Technical Abstract: 1. The larva of codling moth Cydia pomonella (Tortricidae, Lepidoptera) is known as the worm in the apple, mining the fruit for food. We show that codling moth larvae are closely associated with yeasts of the genus Metschnikowia. Yeast is an essential part of the larval diet and further promotes larval survival by reducing the incidence of fungal infestations in the apple. Larval feeding, on the other hand, enables yeast proliferation on unripe fruit. 2. Chemical, physiological, and behavioral analysis demonstrates that codling moths sense and respond to yeast aroma. Female moths are attracted to fermenting yeast and lay more eggs on yeast­inoculated than on yeast­free apples. An olfactory response to yeast volatiles strongly suggests a contributing role of yeast in host finding, in addition to plant volatiles. 3. Codling moth is a widely studied insect of worldwide economic importance and it is noteworthy that its association with yeasts has gone unnoticed. Tripartite relationships between moths, plants, and microorganisms may, accordingly, be more widespread than previously assumed. It therefore is an urgent task to study the impact of microorganisms on host plant ecology and their contribution to the signals that mediate host plant finding and recognition. A better comprehension of host volatile signatures will facilitate further development and areawide application of semiochemicals for sustainable insect control.