Orientus ishidae (Hemiptera: Cicadellidae): Biology, Direct Damage and Preliminary Studies on Apple Proliferation Infection in Apple Orchard

SIMPLE SUMMARY: A recent outbreak of the mosaic leafhopper (MLH), Orientus ishidae, in an apple orchard in Trentino made it urgent to assess the actual capability of this insect to cause damage to this crop. During the biennium from 2020–21, experiments were conducted both in field and semi-field conditions to ascertain whether MLH can (1) Complete the entire life cycle on cultivated apple trees; (2) Cause direct damage by feeding; and (3) Acquire the phytoplasma responsible of the Apple Proliferation (AP) disease. Our results proved that apple trees are suitable host plants for MLH, and that severe direct foliar damage is associated to its trophic activity. Such damage consists of diffuse chlorosis followed by necrosis that eventually take to leaf fall. The ability of MLH to acquire AP was confirmed in field trials. Our conclusions are that MLH is potentially a new pest of apple, although its role as a vector of AP must be confirmed with transmission bioassays. It will also be important to monitor this species in the apple orchards of other regions to prevent future outbreaks and consequent damage to production. ABSTRACT: The mosaic leafhopper, Orientus ishidae (Matsumura), is an Asian species widespread in Europe that can cause leaf damage in wild trees and transmit disease phytoplasmas to grapevines. Following an O. ishidae outbreak reported in 2019 in an apple orchard in northern Italy, the biology and damage caused by this species to apples were investigated during 2020 and 2021. Our studies included observations on the O. ishidae life cycle, leaf symptoms associated to its trophic activity, and its capability to acquire “Candidatus Phytoplasma mali,” a causal agent of Apple Proliferation (AP). The results indicate that O. ishidae can complete the life cycle on apple trees. Nymphs emerged between May and June, and adults were present from early July to late October, with the peak of flight between July and early August. Semi-field observations allowed for an accurate description of leaf symptoms that appeared as a distinct yellowing after a one-day exposure. In field experiments, 23% of the leaves were found damaged. In addition, 16–18% of the collected leafhoppers were found carrying AP phytoplasma. We conclude that O. ishidae has the potential to be a new apple tree pest. However, further studies are required to better understand the economic impact of the infestations.