Identification of volatile compounds in dried fruits and nuts attractive to <em>Plodia interpunctella</em> (Hübner, 1813) (Lepidoptera: Pyralidae)...

..and in legume seeds attractive to <em>Callosobruchus maculatus</em> (Fabricius, 1775) (Coleoptera: Chrysomelidae)

Abstract

The goal of the present study consisted of identifying volatile organic compounds (VOCs) in dried apricot, apple, and almond as well as those in dried cowpea and green pea seeds, attractive to two economically important insect species, Plodia interpunctella (Hübner, 1813) (Lepidoptera: Pyralidae) and Callosobruchus maculatus (Fabricius, 1775) (Coleoptera: Chrysomelidae), respectively. The antenna of P. interpunctella used as biological detector in a gas-chromatography-mass spectrometry/electroantennography technique showed that the moth was able to perceive eight volatile compounds from dried apple, another 10 from dried apricot, and again 10 compounds from roasted almond. Among all those compounds, 12 were found for the first time as electrophysiologically active in P. interpunctella namely 3-methyl-1-butanol acetate, 1-butanol, 1-etoxy-2-propanol, 2-methyl-1-butanol, 2,6,11-trimethyldodecane, 3-hydroxy-2-butanone, 2,5-dimethylpyrazine, (E)-2-heptenal, 2,3,5-trimethylpyrazine, acetic acid, furfural and ethyl benzoate. The pulse beetle antenna perceived a total of 6 compounds, five from green pea seeds, 3-carene, 1-pentanol, 1-octen-3-ol, nonanal, and (E)-2-octenal and just one from the cowpea seeds, acetic acid. All those compounds were newly described as EAG-active for the beetle.

The behavioural assays of C. maculatus using single compounds in Petri dishes revealed that all compounds except 1-octen-3-ol elicited an attraction index inversely proportional to the increase of the concentration, indicating that these compounds could be attractive at lower concentrations and repellent at higher concentrations. 1-octen-3-ol showed an attraction index lower than that of the untreated control at all tested concentrations, indicating an eventual repellent effect even at low concentrations. The test of mixtures of these substances in Y-tube olfactometer showed that females were attracted to five mixtures namely

• mixture of 1-pentanol, nonanal, (E)-2-octenal and 3-carene, ratio 14:29:14:43 (quaternary mixture);
• mixture of 1-pentanol, (E)-2-octenal and nonanal, ratio 25:25:50 (ternary mixture 1);
• mixture of 3-carene, nonanal, and (E)-2-octenal, ratio 50:33:17 (ternary mixture 2),
• mixture of nonanal (10^-5 w/w and (E)-2-octenal (10^-6 w/w), ratio 67:33 (binary mixture 1);
• mixture of nonanal and (E)-2-octenal at 10^-6 w/w either, ratio 67:33 (binary mixture 2).

Males were attracted only to two binary mixtures consisting exclusively of aldehydes, the binary mixture 1 and binary mixture 2. Our results showed that the mixture of alcohols (1-pentanol and 1-octen-3-ol, ratio 50:50) caused male and female C. maculatus to respond negatively, and was thus regarded as repellent. It was also found that the compound 3-carene might have an antagonistic effect against the attractiveness of other compounds in the mixtures tested since when present it caused a repellent effect in male C. maculatus but not in female. Further experiments in natural storage conditions need to be carried out with the attractive mixtures before using them in integrated pest management.

The behavioural bioassays of P. interpunctella in an empty room using mixtures of compounds and a sticky trap showed that male and female P. interpunctella were mostly attracted to the mixture of substances identified in roasted almond and to that found in dried apricot in comparison to the control. The highest numbers of moths trapped were observed in mid-July and early August when high temperatures prevailed. However, compared to the initial number of active moths present in the room, the attractiveness was rather low. This could be due to many reasons such as the variability in response within individuals, the fluctuation of the temperature and relative humidity, the quick evaporation of the tested volatiles, or simply the naturally weak attractiveness of the mixtures tested in the formulation with paraffin oil. In addition, the odour perceived when mixing EAG-active VOCs from one substrate might not be the same as that perceived by the moth when exposed to the crude substrate itself. P. interpunctella might need more complex odour to respond to. The least attraction was found when excluding alcohols from the mixture of substances found in roasted almond, indicating an eventual importance of alcohols in the mixture of volatiles used by the moth for its orientation. Further investigations should be carried out at conditions with controlled optimum temperature and relative humidity, using a slow-release dispenser, mixing all EAG-active VOCs together for a better synergic effect, or combining dried fruits and nuts volatiles with cereal volatiles to form a “müsli-odour” in order to improve the mixtures before using them in lures or traps for the monitoring and / or control of stored-product insects.

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