Gardeners the world over dread the appearance of aphids on their plants. There are around 4,000 species of these sap-sucking insects and about 250 are pests that can wreak havoc on crops in a garden or orchard. Gardeners the world over dread the appearance of aphids on their plants. There are around 4,000 species of these sap-sucking insects and about 250 are pests that can wreak havoc on crops in a garden or orchard. Ecology Agriculture Phys.org – latest science and technology news stories