This secretion repels generalist predators.
Willows vary greatly in the concentrations of salicyl glucosides in their leaves.
One may predict that P. vitellinae prefers and survives better on plants that contain more salicyl glucosides. We determined the amount of larval secretion, host preference, larval growth, and larval survival of P. vitellinae on Salix myrsinifolia , S. pentandra and S. phylicifolia . We also measured feeding rates of three natural predators on P. vitellinae larvae feeding on different hosts. Salix pentandra and S. myrsinifolia contained substantial amounts of salicyl glucosides, but S. phylicifolia contained very little of them. Phratora vitellinae larvae produced more secretion on S. pentandra than on S. myrsinifolia .