. . . . . . . . is an important major pest species in the complex of pests attacking a crop.

Key pest is an important major pest species in the complex of pests attacking a crop.
A pest is any animal or plants harmful to humans or human concerns. It is any living thing, whether animal, plant, or fungus, that humans consider troublesome to themselves, their possessions, or the environment.
Agricultural and horticultural crops are attacked by a wide variety of pests, the most important being insects, mites, nematodes and gastropod molluscs. The damage they do results both from the direct injury they cause to the plants and from the indirect consequences of the fungal, bacterial, or viral infections they transmit.