Electrophysiological and Behavioral Responses of Sorghum Chafer, Pachnoda Interrupta (Coleoptera: Scarabaeidae) to Host Plant Volatile Compounds
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Date
2008-03
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Addis Ababa University
Abstract
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.
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Biology