Borers are Easy to Misdiagnose in the Landscape

July-August 2013


I first start by taking my knife and slicing shallowly into the bark just below the part of the plant that is showing signs of trouble. I’m looking to see if the cambium is still bright green and showing signs of life. Dark brown-reddish brown cambium usually means that it is dead (although some plants, like eastern red cedar, do have some purplish tissue under the bark). Then I will keep slicing down along the stem, all the way down, to determine where the killed cambium stops and starts. If possible, I will slice longitudinally down the centre of the stem to look for signs of borers or internal rot. I usually do the same on the stem/crown below the soil line (that is where a lot of fungal diseases and boring insects can start). 

I’ll go from there to examining the roots, examining as much of the original root system as I can to see if there is a decent fibrous root system there to support the canopy. I’ll roll some of the feeder roots between my fingers and see if they snap when I break them off and if the centres are nice and white. This is a good time to check the Crop Protection Guide for Nursery-Landscape Plants and your favorite guide to landscape pests and diseases. 

All of this said, we don’t always have the luxury of sacrificing landscape plants to do a proper investigation into what might be causing them harm. Quite often it is a one-and-only special tree that’s irreplaceable and the landowner is standing there tapping their toe. Sometimes we have to go on gut instinct, which isn’t always very accurate. Many of you will take some photos of the symptoms and email me, and I am always here to help give another opinion. All I ask is that you take those photos in focus! Take some of the symptoms, the main stem, and the overall tree. The more photos, the better. I might see something diagnostic in the photos that can help you come to your diagnosis with a little more confidence. 

Usually at this time of year we see tops of ornamental willows crashing. It honestly looks remarkably like a foliar and stem disease – almost bacterial. It seems to happen when we get to early summer, especially after hot, humid conditions. If you send those samples in for disease analysis, the report will come back with no causal agent. If you slice down the bark shallowly and examine the cambium, you will see nothing, just green cambium. However, if you cut a cross-section of the stem, you will see that it is hollowed out in the centre. Slice longitudinally down the centre of the stem and you will see sawdust and frass (insect excrement). Keep pulling the stem apart and eventually you will see a small, white legless grub that resembles a weevil larva. If you take a second look at the bark, you will see that it is quite uneven at the point where the borer is found and that there are noticeable signs of frass and bark loosening. And there you have it, the Poplar-Willow Borer (Cryptorhynchus laphathi). Adults are weevils with a prominent snout and a body length of about 8-10 mm. They have a short broad beak and elbowed antenna. They are mostly black in colour with pink scales covering the last third of the their body and some entomologists I know think that they are very pretty. You probably won’t see the adults but you will certainly be able to find the larvae at this time of year. They will be pupating soon so if you see ornamental willows “crashing” in the landscape, examine them closely for signs of boring insects before you diagnose it as disease. 

Because it bores into the centre of the plant, it is difficult to get systemic insecticides to the boring larvae. There is no conductive tissue there as the centre of a woody plant is usually the most “dead” portion of the plant. For this reason, poplar-willow borer management is very challenging. It is best to remove symptomatic stems and destroy them prior to adult emergence. There are some excellent images of this pest and host injury on Dave Cheung’s website: Nursery and Landscape Pests ( 

Nursery-Landscape Report is Now a BLOG!
If you have taken the time to read my weekly report, most of you will have noticed that I don’t attach it as a word file to my weekly emails and that it can’t be found on our Ministry website anymore. The weekly Nursery-Landscape Report can now be found on my new blog: And the nice thing about it is that if you are too tired to read, you just have to look at the pictures! Seriously, life is not all work and I think there should be entertainment and wonder in education. You can “search” all website content with keywords and you can even scan back issues for certain words and highlight those words by using the “Ctrl + F” pop-up search function. I have put a lot of effort into making my weekly Nursery-Landscape Report a more valuable and rich resource for you. I hope you find it useful and I hope to hear from you with your ideas on how to make it better J. 

Arborists are Awesome & So Are Editors
The Ontario arboriculture industry is packed full of some of the most interesting, innovative, energetic and passionate people that I have ever come across. And a lot of great information comes out of the ISA Ontario Arborist every issue. The backbone of this magazine is Joy Black, our editor-in-chief. Joy is leaving us after this issue to pursue a different passion, helping to build her family’s solar energy business. We all would like to thank Joy for her attention to detail in producing such a great resource for our industry. I would like to thank Joy for her wonderful editing skills, her goofy sense of humour, and her forgiveness over missed deadlines. She’s one of those magical people with a big heart and it is no wonder why her parents named her “Joy.” We will miss you and we wish you the best in your new endeavours.  


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