The codling moth (Cydia pomonella) and navel orangeworm (Amyelois transitella) are insect pests that inflict serious economic damage to California walnuts. Feeding by these larvae causes physical damage to the nut and can lead to contamination by aflatoxigenic fungi. Over the years volatile natural products have played a critical role in efforts to control or monitor these and other insect pest moths.

The ex situ volatile emissions from intact and mechanically damaged Howard variety walnuts from the California Central Valley were evaluated over the course of a typical growing season. The volatile profiles were compared and differences in emission considered as a means to identify candidate volatiles for use in host plant-based attractants or in conjunction with pheromone blends to enhance attractancy.

Walnut volatiles were extracted by headspace solid phase microextraction (HS-SPME) in a semi-closed system and analyzed by gas chromatography mass spectrometry (GC–MS). Ninety two volatiles were identified, including monoterpenes as the predominant class of compounds. A multivariate analysis of the data highlighted two sampling periods (late July–late August) where intact walnuts and mechanically damaged walnuts can be distinguished due to their volatile profile composition.

The results of this study provide relevant information regarding potential host plant-based semiochemicals of two insect pests, valuable data regarding the ambient odors these insects encounter in walnut orchards and add some potentially interesting volatile compounds to the existing literature.