Huge clouds of the tiny pests have been constantly spotted around the city, sticking to clothes and causing havoc in gardens.
But, why have they suddenly appeared and how long will they be sticking around for?
Put simply, the greenflies have invaded Sheffield because of the weather.
Light winds, warm temperatures and the rapid onset of spring have created the perfect conditions for greenflies in Sheffield.
Greenfly eggs usually hatch in the early spring and begin sucking on sap from leaf joints or the underside of leaves.
But, as the weather gets warmer, the greenflies sprout wings and become mobile; travelling to new plants.
In these favourable conditions, greenflies mate at a much greater rates with one insect able to produce 50 offspring.
Greenflies are hugely affected by the weather; in particular rain, temperature and wind.
The insects can be easily killed by unfavourable weather, such as late spring freezes while rain prevents them from flying across the country.
However, at the moment it seems the greenflies are concentrated in bigger groups around Sheffield.
This is because the wind has not been strong enough to carry them across the country in search of new sources of food.
Gardeners in particular will be hoping to see the back of the greenflies sooner rather than later as they spread diseases from plant to plant.
Sadly, with Sheffield set for balmy conditions over the next few days, it could be a while before they disappear.
Greenflies only disperse when the wind is strong enough to carry them so you may have to resort to other tactics if you're keen to get rid of them.
Soapy water in an empty spray bottle does the trick or there are a number of sprays you can buy.