Aphids in the greenhouse can affect crop appearance and marketability. They can damage plants through direct feeding, the excretion of honeydew, and transmission of viruses. In addition, aphids have become resistant to many insecticides. Aphids enter the green house by 1) migrating into the greenhouse from the outside; thus infecting the crop, 2) moving onto a crop from weeds or adjacent infected crops within the green house, or 3) moving with a crop when it is introduced into the greenhouse. The two most common aphids found in greenhouses are the green peach aphid and the melon aphid.
Source: • Floriculture Indiana Volume 8. No. 2