My lemon tree has two main stems. The first is dead from borer and it has now started on the second. How do I?

Thank you for contacting Yates. 

Unfortunately,as yet we don't have a product that is registered to treat borers in trees. To reduce and prevent borer infestation, we recommend improving the health of the plant and try to prune the stems/branches that are infested where possible. You could also make a lance out of a metal coat hanger and lance the borers down the hole/s, however, this is a rather crude form of control. Arborists may also be able to further assist you with advanced trees as they may have access to products for chemical control.