A romp through the North American countryside this time of year is quite enjoyable. So many plants are coming into bloom and life abounds everywhere you look. One particularly lovely sight to see is a large stand of catalpa trees in full bloom. With their stunning display of large flowers all clustered onto spikes, it is no wonder why this genus has become such a popular landscaping choice.
The genus name Catalpa is actually a derivation of the Muscogee word "kutuhlpa," which translates to "winged head". This is probably in reference to the winged seeds that emerge from the long, bean-like pods. Either way, these trees have an interesting story to be told that goes far beyond their horticultural use.
North America has two native species of catalpa, Catalpa bignonioides and C. speciosa. When described by European botanists, the former was growing in a narrow swath of the southeast and the latter in an even narrower range near the confluence of the Ohio and Mississippi Rivers. As many of you realize, these trees do really well when planted outside of these areas. This fact is not lost on botanists and ecologists and indeed many have speculated that the genus was undergoing a range contraction long before Europeans made it to the continent. An archaeological dig in West Virginia added some credence to this theory when evidence of C. speciosa was found far from where this tree was originally thought to grow.
Catalpas are the sole host for the larvae of the catalpa sphinx moth (Ceratomia catalpae). Large infestations of these caterpillars can even defoliate the trees. Because of this, catalpas have evolved an interesting defense mechanism. The leaves of catalpa have what are called extrafloral nectaries. These are glands that excrete sugary nectar. The nectar attracts ants. When the leaves sense damage from the catalpa sphinx moth caterpillars, production of nectar increases dramatically. This focuses the ants attention towards leaves that are in need of defense. Because ants are so apt to defend a reliable food source, they quickly go to work on driving away the caterpillars.
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