I use them, very effective but don't use until the end of march, when it's warmer, they don't keep for long as they are a living organism, you then have to keep them in the fridge until you are ready, then mix with water, the packet tells you what to do, after applying you have to run a hose pipe over the ground because some will be on the leaves, they must be in the ground, if the
weather is dry, they need watering, they thrive in damp soil.
They will only last for 6 weeks though.
Last year amazon was the cheapest place to buy, I haven't looked this year.