SW Yard & Garden: Don't let pine sawyer beetles saw down your tree
Question: One of my Austrian pines started turning brown at the top this spring, and as can be seen is now 3/4 brown. The trees are drip irrigated and were fertilized lightly for the first 20 years. Today, I found an inch-long bug on the bark. Could these bugs be pine sawyers and, if so, what’s the best approach to control them?— Richard C., Question Submitted via Bernalillo County Horticulture Extension Agent, Sara Moran
Answer: Dr. Carol Sutherland, NMSU Extension Entomology Specialist and NMDA State Entomologist, provided a detailed response:
“The pictured beetle is indeed the pine sawyer. That would be Coleoptera, Cerambycidae, Monochamus clamator. The beetle in the photo could have come from the trees on this property or elsewhere, too. They are all strong fliers and very alert to finding not only mates but also trees in distress. With drip irrigation, there are always questions about how good that system is for trees: Does it deliver the volume of water the tree needs, especially over time? Has it been periodically modified and water delivery increased to the ever-growing root system? That, plus root competition and overall tree health (just like people), are at work here in determining tree longevity and susceptibility to attack.
Weak trees likely produce certain attractive scents. Mated females are attracted to those trees where they find niches in the bark to lay an egg or two here and another egg or two there. Adults leave, and the larvae develop, hatch, and bore into the bark. These cylindrical larvae often bore between the bark and wood first, killing a patch of vascular tissue that then dooms the canopy above and roots below that injury. That makes the tree more susceptible to other wood borers—more Monochamus and a regular zoo of other beetles, both pests and predators.
The life cycle should last about a year per beetle. As weak and dead spots increase on the tree, more beetles attack — and that’s it. Most of these attacks will likely be in summer when trees are at their weakest. Dying trees probably need to be removed sooner rather than later, along with branches and other debris. It’s likely that cut stumps and branches are also attractive to ovipositing beetles, so don’t let them sit around.
It's very hard to impossible to look at a weak tree or one with die-back and guesstimate what might be feeding on or in it — as well as how many beetles there really are inside the tree and what level of damage they have already done. We don’t have X-rays or MRIs for trees, alas, and even then it might be too late to save the tree. Really, there is no way to save infested trees; bark and wood protect the larvae from contact insecticides, while cut vascular systems just won’t translocate any systemic. If tree care has been inadequate for some period of time, trees are not capable of ‘making up for lost time’ when care is improved.
The degree of infestation in a given tree likely cannot be determined until the tree is down, cut into ‘rounds’ for firewood, and seasoned far away from the area where they were harvested. Chances are you’ll be able to hear the larvae chewing, grinding, ticking, or squeaking as they feed and burrow. If bark is loose, it might be peeled away to expose tunnels along the wood surface and holes where larvae have bored into the sapwood. These beetle larvae produce lots of frass, sawdust, and wood strands, packing their tunnels behind them as they feed onward. Empty pupation chambers will be just below the bark; exit holes are round for these beetles.
This is a timely issue, as I’ve been getting a lot of pine sawyer samples this year.”
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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.