At just 1mm long, the greenfly is almost impossible to see on its own, but millions in the air at any one time cannot be missed.
They have been landing on shirts and trousers and can be seen flying in large clouds around the towns of Cromwell and Alexandra.
Central Stories museum and art gallery director Brian Patrick, who has long been a student of entomology, said he had never before seen a mass emergence of greenfly.
The greenfly has a strange way of surviving, with the female cloning itself many times during the summer. So prolific is the reproduction process that the female can be gestating an egg which is also beginning to form another generation inside itself.
At the end of the summer a male greenfly appears and sexual reproduction takes place.
That combination gives the population strength, Mr Patrick explained.
‘‘It is an elegant strategy to build up large numbers during the summer, and while the food's there the females can produce millions and millions of clones.''
Mr Patrick is at a loss to explain the phenomena except to say Alexandra is a green oasis in the middle of a dry desert with all the irrigation that takes place during the summer.
‘‘That, combined with a warm dry summer, has meant an excellent breeding season.''
The greenfly is fragile and only lives for a few days.
Mr Patrick said he was surprised there had been no complaints of roses and other flowers and greenery being eaten, as the present mass emergence meant that somewhere during the summer the larvae had been feeding off anything that would sustain them.
