Every year an area about 5 square foot in my front garden is COMPLETELY taken over by Nasturtians, a beautiful display of yellow orange and red flowers. Bees hum over it for weeks, as do butterflies, the cabbage whites especially. When the caterpillars start to hatch slowly the leaeves are eaten away. Then suddenly, as the caterpillar colony have nearly all hatched and are all in caterpillar stage the leaves and Nasurtians disappear in a space of 2 days, litereally, just hundreds of caterpillars eating leaves and leaving lots of stalks sticking up with the odd flowers left. It is a well known phenomena in my area, and children knock on the door every year asking if they can collect some caterpillars, I even have to ration them there are so many children come to see them.
This year, I have seen personally only SIX 6 caterpillars on the leaves. Only 2 cabbage white butterflies, and one brown one that disappeared before I could identify it. The Nasturtians are still in full swing, not many bees but some.
Something has gone badly wrong with the cabbage white this year. My back garden is 100' long and usually a haven for all butterflies, I have only spotted about 6 !
A few days ago I did notice some black husks that looked like dead baby caterpillars (1/4" long max) - but I collect the Nasturtian leaves to feed my tortoises and to add to salads and always check for the butterfly eggs, again, I have only seen a couple of patches of eggs this year whereupon usually I have trouble finding leaves that HAVENT got butterfly eggs on - so this is a massive change this year, very dramatic from my point of view.