There used to be loads of small tortoiseshell butterflies fluttering around our gardens, but in recent years their numbers have fallen, and experts think they know why...
They think this insect - the Sturmia bella fly - could be to blame. It was first spotted in the UK in the 1990s, and it's possible that it arrived in Britain because of climate change.
The fly lays eggs on the nettles the tortoiseshell caterpillars feed on. The baby butterflies eat the eggs, but when they hatch the little caterpillars are killed.
Experts are really worried about the winged creatures. They say numbers of the small tortoiseshell butterflies have fallen by up to 80% in some parts of the country.
Tony Hoare said: "The small tortoiseshell is one of my favourite butterflies, and I feel really sad that it's almost disappeared from the garden."
And they're not the only butterflies under threat, with the numbers of lots of other types of British butterfly also falling.