from concentrates tend to separate and can clog the finer parts of a spray mister, causing it not to work next time you go to use it. Always clean out spray misters directly after use. You can tip out the rinse water in an area of the garden that won't end up with the pesticide going into the waterways.
How to safely dispose of leftover pesticide
You've made up too much and have leftovers. Now what? If you read the label (which you ALWAYS should before use), you'll see pesticides normally warn to keep them out of any waterways.
That means no washing leftover pesticide down the sink or flushing it down the toilet. Even tipping them out in the yard needs to be considered to make sure the run-off doesn't go into the waterways.
Find other plants
What I normally do instead is find another suitable plant or plants (normally outside), to spray the leftovers on. If the pesticide you're using is for indoor plants, it can typically be applied to ornamental plants, roses and shrubs outside too BUT (again), always check the label first.
As a general rule with pesticides, if the label doesn't specifically list the type of plant, it cannot be applied to it. Fruit, herbs and vegetables are common ones to avoid unless the label says it is okay. And if the label does say it is okay, look up the withholding period before you eat anything you've sprayed.
If you spray it on a flowering plant, avoid buds and flowers as a general rule, ideally only apply to plants not in flower. And, as with pesticides in general, spray when bees are not active, such as early morning or late evening.
The withholding period is how long you should wait between the last treatment and consuming the crop or plant.
Check with your council
There are businesses that take commercial amounts of pesticides, but for the garden, your council will normally have information on safe disposal of household chemicals. Here's what my local council advises > which is to drop it off to a local, council-provided hazardous waste point.
How to safely dispose of pesticide containers
You've used up your pesticide, now what? For pesticides, first use up the contents according to the label. Then rinse the container with water. Dispose of the rinse water either in an area of ground where it won't go into the waterways, or again, spray permitted plants with the rinse water. Repeat this again twice more (three times in total). Then the container can go in with the general waste.