With big, heart-shaped leaves, dense clusters of white flowers, and long, slender seed pods, catalpa trees (Catalpa spp., Bignoniaceae) are easy to recognize. Two species are found in North America: northern catalpa (C. speciosa) and southern catalpa (C. bignonioides). These trees have different native ranges within the United States, but both have been introduced as ornamentals and subsequently naturalized throughout much of the continent (Kershner et al. 2008). Since their current ranges overlap quite a bit and they are very similar, it can be difficult to distinguish between them.
While hiking at Secor Metropark west of Toledo, Ohio, I found a nice example of a northern catalpa. In early June this tree displayed the distinct leaves, flowers, and seed pods all at once.
Northern catalpa is the food of choice for larvae of the hawk moth Ceratomia catalpae (Lepidoptera: Sphingidae) (Stephenson 1982). These insects can seriously damage the leaves of these trees, but the trees have evolved an interesting defense. When the moth larvae feed on the leaves, the foliage secrete extrafloral nectar that attracts a number of other insects. These insects happen to be predators of hawk moth eggs and larvae (Stephenson 1982). The net result is that the predaceous insects kill many of the hawk moths, and the trees can then grow more robust foliage and fruit, helping in their reproductive efforts. The predaceous insects, in turn, benefit from this situation by being called to a great source of food.
Kershner, B., D. Mathews, G. Nelson, and R. Spellenberg. 2008. National Wildlife Federation Field Guide to Trees of North America. Sterling Publishing Company, Inc., New York, NY.
Stephenson, A.G. 1982. The Role of the Extrafloral Nectaries of Catalpa speciosa in Limiting Herbivory and Increasing Fruit Production. Ecology 63(3):663-669.