What are aphids?
Aphids form a single, very large group of insects: the super- family Aphidoidea, belonging to the order Hemiptera. The aphids discussed here all belong to the family Aphididae, a family containing many species that cause damage in cultivated crops. The most significant aphids that occur in greenhouses are:
- Myzus persicae subsp. persicae (peach potato aphid) and Myzus persicae subsp. nicotianae (tobacco aphid) on various vegetable and ornamental crops.
- Aphis gossypii (cotton aphid), mainly on Cucurbitaceae, but also on chrysanthemums, and sweet pepper.
- Macrosiphum euphorbiae (potato aphid), mainly on Solanaceae and various ornamental crops.
- Aulacorthum solani (glasshouse potato aphid), mainly on Solanaceae and various ornamental crops, e.g. begonia.
Depending on the species, aphids can inflict various types of damage to a crop:
- Nymphs and adults extract nutrients from the plant and disturb the balance of growth hormones. As a result, the plant’s growth is retarded, giving rise to deformed leaves, or even, if the infestation occurs early enough in the season, the death of young plants. Retarded growth and defoliation reduce yield.
- Plant sap has a low protein content but is rich in sugars. Aphids therefore need to extract large quantities of sap in order to get sufficient protein. As a consequence, the excess sugar is secreted in the form of honeydew, making the crop and its fruit sticky. Black fungal moulds (such as Cladosporium spp. and Capnodium spp.) grow on this honeydew, contaminating fruit and ornamental crops and rendering them unmarketable. At the same time, photosynthesis in the leaves is reduced, affecting production.
- The aphid’s saliva can induce strong ‘allergic’ reactions such as malformations of the growing tips.
- Aphids can transmit pathogenic organisms, particularly viruses. Viruses are mainly transmitted by the winged individuals. Potato virus Y (PVY) is transmitted by aphids in tomato in this way, as is the cucumber mosaic virus (CMV) in cucumber.