Tree Recovery Following Defoliation Vol. 9, No. 12

Tree Recovery Following Defoliation

Hardwood trees can be defoliated by many different caterpillar pests: forest tent caterpillars, Eastern tent caterpillars, yellownecked caterpillars, fall webworms, walnut caterpillars, variable oakleaf caterpillars, pink-striped oakworms, green-striped mapleworms, and more.  These are all relatively common species with sporadic population cycles, meaning they can be more abundant in some years than others, occasionally occurring in heavy outbreaks that cause severe defoliation.  These caterpillars are all present during non-outbreak years but go largely unnoticed.
 
During heavy outbreaks, any of these caterpillars can completely defoliate host trees.  Usually, this comes as a surprise to the owner or manager of the landscape in which this occurs.  The reason for this surprise is two-fold.  First, such outbreaks are sporadic, meaning many years may elapse between outbreaks.  Second, most caterpillars do eighty to ninety percent of their eating in the last three or four days of their lives.  A tree infested with tens of thousands of small to medium-sized caterpillars may look fine on Wednesday and be completely defoliated by Sunday. 
 
Fortunately, most hardwood trees can tolerate being completely defoliated by caterpillars with little long-term damage.  Certainly, there are short-term effects that result in slowed growth and can make the tree more susceptible to other stresses, but severely defoliated hardwoods usually recover by the next year.  The young Nuttall oak in the photo is a good example.  It was completely defoliated by variable oakleaf caterpillars in September of 2017.  There was not enough time for the tree to releaf before winter, but it leafed out on schedule the following year and in June it appeared as healthy as other trees in the planting that were not defoliated the year before.  This is not usually the case with needle-bearing species such as junipers and Leyland cypress.  When these trees are completely defoliated by bagworms they must often be replaced because they do not recover well.
 
What does one do for a hardwood tree that suffers serious defoliation?  Just have faith the tree will recover and provide proper care and culture until it does.  Don’t go overboard; excessive water and/or fertilizer can be more damaging than defoliation.  But do keep the tree adequately watered during dry periods.  Also, if the tree is defoliated early in the season, try to keep it from being defoliated again.  As Yogi Berra might have said, the cumulative effects of repeated defoliations can add up.
 
Control: It is usually a challenge to effectively and safely treat large trees in urban landscapes for insect pests.  In many cases, by the time the damage is noticed it is too late to spray, and soil-applied systemics do not work well on most caterpillars.  Also, few people have the spray equipment and experience needed to treat larger trees and off-target drift is a key concern that often negates spraying trees in urban areas.  When treatment is practical, as on smaller, recently planted trees (and this is when protection is most important), products containing Spinosad (Conserve SC) or chlorantraniliprole (Acelepryn) will provide good control—if spray coverage is adequate.

Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only.  Always read and follow current label directions.  Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled.


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