More than 300 Chalkhill blue butterflies were recorded at Hexton Chalk Pit near Hitchin
A wildlife trust said some species had been spotted in nature reserves this summer for the first time in years.
Volunteers for the Herts and Middlesex Wildlife Trust recorded a great white egret near Ware and more than 300 Chalkhill blue butterflies at Hexton Chalk Pit near Hitchin, where they also found a glow-worm for the first time in 25 years.
The trust said it was partly due to the unusually hot weather but also the result of increased surveying and monitoring in the county.
Senior reserves officer Dan Townsend said: "This summer we've had some extraordinary records, from species that haven't been recorded in Hertfordshire for decades to new discoveries for individual reserves."
A great white egret was recorded at Amwell near Ware
The St Albans-based trust manages more than 40 nature reserves covering nearly 810 hectares (2,000 acres) in Hertfordshire and Middlesex.
It said grasshoppers had flourished during the hot summer, while butterflies had been able to complete their life cycle more quickly and emerge in large numbers.
By contrast, bumblebees and hoverflies had struggled, the trust added.
Its reserves have provided the setting for several significant discoveries by the Herts Invertebrate Project, external, which was set up in 2015.
The trust said grasshoppers had enjoyed this year's particularly warm weather
At the end of June, the project visited Fir & Pond Woods, near Potters Bar, and recorded Euides basilinea, a planthopper associated with reeds, which had no previous records in Hertfordshire.
At Aldbury Nowers, near Tring, the leafhopper Zygina hyperici was recorded for the first time in the county since 1916.
Arboridia parvula, a leafhopper associated with Common Rockrose, was also recorded for the first time in Hertfordshire since 1976. The survey also uncovered Agyneta mollis, a tiny nationally rare money spider.
Jersey Tiger moths, once more strongly associated with the Channel Islands, have increasingly been recorded further north, while Hummingbird Hawk-moths are also being seen regularly in Hertfordshire.
