When the Scrin arrived on Earth during the Third Tiberium War, they brought their own harvesting technology with them. Scrin Harvesters, like many of their vehicles, are bio-mechanical constructs that hover and are able to absorb Tiberium through a 'mouth', with the aid of 'claws' to bring the crystal closer. In addition, they appear to self-repair wherever Tiberium is present.
When the invasion started, due to the Scrin expecting the planets inhabitents to have mostly died out.Scrin harvesters were weaponless against the human population and not suited for combat over tiberium fields in a three-way war with GDI and Nod on Earth. It seems that the Supervisor and Overlord agreed to build up Warp Spheres and Extractors to mine the natural resource. New harvesting deployments were found in Cologne, Africa, Berne and Italy.
The Scrin harvester was first used in London to collect Tiberium from barricaded fences as GDI vehicle and troop productions try to fend them off and destroy Foreman 371's base since its establishment after fifteen civilian buildings were destroyed by savage and brutal force. After their absence in Munich, they came to Croatia and lastly Threshold 19 to carry Tiberium from the field to their Extractor as Foreman 371 tries to do his objectives against GDI and Nod in the theatres of operation.
A harvester comes free of charge with every new refinery. It automatically starts searching for available Tiberium fields and collecting crystals before returning back to the refinery to have them processed. When lost, warp spheres and warp chasms can build a new replacement.
They are completely unable of defending themselves from enemy attacks.
As Scrin units can heal themselves in tiberium field, while harvesters are gathering tiberium, they can repair automatically making them last longer than other harvesters.