Borers in Alder Trees

Question from Jennifer:

We have had a major problem with flat-head borers in our alder trees and have already had quite a few trees removed. We have three left, one which is hopeless but the other two can probably be saved if there is a solution. So my question, is there a solution for these guys? They are so big and beautiful in our yard that I’d hate for them to have to go.

Another question… Can you recommend a fast growing tree? We live in Valley Center so anything that can deal with warm summers and occasional frost in the winters.

Answer from Pat:

Alder trees (Alnus) are strongly succeptible to flat-head borers. Once flat-head borers are established in a tree, getting rid of this pest is difficult if not impossible. Some tree companies offer systemic methods of control involving drilling into the tree and plugging in tubes of insecticide, but I have not heard of these methods working.

Here is a link that describes various methods of control: http://www.weekendgardener.net/garden-pests/flathead-borers-011001.htm, but the main way is to keep the trees well fed and well-watered. Stressed and drought-damaged trees are the first to succomb.

Unfortunately, many trees get borers but some, especially alders, are worse than others. A lovely flowering shade tree you might consider growing in the place of those you are losing is tipu tree (Tipuana tipu.) It is reasonably fast-growing and trouble-free but even it might get borers unless well-irrigated, fed and cared for. It needs regular water, is however very beautiful. Protect trunks of young trees by painting with white latex paint and other suggestions in above link. Don’t allow tree men to cut out any natural grafts that occur when branches cross in the lower branches, which often happens as this tree grows. Nature puts them there on this tree to strengthen the tree so it can stand up to wind.

Pepper tree (Agonis flexuosa) does well in poor soil, wind, heat and drought. Bottle tree (Brachychiton populneus) has no significant pest problems, withstands heat, sun, dust and drought. Needs well-drained soil. One of my top favorite flowering trees is Cape chestnut (Calodendrum capense). It has no significant pest problems and likes light clay, loam, well-drained and deep soils. Flower litter might bother you but is worth it in my opinion for such a breathtakingly beautiful tree.