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Lemon Tree Borer (also known as Citrus Tree Borer) is a very destructive native New Zealand beetle which has become a common pest for home gardeners. Although Lemon Tree Borer will happily feed on a wide range of native and exotic trees, they developed an instant liking for citrus when orchards were first introduced into NZ. Their favourite targets include lemons, apples, grape vines, walnuts, persimmons, and cherry trees.
Classic warning symptoms of Lemon Tree Borer infestation are wilting leaves or dieback of individual branches. Observation of 2-3mm round holes in branches, with visible clumps of powdery sawdust (actually, it’s frass – excrement from the borer larvae) confirms an infestation.
The adult beetles are flying at night from mid-spring to autumn, on the lookout for trees to lay their eggs. The scent of freshly pruned citrus trees attracts them like a magnet, so it’s wise to restrict pruning to between April and August. Whenever you are pruning, make sure to tidy up and burn the offcuts.
When Lemon Tree Borer eggs are laid into fresh wounds on the bark, the larvae hatch out and eat into the tree, tunneling their way along inside the centre of branches towards the stem. This prevents sap from flowing, so the affected branches often dry out and snap off under their own weight. If larvae enter the central trunk, it can be fatal to the tree. For this reason, make certain never to scuff the base of trees with your line trimmer; it makes an inviting entry point for adult beetles to lay eggs.
A good solution for protecting fresh pruning cuts is Yates PruneTec, which seals off the wound and forms a physical barrier against larvae boring into the tree.
Oemona hirta larvae can grow inside a tree for 1-2 years before emerging as adult beetles, so they can cause a lot of damage in that time. Hiding away deep inside the wood makes them difficult to control, as they are out of reach of most insecticides.
The old-fashioned organic method is still best – get a wound guitar string or a length of thin wire, search for the borer holes and poke it in firmly as far as you can go. This takes a little time, but it’s very effective.