ALERT ! Wasps and a sudden dearth.

It was obvious today that there is little nectar coming into my hives at the  present moment, stores are reducing and in one alarming case - where wasps have plaguerised a colony - there was no nectar or pollen and the queen has stopped laying completely; this is a colony which still has 5 (14x12) frames of sealed brood and was very strong. I have put fondant on top and reduced the entrance right down to a small hole. Going back just a month there nectar was pouring into all my hives !