A number of aphid species may become pests of greenhouse crops; however, these are usually seasonal in nature and more common in spring and early summer as temperatures begin to increase. Aphids are small, approximately 1–3 mm in length, with a soft, oval body (Fig.1.). Colour of these pests is dependent on species and may range from light green to yellow, tan or black. Common species infesting greenhouse crops include the green peach aphid (M. persicae), the melon/cotton aphid (Aphis gossypii), the chrysanthemum aphid (Macrosiphoniella sanborni), the potato aphid (Macrosiphum euphorbiae), the rose aphid (Macrosiphum rosae) and the foxglove aphid (Aulacorthum solani) (Parrella and Lewis, 2017). Damage caused by aphids include weakening of the plant through removal of plant sap, secretion of sticky honeydew and the development of sooty mould on foliage and fruit, transmission of virus diseases, and for some species, severe distortion of the newer leaves as a result of feeding damage (Fig. 2.). Hydroponic crops such as capsicum are particularly sensitive to developing twisted and deformed foliage in the tops of the plant as a result of aphid feeding.
Early infestations of aphids are often seen in proximity to vents and other entry points where winged adults fly in from outside. Once established, populations of aphids may consist of both winged and wingless forms and the life cycle can be complex. Aphid infestations typically originate from host plants outside the greenhouse, with only a few individuals required to start an infestation because aphids reproduce parthenogenically with young developing within the parent. Under high population pressure, winged forms then develop as a result of overcrowding and spread the population further within the crop. Aphids are attracted to the young growing points of most plants and may be found in the centre of lettuce heads and on the undersides of young leaves.
Control of aphids is relatively straightforward as most remain susceptible to a number of systemic insecticides as well as growth regulators and biological controls including entomopathogenic fungi and insect predators and parasites. The use of yellow sticky traps positioned close to vents and entry ways may be employed to provide an early indication of the arrival of winged adults into a growing area (Fig. 3). Important insect predators and parasites include the gall midge (Aphidoletes aphidimyza), the ladybird (Adalia bipunctata), green and brown lacewings, and parasitic wasps of the genus Aphidius, which appear to be the most effective (Parrella and Lewis, 2017). As with all natural controls, a balance needs to be maintained between pest and beneficial insects for effective control.
Reference : A Practical Guide Hydroponics and Protected Cultivation by Lynette Morgan