Electrophysiological and Behavioral Responses of Sorghum Chafer, Pachnoda Interrupta (Coleoptera: Scarabaeidae) to Host Plant Volatile Compounds

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Addis Ababa University


The sorghum chafer, Pachnoda interrupta, (Coleoptera: Scarabaeidae) is a polyphagous herbivore and an important pest of sorghum (Sorghum bioclor). In this thesis work, plant volatile compounds from different hosts were identified with the purpose of using it as bait in traps for monitoring and/or control of P. interrupta adults. Behavioral and antennal responses of the sorghum chafer, Pachnoda interrupta (Olivier) were tested to eugenol, methyl salicylate, methyl anthranilate, isoamyl acetate and butyl butyrate. In the field, all odor-baited traps except isoamyl acetate applied on cotton dispensers, were significantly more attractive than unbaited traps. All compounds, except isoamyl acetate, elicited dose dependent electrophysiological responses from both male and female antennae. Dispenser type (cotton wick or rubber septum), trap location (inside or outside sorghum fields) and season (mating/July or feeding/September) affected the performance of different compounds as lures. In July, methyl salicylate applied on cotton was the most attractive lure, whereas the most attractive treatments in September were eugenol and isoamyl acetate on rubber septum. Headspace volatiles were collected from sorghum, Sorghum bicolor (Poaceae) and abutilon, Abutilon figarianum (Malvaceae). Combined gas chromatographyelectroantennographic detection (GC-EAD) analysis was carried out on the antennae of both sexes of P. interrupta. Compounds active in GC-EAD were identified by coupled gas chromatography-mass spectrometry (GC-MS) as (Z)-3-hexen-1-ol, tridecane, 1-octen-3-ol, and 1-octanol from sorghum. From abutilon, (Z)-3-hexen-1-ol, tetradecane, methyl salicylate and methyl anthranilate were identified. Similarly, EADactive compounds were identified from ripe banana as: ethyl butanoate, butyl isobutanoate, isobutyl isobutanoate, isopentyl isobutanoate, isoamyl butanoate, cis-3- hexyl-2-methyl butanoate and 2-heptyl butanoate. In the field, there was no significant difference in number of beetles caught in traps baited with individual sorghum compounds as compared to a blend of all four compounds in ratios mimicking the ones found in the sorghum extract. However, when methyl salicylate and/or eugenol were added to the blend, trap catches increased significantly. Similar results were obtained when eugenol was added to the synthetic abutilon blend in this study. Behavioral responses tested using the Y-tube olfactometer showed no significant different in response between the banana extract and the blend of synthetic compounds. This indicates that the compounds identified are behaviorally relevant and can be used as potential candidate for further field test. Scanning electron microscopy (SEM) revealed four morphological types of sensilla in the sorghum chafter, Pachnoda interrupta: grooved peg, smooth peg, grooved placodea, and smooth placodea. Olfactory receptor neurons (ORNs) were screened for response in single sensillum recordings with a set of 80 food-related compounds. From the 50 responding ORNs, 23 classes were identified. The classes were tuned to single compounds or sets of structurally and functionally similar compounds. The different ORN classes responded to green leaf volatiles (GLVs), flower and fruit odours and fermentation products. This indicates a sensory strategy for food search where multiple classes of ORNs each have a narrow detection window. Such a strategy could rely on common compounds present in hosts such as fruit or flowers, which advertise to symbionts for pollination and seed-dispersal with stereotyped odour signals.