Catalpa
The genus Catalpa comprises approximately ten species of fast-growing, deciduous trees found naturally in subtropical and temperate regions of East Asia, the eastern and south-eastern United States and the Caribbean. In Britain, the two North American species, Catalpa bignonioides (southern catalpa) and Catalpa speciosa (northern catalpa), have been widely planted as ornamental trees in parks and gardens since they were first introduced in 1726 and 1880, respectively.
Catalpas are distinctive in leaf, flower and fruit. Heart-shaped leaves, up to the 30 cm long, emerge in the spring. As they grow, defence chemicals, including irioid glycosides such as catalpol, are produced which are stored inside the cells of the leaves. Catalpa leaves are the sole food of catalpa hawkmoth (Ceratomia catalpa) caterpillars. As these black-and-yellow larvae feed and grow they accumulate irioid glycosides inside their bodies. Catalpol accumulation is potentially advantageous for the caterpillars as it appears to be associated with predator deterrence. Catalpa speciosa leaves also have glands that secrete nectar. When leaves are damaged by hawkmoth caterpillars, the glands produce copious amounts of nectar, which attracts other insects, such as ladybirds and ants, that eat the hawkmoth eggs and young caterpillars.
In the summer, white or cream, bell-shaped flowers, marked with purple lines and yellow spots, are arranged in erect, pyramidal spikes. Across the native range of Catalpa speciosa, there is diurnal and nocturnal insect pollination. During the day, large bees such as bumblebees and carpenter bees, are pollinators, whilst at night, moths become effective pollinators. In the autumn, catalpas can produce large quantities of bean-like fruits, up to 50 cm long, hence another of their common names, bean trees. When the fruits mature, they split, producing dry, elongated, flattened seeds that are dispersed by wind.
The generic name Catalpa is derived from Native American name, kathulpa, or catawba, a group of Native Americans inhabiting the Catawba River in the Carolinas. Catalpa bignonioides was introduced in Britain by Mark Catesby (1683-1749), from an expedition funded by men such as Hans Sloane (1660-1753) and William Sherard (1659-1728).
Other than as ornamentals, uses of the genus Catalpa appear to be limited. However, the durability of catalpa wood when in contact water made it a preferred timber for the manufacture of railway sleepers in North America. Charles Sargent reported that trunks of catalpa were intact more than 65 years after forests had been submerged following an earthquake in Missouri, USA, in 1811.
Further reading
Stephenson AG 1982. The role of the extrafloral nectaries of Catalpa speciosa in limiting herbivory and increasing fruit production. Ecology 63: 663-669.
Stephenson AG and Wayt Thomas W 1977. Diurnal and nocturnal pollination of Catalpa speciosa (Bignoniaceae). Systematic Botany 2: 191-198.
Von Poser et al. 2000. The distribution of iridoids in Bignoniaceae. Biochemical Systematics and Ecology 28: 351-366.
Stephen Harris