Mulberry Mysteries

tom morgan

     Red mulberry (Morus rubra) is a common tree in the eastern USA and yet it has mysterious interactions with animals. This tree’s leaf has three to five veins that exude a defensive latex with a bittersweet taste. Tree-climbing woodchucks find these leaves quite tasty. And white-tailed deer enjoy the leaves so much that one observer claimed to see a deer climb a tree. Bison eat the leaves (according to George LeRoux of “The Flint Hills Prairie Bison Reserve” near Alta Vista, KS). Indeed, the leaves of the white mulberry (M. alba) are considered high quality food for livestock (when the branches are cut and carried to the livestock).

     White mulberry has become naturalized in Kansas and is more abundant than the native red mulberry in some areas. White mulberry was named for the pale coloration of the leaf buds, not the fruits (which may vary in color from white to black). The leaves of this mulberry are smooth on the underside or have hairs along the veins, while the undersides of red mulberry leaves are noticeably hairy to the touch of one’s fingers. Mulberry leaves have a variety of shapes, usually with unlobed leaves, resembling those of basswood, predominating in the mulberry’s crown, while leaves with two or three lobes are common on lower branches and sprouts or young trees.

     Native Americans utilized latex from mulberry leaves to treat ringworm infections, so latex does have unusual properties. The intended target of the sap appears to be herbivorous insects, however. Insects known as mulberry borers (cerambycid beetles in the genus Dorcaschema) which tunnel in dead limbs as larvae and then emerge as adults to feed on leaves. The somewhat plump appearing adults, with their abdomens slightly wider below the middle, take care to sever the latex-delivering veins, and then begin feeding on the undefended leaf tissue.

     The white mulberry was introduced to Kansas in the nineteenth century to establish sericulture, since the Asian silkworm (Bombyx mori) prefers the leaves of the Asian white mulberry. This naturalized species is more abundant than the native species in some areas and hybridizes with it, perhaps threatening the survival of the red mulberry.

     The latex of another mulberry (M. australis) contains enormous amounts of “sugar mimic alkaloids” which have little effect on sugar metabolizing enzymes of the Asian silkworm, but are toxic to other insects. Like the Native American medicine men, Chinese physicians discovered useful properties of mulberry leaves, and white mulberry leaves have been used in Chinese medicine for more than a millennium to treat diabetes, apparently because sugar mimic alkaloids inhibit the digestion of carbohydrate. It seems probable that many critters, including woodchucks, might be sensitive to the alkaloids of mulberry sap. However, a critter that eats as quickly as a hungry woodchuck consumes a whole leaf before additional alkaloid-rich latex is pumped into that leaf.

     As I wrote this, I periodically conducted a raid on a bag of salty chips, but what I really want is sandpapery rough, native mulberry leaf. Well, that mulberry fruit I saw at George LeRoux’s ranch was beginning to redden. Yes! I’ll wait for it to a berry to become juicy sweet.