Mulberry leaves (Morus spp.) have been used to rear the silkworm, Bombyx mori for thousands of years. However, we found that mulberry leaves are highly toxic to lepidopteran larvae other than the silkworm, B. mori, (such as the Samia ricini, Saturniidae and notorius pest species, Mamestra brassicae, Noctuidae ), due to the ingredients of the latex. The toxicity of mulberry leaves was lost when latex was washed off, and latex-added artificial diets showed toxicity. Mulberry (M. australis) latex contained very high concentrations of alkaloidal sugar-mimic glycosidase inhibitors reported to have anti-diabetic activities, such as 1,4-dideoxy-1,4-imino-D-arabinitol (D-AB1), 1-deoxy nojirimycin (DNJ), and 1,4-dideoxy -1,4-imino-D-ribitol. Their concentrations, altogether, in latex reached 1.5-2.5% (8-18% to dry weight) in several mulberry varieties, which were 100 times the concentrations previously reported from whole mulberry leaves. These sugar-mimicking alkaloids showed toxicities to caterpillars, but not to the silkworm, B. mori. Our results suggest that mulberry latex and sugar-mimicking alkaloids in it play key roles in defense of mulberry against insect herbivory, and also suggest the existence of some adaptive mechanisms in the silkworm, B. mori. This study shows, for the first time in long history of sericulture, the existence of strong latex-borne defense mechanisms in mulberry, and gives practical answer to mulberry-silkworm interactions.

Reference

Konno K. Ono H. et al. (2006) Proc. Natl. Acad. Sci. USA, 103, 1337-1341.