The potato aphid (Macrosiphum euphorbiae) is of North American origin, but is these days distributed throughout the world. It is a highly polyphagous species with a preference for Solanaceae (particularly potato), but has also been identified on more than 200 plant species from more than 20 families. Various vegetable and ornamental crops grown in greenhouses are colonized, including tomato, aubergine and roses.
Life cycle and appearance of Potato aphid
Aphids have a complex life cycle, with both winged and wingless forms of adults and a great variety in colour. When reproduction is asexual, the young aphids are born as developed nymphs. They immediately start to feed on plant sap and grow rapidly. When reproduction is sexual, the aphids lay eggs that overwinter. In greenhouses reproduction also takes place by parthenogenesis, with unfertilized viviparous females continuing to produce new generations of females. Aphids moult four times before reaching adulthood. With each moult they shed white skin, betraying their presence in the crop.
The adult potato aphid is a large, slender aphid with long, green cornicles, long legs, and a long cauda. Adults are usually green, but may also be pink or red (especially in tomato), depending on the food source. Even the winged forms can be red.
Wingless potato aphids are 1.7 - 3.6 mm long, elongated, with a dark longitudinal stripe running along the dorsal surface. In green individuals, this stripe is dark green, while in pink specimens it is dark red. This longitudinal stripe is specifically characteristic of the potato aphid and is particularly visible in the nymphs. Immatures have a light dusting of whitish-grey wax. The aphids instantly fall off the plant when touched.
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, if the infestation occurs early enough in the season, the death of young plants. Retarded growth and defoliation reduce yield.
Plant sap is rich in sugars, but has a low protein content. Aphids therefore need to extract large quantities of sap to get sufficient protein. The excess sugar is secreted in the form of honeydew, making the crop and its fruit sticky. Black fungal moulds (Cladosporium spp.) grow on this honeydew, contaminating fruit and ornamental crops and rendering them unsuitable for market. At the same time, photosynthesis in the leaves is reduced, affecting production.