This is actually Very common When Flowering the Bottom of the plant will start to turn yellow. I looked at the Pic of your plants and they look pretty health. But something I did notice is your pots seem a bit small and this could be turning the leafs yellow also. It happens when the Roots run...
There is a few things you can do to save your Plant, yes the Sand works. Only put about a 1/2-1 inch on the top of the Soil. This will Surely rip apart any More of the larva that hatch. You can Also Put raw patato stips on the top of the soil and leave them there for a day or too and remove and...