Mussel scale, Lepidosaphes ulmi - Host plants

Mussel scale has a wide range of host plants, mainly deciduous trees. The table below refers to published host plants, both in New Zealand and overseas. Within the trees, most of the population lives on the trunks, stems, and branches, but the fruits and leaves are also attacked. Host plants have been reported from 85 genera in 33 families. It is most destructive on apples and pears. Although stone fruits are included in its host range, it is not a major pest of these crops in New Zealand. Mussel scale has also been reported to occur on quince, currants, blueberry, grapes and walnuts overseas. Poplars, willows, birches, elms, alders, rowans (mountain ash), and other common ornamental trees serve as reservoir hosts for this pest outside orchards. However, the densities of mussel scale on these hosts rarely reaches the high levels seen with oystershell and San Jose scales. This may be due to a combination of climate, competition with these other scale insects, and a range of effective natural enemies keeping mussel scale at lower levels. Nevertheless, shelter and ornamental trees are a source of mussel scale dispersing in the orchard environment, and the correct choice of shelter species can make a contribution to cultural control. The host plant of mussel scale is known to affect the sex ratio of populations and the number of eggs produced by the females. It is thought that these strain differences may even be a complex of closely related species in Europe, some specific to particular hosts. Mussel scale causes swollen stems (galls) on some host plants, such as the heather, Calluna vulgaris.