| Tribolium castaneum (Herbst), Rhyzopertha dominica (F.) and Sitophilus granarius (L.) are among the top list of serious insect pests of stored grains worldwide. Rapid, reliable and simple methods to detect the quality and change in quality of stored grain are critical for international quarantine inspection, domestic storage and the whole supply chain. Changes to headspace volatile patterns may be an early observable symptom of reduced grain quality and infestation by pests. Understanding the mechanism of chemical communication between the stored grain insects and stored grain, and that of host locating in stored grain pests could provide scientific and practical guidance for timely adjustment of storage conditions and accurate pest management.In the present study, headspace solid phase micro-extraction coupled with GC (SPME-GC) and GC-MS techniques were used to evaluate the influence of moisture content, storage temperature and storage length on the patterns of volatile compounds produced by wheat grain. The volatile chemicals from wheat, T. castaneum, R. dominica and S. granarius and wheat infested by each of the above three species were separated and identified. Electroantennogram (EAG) assay and olfactometer studies were carried out to identify and confirm particular behaviourally active volatile compounds, and to understand if and how these volatiles function on chemical communication and host location in stored grain pests. The main results are shown as follows:1. An innovative method for the rapid and reliable analysis of the volatiles of stored grain and insects was established. The HS-SPME technique coupled to GC-MS proved a useful tool for diagnosing volatiles in wheat, stored grain pests as well as those from infested wheat. The CAR/PDMS coating fibre was recommended for the extraction of volatiles in grains infested with the tested species R. dominica, T. castaneum and S. granarius.2. Volatile chemicals from wheat with two levels of moisture content stored under different temperatures and storage times were tracked. Moisture content, storage time, and temperature together exerted complex impacts on volatile profile of wheat. Moisture content played an important role in the volatile emanating from wheat. Even at low temperature, i.e. 15 oC, the volatile profiles of wheat with standard moisture content could be easily distinguished from that of wheat with higher moisture content. When stored at 25oC, the volatile profiles of wheat with different moisture contents were evidently different no matter the storage time. The higher the moisture content, the more complicated and difficult to precast the condition. The difference went significantly with time.3. Volatile chemicals from wheat, insects and those from wheat infested by insects were collected, separated and identified with HS-SPME-GC-MS technology. In total, 15 main volatile components from wheat were identified, including dimethyl sulphide, acetone, methanol, ethanol, 2,3-butanedione, 2-butanol, hexanal, 2-methyl-1-propanol, 1-butanol, 1-penten-3-ol, 3-methyl-1-butanol, 1-pentanol, 3-methyl-2-buten-1-ol, 1-hexanol, and 1-pentadecene; methylbenzoquinone, methylhydroquinone and 4-ethyl-1,3-benzodiol from T. castaneum; (S)-(+)-1-methylbutyl (E)-2-methyl-2-pentenoate (dominicalure1) and (S)-(+)-1-methylbutyl (E)-2,4-dimethyl-2-pentenoate (dominicalure2) from R. dominica. The compound 3-hydroxy-2-butanone was reported in adult grain weevil for the first time.4. The influence of particular volatile chemicals functioning in tuning host finding behavior in T. castaneum was investigated. The adults were not attracted by clean flour or clean wheat grain. Wheat flour + T. castaneum adults proved the most attractive for T. castaneum. Wheat grain + R. dominica served a good attractants as well (p = 0.016). Wheat grain + S. granarius functioned as a repellent for adult T. castaneum (p = 0.004). The larvae were more sensitive to potential food and infested flours than adults did.5. Methylbenzoquinone triggered the highest antennal response in adult male T. castaneum as 2.63±0.90 mV at 1000μg/10μl, followed by 4-ethyl-1,3-benzodiol, 2.23±0.41 mV, 3-hydroxy-2-butanone 1.95±0.69 mV, methylhydroquinone, 1.19±0.34 mV. Antennae of female adult T. castaneum made no response to 4-ethyl-1,3-benzodiol, but made noticeable and the highest response to methylbenzoquinone as 2.04±0.81 mV at 1000μg/10μl, followed by 3-hydroxy-2-butanone 1.82±0.68 mV, methylhydroquinone, 1.54±0.32 mV. The intensity increased with the concentration.6. The chemical 3-hydroxy-2-butanol as repellent or alarming pheromone in S. granarius was recorded and confirmed. In behaviour bioassay, S. granarius responded positively to 3-hydroxy-2-butanone at 1 ng/10ul, but turned negatively as the concentration increased by 10μg/10μl, and responded constantly negatively to 1-pentadecene with the concentrations increasing from 1 ng/10μl to 1000μg/10μl. 7. The antennal response of male grain weevils to 3-hydroxy-2-butanone intensified with the increase of concentration from 5.35±0.82 mv at 1000μg/10μl, 3.66±0.61 mV at 10μg/10μl, 1.92±0.38 mV at 0.1μg/10μl, to 0.95±0.11 mV at 0.001μg/10μl. For female grain weevil, the intensity was recorded as 4.99±0.64 mV, at 1000μg/10μl, 3.36±0.48 mV at 10μg/10μl, 1.88±0.46 mV at 0.1μg/10μl, 0.94±0.26 mV at 0.001μg/10μl.For the first time, a hypothesis for the chemical mechanism of host locating in stored grain insects in a relatively closed system of stored grain was proposed and confirmed. The GC chromatogram profiles of volatile from wheat, T. castaneum, R. dominica, S. granarius, and those of wheat infested by T. castaneum, R. dominica, S. granarius each were established. A new simple and efficient method to detect stored grain pest was established. The chemical communication and host locating mechanism in stored grain pests were investigated through EAG and behaviour study. In addition, taking wheat as an example, the effect of moisture content of grain, storage temperature, and storage time on the volatile chemicals of grain were examined.
|
PDF Full Text Request