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Classification and host plants
Species: D. plantaginea Pass.
“Phytopathology, agricultural entomology and applied biology” – M.Ferrari, E.Marcon, A.Menta; School edagricole - RCS Libri spa
Host plants: Apple tree.
Identification and damage
Gray Phid (Ash aphid) is one of the most dangerous phytophages for the apple tree. The adults of this insect have a size of about 3 mm, a gray-violet color and are covered with a grayish waxy layer with a powdery and pruinose appearance.
The juvenile stages are clearer, tendentially pinkish. They live in colonies on the shoots and under the leaves; their nutritional bites cause serious and irreversible leaf curling; in the shoots deformations and developmental arrest occur, with consequent loss of the cimal. The gray aphid also stings the flowers and fruit in the fruit setting phase. On the flowers it causes flower abortions and leaking: on the fruit there are serious malformations, moreover they remain smaller and, therefore, are commercially very depreciated. An early attack, during the fruit setting phase, can also cause the fruit drop. The damage described is also aggravated by an abundant honeydew production that damages the vegetation both directly (causes asphyxia, slow burn effect) and indirectly, due to the establishment of saprophytic fungi (fumaggini) which also induce photosynthetic efficiency of the leaves .
The gray aphid is an insect with a dioecious cycle; the apple tree is the primary host, the plants of the genus Plantago are instead secondary hosts.
Linsetto overwinters, on the apple tree, at the egg stage; this is blackish in color and is deposited on the branches of the host plants.
In spring, generally in March-April, the parthenogenetic founders are born, which give rise to generations of fondatrigenie (up to 3-5 generations) always on the Apple tree. The founders give birth directly to the nymphs. These generations attack the shoots from the budding and continue until the beginning of the summer (June and sometimes even July). Subsequently, the first winged migrants appear, who go to the secondary host (gen. Plantago), where they carry out some (always parthenogenetic) summer generations.
At the end of the summer, early autumn, the sessupare are formed, which migrate again to the primary host (Melo), on which the amphigonics originate; the females of these will lay their eggs for wintering.
Gray apple tree aphid - Dysaphis plantaginea Pass. (photo www.sopra.admin.ch)
Damage to fruits caused by gray apple aphid (photo www.inra.fr)
The fight against gray aphid is mainly chemical; The aphid is too dangerous even with a very low presence (the damage and therefore intervention thresholds are very low). The damages, often substantial and irreversible, advise to treat anyway, in a preventive way or at the appearance of the founders, in the pre-flowering or at the end of the flowering (fall of the petals).
In the subsequent stages of the bunches, up to the fruit-nut, it is advisable to sample the shoots; the intervention is convenient even with the presence of only 1 or a few mobile forms, or with honeydew damage. The natural enemies of Aphid are many, however they cannot control the populations of the phytophagus both for its very low damage threshold and for the dyscrasia of the biological cycles (natural enemies are present in mass only in early summer, when the greatest damage is already done). Early spring treatments are therefore not particularly harmful to useful biocoenosis. The natural enemies of this aphid, like any other aphids, are:
- Coccinellid beetles (predators): Adalia bipunctata, Coccinella septempunctata and C. decempunctata, Scymnus spp., Harmonia conglobata and others;
- Chrysopid Neuroptera: Chrysopa carnea;
- Cecidomidi Diptera: Aphidoletes aphidimyza;
- Sirphid Diptera, excellent predators, at the larval stage, of Aphids;
- Parasitoid hymenoptera of the genus Ephedrus and Aphidius;
- Predatory mites of the genus Allothrombium.
The insecticide treatments carried out in the spring-summer months (from May onwards) must take into account the possible presence of these natural enemies; the choice of active ingredients must be made trying to safeguard the presence of useful biocoenosis as much as possible.