Comparison of ex-situ volatile emissions from intact and mechanically damaged walnuts

Authors: SAN ROMAN, ITXASO - University Of Basque Country; BARTOLOME, LUIS - University Of Basque Country; Gee, Wai; Beck, John; ALONSO, ROSA - University Of Basque Country

Submitted to: Food Research International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2015
Publication Date: 4/11/2015
Publication URL: http://handle.nal.usda.gov/10113/61040
Citation: San Roman, I., Bartolome, L., Gee, W.S., Beck, J.J., Alonso, R.M. 2015. Comparison of ex-situ volatile emissions from intact and mechanically damaged walnuts. Food Research International. 72:198-207.

Interpretive Summary: The codling moth and navel orangeworm are insect pests that inflict serious economic damage to California walnuts. Feeding by these larvae causes physical damage, resulting in lower kernel quality, and can lead to fungal contamination by the toxin-producing fungus Aspergillus flavus, which represents a serious food safety concern. In this way, control of insect pests in tree nut orchards is a major strategy to reduce contamination by Aspergillus flavus. Over the years odors emitted from insects or plants have played a large role in efforts to control or monitor insect pest moths. The two most important sources of relevant odors have been sex pheromones emitted by the insects themselves, and odors emitted by host and non-host crops, for example the pear ester for codling moth and a five-component blend of host plant volatiles for navel orangeworm. The odors from intact and mechanically damaged Howard variety walnuts were evaluated over the course of a typical growing season. The emitted odors 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) and analyzed by gas chromatography mass spectrometry (GC-MS) and by statistical techniques. Ninety-two volatiles were identified in 35 samples, including monoterpenes, a class of compounds responsible the pleasant odors of many crops, as major groups of compounds and followed by sesquiterpenes. Statistical analysis results showed that the samples corresponding with the first three sampling periods (mid May - mid July) did not demonstrate differences between two groups. The samples corresponding to the last two monitoring (late July – late August) showed significant differences in odors emitted between intact and damaged walnut samples.

Technical Abstract: 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, resulting in lower kernel quality, and can lead to fungal contamination by the aflatoxigenic fungus Aspergillus flavus, which represents a serious food safety concern. In this way, control of insect pests in tree nut orchards is a major strategy to reduce contamination by aflatoxin. Over the years volatile natural products have played a large role in efforts to control or monitor insect pest moths. The two most important sources of relevant natural products have been female sex pheromones and odors emitted by host and non-host crops, for example the pear ester for codling moth and a five-component blend of host plant volatiles for navel orangeworm. The volatile emissions from intact and mechanically damaged Howard variety walnuts 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) and analyzed by gas chromatography mass spectrometry (GC-MS) and by multivariate statistical techniques (such as principal component analysis (PCA) and correlation analysis). Ninety-two volatiles were identified in 35 samples, including monoterpenes as major groups of compounds and followed by sesquiterpenes. The PCA results showed that the samples corresponding with the first three sampling periods (mid May - mid July) did not showed differences between two groups. Nevertheless, the samples corresponding to the last two monitoring (late July – late August) showed significant differences in volatile emission between intact and damaged walnut samples.