Conducted by the Butterfly Conservation, the count shows that numbers of common species, including; Gatekeeper, Comma and Small Copper were down by 40%, 46% and 30% respectively compared to last year.
This comes despite the UK experiencing weather conditions that usually help butterfly species to thrive.
The Small Tortoiseshell saw a 47% drop in numbers and Peacock slumped by 42% with both species recording their second worst years.
Numbers of the colourful Peacock have now dropped from an average of 3.6 individuals per count in 2013 to just 0.5 per Count in 2016, a six-fold decrease over three years.
Participants also saw the lowest number of butterflies per count since the scheme began 40 years ago, with an average of just 12 butterflies spotted.
These figures were even lower than those experienced during the cold and wet disaster summer of 2012 – the worst year on record for UK butterflies.
Reasons why butterflies have struggled despite favourable summer weather conditions are as yet unclear, say the Butterfly Conservation.
Butterfly Conservation’s Head of Recording, Richard Fox, said: “The drop in butterfly numbers this summer has been a shock and is a bit of a mystery. When we have cold, wet summers, as in 2012, we expect butterfly populations to plummet, but that wasn’t the case this year.
“The summer months were warmer than usual, yet most Big Butterfly Count participants saw fewer butterflies. Perhaps the very mild winter had a negative effect, or the cold spring, or perhaps the impacts of intensive farming and pesticides are really hitting these common species now.
“The importance of Big Butterfly Count is that it takes place every year over a long period; the longer it goes on the more we can learn about the causes that are driving the declines and in some cases, increases of our beautiful butterfly species. We are really grateful to the many thousands of people across the UK who do their bit to help butterflies by taking part in the Big Butterfly Count each summer.”
The Red Admiral and Green-veined White were two species which bucked the negative trend to experience good years. The Red Admiral was up 70% compared to 2015, had the largest year-on year-increase of any species and achieved its second highest abundance since the Count began.
Results can be found at www.bigbutterflycount.org
Image: Small Copper in Derbyshire, Getty