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Flowers, Fruits, and Frass

Why are trees dying in Illinois? by Phil Nixon

Numerous trees are dying across Illinois. It is easy to find dead and dying maples, oaks, walnuts, ash, and other trees. Many of the dying ash are due to emerald ash borer. Many of the dying pin and red oaks in southern Illinois are due to horned oak gall. But many of the dying ash, oak, and other trees are due to the season-long drought of 2012, the July-September drought of 2013, and the severe winter of 2013-2014.

After a very severe drought such as that in 2012, it is common for trees to succumb to its effects for about six years. The more years that pass after a drought, the less likely clients are to accept that analysis, making life harder for arborists and landscapers. During extended dry conditions and severe winters, trees lose root mass, resulting in a reduction of sap flow and dieback of branches. The reduced sap flow provides opportunities for disease and borer attack that would not be present otherwise.

Insect borer adults are tuned into aromatic chemicals released by trees with dieback. Generally, this comes from the portion of the tree with storm injury, other limb breakage, frost cracking, or pruning wounds. Trees dying from old age or severe weather produce similar compounds. These chemicals indicate reduced sap flow.

Eggs laid into niches made by adult beetle borers or other small wounds on healthy trees are likely to be washed out of the wound by strong sap flow. Eggs managing to stay in the niche or wound will hatch into larvae that will likely to be drowned in sap or be crushed by internal sap pressure in healthy trees. While these natural defense mechanisms by the tree are effective in eliminating borers, it is also in the borers' best interests to not expend resources and energy by laying eggs into unsuitable hosts. Selecting trees or parts of trees that are dying are a better option for mother borers.

Bark beetle infestations increase with drought along with other beetle borers. Flatheaded appletree borer attacks rose family trees including crabapple, hawthorn, serviceberry, mountain ash, and ornamental pears, plums, and cherries. It also attacks maple, ash, and a variety of other trees. Two-lined chestnut borer, another flatheaded borer, attacks beech family trees including chestnuts, oaks, and beeches. Redheaded ash borer is a roundheaded borer that attacks many tree species in addition to ash. Other, more specific roundheaded borers include linden borer in linden, cottonwood borer in poplars and cottonwood, and ash and privet borer in ash and privet. Ash/lilac borer is a moth that attacks declining trees. Dogwood borer and carpenterworm are moths that attack a wide range of hosts. All of these borers attack declining trees.

Insecticide treatment is most effective against borers that attack healthy trees such as emerald ash borer and Asian longhorned beetle. These exotic borers are able to attack healthy ash, maple, and other trees because native North American trees did not co-evolve with these insects and develop protective chemical and other mechanisms. Even if we plant trees native to the exotic pests' native country, in these cases China, the different soils and weather in this country cause those trees to be less healthy and more susceptible to these borers than they would be in their native lands.

Newly planted trees become weakened and susceptible to even native borers as they recover from transplant shock and adapt to their new environment. They profit greatly from insecticide applications to prevent and control borer attack.

Established trees that are declining due to drought, cold weather, girdling roots, soil regrading, soil compaction, and other factors are typically attacked by borers when their health has declined beyond recovery. These, and trees dying of old age, are unlikely to be saved by borer insecticide treatments. The borers are merely hastening death, not causing it. Insecticide treatments are unlikely to prolong the life, or as I like to say prolong the death, more than two to three years. As such, insecticide borer treatments are generally an unnecessary cost to the environment and client. If insecticide treatments prolong the tree's life enough for a new tree to replace the loss of shade and aesthetic appearance, they may be justified. (Phil Nixon)


Picture of Red headed Ash Borer larvae by Phil Nixon