How do you keep wasps out of the swimming pool?

Unless you see a hive inside or near the pool then the pool isn't the problem. You are going to have to hunt for the hive. Hives are usually built by the wasps anywhere in trees (long cone-shaped hard-shelled hives and can get massive) and they can also built right on your house under the eaves so check out cabanas, the house, carport, garage or any building structure in the area (it won't be far.) Once you've found them call in an expert and don't try to handle this yourself. Once the hive(s) are destroyed, the problem will go away.

Just in case you aren't use to seeing wasps they have elongated light colored bodies with dark stripes while bees are fat, butter yellow to pale yellow with dark brown or black stripes. Bees will usually leave you alone, but wasps are nasty and will often come around where there is fruit trees, food (if you are eating outside) and actually will attack a person. If you see a wasp move out of the way and try not to swat at it.

Also, while the exterminator is there ask him to check to be sure you don't have mud wasps. Mud wasps burrow underground and don't build hives like the other wasps.

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I live in southern Spain and I am suffering from exactly the same problem. The wasps that we have are known as paper wasps. I have tried all the normal remedies (cut down plastic bottle full of sugar water, purpose built traps etc. (totally ignored) Pastels in a floating dispenser (they loved it and used it as a diving platform). Then I discovered something. WASPS CAN'T SWIM. The land on the surface of the water and use surface tension to stay afloat. Break the surface tension and they drown. To do this is dead easy. Get yourself a garden spray, any type will do, the one i use contains about half a litre of water. Add to the water one or two pumps of liquid hand washing soap or shampoo. When the wasps land on the water squirt them and they will sink into the water and end up in the skimmer filter. It is not a permanent fix as the little s*ds reproduce at an alarming rate but the satisfaction of revenge on the little devils after all the swollen arms and legs I've had make it worthwhile. I have reduced the number of wasps in the pool by about half in two days (The first day i got over 100 of them) and strangely enough the wasps don't think that you are attacking them and so far they haven't attacked me. (No promises that this will be the same for you.) I am not sure what the situation is with the chemical balance in the pool but the amount of soap used is so small i can't see that it would be a problem.