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Rhonda Ferree's ILRiverHort

Rhonda Ferree's Horticulture Blog

Tree Insects Increase Likely After Drought

Insect borers and some scale insects are likely to be more numerous than normal on trees for the next several years", says Phil Nixon, University of Illinois Extension Entomologist. "During extended dry conditions, trees lose root mass, resulting in a reduction of sap flow and dieback of branches".

"The reduced sap flow provides opportunities for borer attack that would not be present otherwise". This increase is likely to be noticeable next year and will build for about three years. Borer infestations are likely to fall over the following two years until reaching normal levels around 2018.

Insect borer adults are tuned into aromatic chemicals released by trees when they start to dieback. These chemicals indicate reduced sap flow. Borers prefer to lay eggs in dying trees or tree parts because less sap flow means less eggs are washed away. This includes beetle and moth borer.

Bark beetle infestations will increase along other beetle borers. Flatheaded appletree borer attacks 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 declining 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.

Moth borers will increase in numbers, particularly ash/lilac borer, dogwood borer, and carpenterworm. Dogwood borer and carpenterworm attack a wide range of hosts. (Phil Nixon)

Similarly, Nixon says that there are a couple of scale species that become much more numerous after droughts and other tree-weakening events. European fruit lecanium scale becomes very common on stressed trees and remains in high numbers for several years until the tree recovers. It is common on maples, oaks, hazlenuts, crabapples, and many other tree species. Large infestations produce high amounts of honeydew such that it feels like it's raining under the tree.

Mature lecanium scales appear similar to small army helmets or turtle shells. They are round, slightly-raised, brown domes about one-quarter inch in diameter. They produce pink crawlers that emerge in mid-June in central Illinois when Queen Anne's lace is blooming. Being soft scales, they are controlled with dormant oil sprays.

Scurfy scale becomes more common after droughts as well. It attacks many hosts including hawthorn, quince, crabapple, firethorn, mountain ash, horsechestnut, elm, hickory, maple, willow, and dogwood. It is grayish-white and about one-eighth inch long. When numerous, it coats trunks and branches, making them appear to have white bark. Purple crawlers hatch in spring when bridal wreath spirea, Spirea X van houttei, is in early bloom. Crawler sprays are most effective at that time. Being an armored scale that overwinters in the egg stage, it is not very susceptible to dormant oil sprays and does not produce honeydew.

University of Illinois Extension Educators expect substantial tree dieback and death due to the drought over the next several years. Many trees will take three years to die, and some will hang on until five years after the drought. As these trees decline, they are more susceptible to insect and disease infestations.

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