How the oligophage codling moth Cydia pomonella survives on walnut despite its secondary metabolite juglone

Besides apple, its primary host, the codling moth Cydia pomonella uses walnut as a secondary host. Abundance of toxic naphthoquinones, among which juglone prevails, does not restrain this economically important pest insect from infesting walnut, but processes underlying the suitability of this host were yet unknown. Larvae feeding on an artificial diet supplemented with juglone at naturally occurring concentrations survived to adulthood at a similarly high proportion as those in the juglone-devoid control. However, their development time was prolonged, their weight gain was reduced, and adult sex ratio was distorted. Results from the natural system with walnut and apple fruits were in line with data gained on artificial diet. Remarkably, a twofold increase of the maximal juglone content reported from the walnut husk was lethal to the larvae. Chemical analyses showed that larvae feeding on the artificial diet supplemented with juglone concentrations present in walnut contained 1,4,5-trihydroxynaphthalene and excreted it in their frass, whereas the hemolymph contained neither detectable amounts of juglone nor the product of its reduction. Hence, effective metabolism of juglone in the intestinal system of the larvae underlies their survival on host plants containing this defensive compound.