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Abstract
This study investigated a potential new method, solid phase microextraction (SPME) for detecting insect infestation in stored wheat. The results show that volatile pheromones isolated from the common stored grain insects can be used to indicate infestation. The amounts of pheromone released varied between species. This resulted in different sensitivity when SPME was used for detection. A single specimen of lesser grain borer, LGB (Rhyzopertha dominica) and red flour beetle, RFB (Tribolium castaneum) could be successfully detected in 1 kg of wheat whereas groups of 15 and 20 of rusty grain beetle, RGB (Cryptolestes ferrugineus Stephen) and flat grain beetle, FGB (Cryptolestes pusillus Schonherr) male beetles were required to be detected in the same quantity of grain.
Extraction period and temperature affected the sensitivity of SPME method. Longer exposure time and higher temperature allowed more analyte to be collected. Consequently extraction for 4 h at 50 °C appeared to be the condition for extracting pheromone produced by the beetles covered by this study. The method was further adjusted to suit industrial condition and this resulted in changing the sensitivity. Detection of a single LGB can still be successfully achieved whereas for the remaining species, the minimum number of insects required had to be increased by about 40-60% in order to detect VP in headspace.
Method validation was conducted for LGB. The validation showed acceptable results since the method presented linear calibration curve, high accuracy, low detection limit and was specific to the pheromone of LGB. However, poor precision was observed due to matrix interferences. This was improved by using standard addition method.
SPME can also be used to quantify the LGB pheromone by internal standard technique but no significant difference of pheromone amount produced relating to increased number of insects was observed. Therefore, it may not be used to predict the size of the population present in the matrix. Instead, a range of concentration can be established to estimate the population. Lastly, SPME also demonstrated potential to be used in the larger scale where a total of 7 males insects can be detected in 20 kg of grain.