How do oaks wage war?
By Chelsie Vandaveer
November 8, 2001
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The oaks (Quercus spp.) in a park do not appear strange; rather they are considered stately and give one a sense of relaxed graciousness. Oaks appear to shrug off the indignities of nature and simply to endure. But oaks can and do wage war.
Every few years there is an outbreak of gypsy moths (Lymantria dispar). In a matter of days, gypsy moth caterpillars can defoliate an oak,
depriving the tree of its ability to manufacture sugars and other compounds necessary for life. But the oak does not take this attack lightly.
The oak sets about growing a new set of leaves and they appear much the same as the first set. But these leaves are different. The oak has upped the ante and the challenge is now to the caterpillar. The new leaves are tougher, are higher in tannins and phenols, and contain less water.
The tougher leaves means the caterpillar has to chew more, this wears away at its mouthparts. The tannins and phenols combine with the proteins slowing and often preventing the digestion of the caterpillar's meal. And less water means the caterpillar is going thirsty. The caterpillars raised on the second set of leaves are fewer, smaller, and less likely to reproduce. Further outbreaks of gypsy moths are reduced.
But the tough leaves are an additional energy expense for the oak. In the years following the gypsy moth attack and subsequent population crash, the oaks go back to producing regular leaves. The oaks seem to 'know' that the additional compounds are unnecessary when the leaves are not under attack. After a few years, the gypsy moth population rebuilds and the battle begins again.
The Entomology Web at Clemson University has photographs of male and female gypsy moths. To view these pests, click on the link:
http://entweb.clemson.edu/museum/moths/exotic/moth7.htm
The Florida Champion Tree Project has a photograph of the largest live oak in Florida called the Cellon Oak for the family that preserved this beauty. The trunk is 30 feet in circumference, the crown diameter averages 160 feet. To view this photograph, click on the link:
http://www.championtrees.org/champions/oakliveFL.htm
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Suggested Reading:
What experiment turned into a yearly disaster? Plants that Changed History - December 25, 2001
What tree poisons competitors to preserve its territory? Weird Plants - August 16, 2001
What hardwood tree has no growth rings? Weird Plants - July 11, 2002
What is a Kentucky coffee tree? Weird Plants - December 11, 2003
How did the Brazil nut tree get its name? What's in a Name? - May 3, 2002
What is the plane-tree? What's in a Name? September 13, 2002
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