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Statistical Analysis of An Experiment to Evaluate Bait-Formulated Insecticides in Controlling Red Imported Fire Ant-A Case Study of Longitudinal Data Analysis
|關鍵字:||red imported fire ant;入侵紅火蟻;repeated measurement;longitudinal data;重複量測;縱向性數據||出版社:||農藝學系||摘要:||
台灣地區在二0 0 0年左右遭受入侵紅火蟻（Red imported fire ant）的入侵，在受感染的地區不僅會在醫藥、經濟上造成損失，也會對生態造成嚴重的危害。是故為了針對政府所核准的三種餌劑型殺蟲劑－芬普尼、百利普芬及賜諾殺進行藥效的評估，因此行政院所屬的桃園區農業改良場及台南區農業改良場分別於受感染的桃園與嘉義兩地進行試驗。在兩試驗地點，皆設有三個藥劑處理及一個對照處理，每處理均重複配置於三個10×10平方公尺的試區，在藥劑施用後八週內重複地量測每試區的活動蟻塚數。對於這些縱向性數據而言，我們可以探討以下四個問題：一、這些藥劑在防治入侵紅火蟻上是否有效？二、這三種藥劑在防治上是否有同樣的效率？三、藥劑處理前的活動蟻塚密度是否會對藥劑的效果造成影響？四、處理之間的活動蟻塚密度是否有不同的時間趨勢變化？基於上述幾點問題本文試以SAS/STAT所提供的不同程序予以分析，並比較不同程序的適當性。
其次，使用Proc GLM執行裂區設計的時間分析，從分析結果中發現百利普芬餌劑在防治入侵紅火蟻的效果上較比芬普尼餌劑和賜諾殺餌劑為差。然而因桃園地區的試驗數據違背Huynh-Feldt的假設，因此以裂區設計的變方分析模式所分析的結果並不可靠，而在Proc GLM中增加REPEATED指令事實上有相同的分析結果。
Red imported fire ant (RIFA) arrived Taiwan around 2000 and has caused more or less medical, economic and ecological problems in the infested area. To evaluate the effectiveness of three bait-formulated insecticides, namely, Fipronil, Pyriproxyfen, and Spinosad, for managing RIFA, Taoyuan and Tainan District Agricultural Research and Extension Stations each conducted a filed experiment, respectively, in Taoyuan and Chiayi counties where infestation of RIFA was found. In both experiments, three 10x10 square meter plots were randomly assigned to each of the three insecticide treatments and a control treatment. Number of ant mounds on each plot was counted repeatedly during the first eight weeks after the application of treatments. From these longitudinal data, we can investigate four questions of interest: 1. Do these insecticides work in controlling RIFA? 2. Do these insecticides have the same effectiveness? 3. Does pre-treatment mound density influence the effects of the insecticides? 4. Does the evolution of ant mound density differ in time trend among treatments? This case study aims to compare the validity of different statistical procedures available in SAS/STAT (version 8) with respect to answering the above questions.
Firstly, the relative mound density, i.e., ratio of mound number to the pre-treatment mound number, was log-transformed and plotted against time of measurement. It revealed that all these insecticides worked effectively in controlling RIFA and the every plot's profile of log-transformed relative mound density could be well approximated by a quadratic function of time.
Secondly, Proc GLM was then invoked to perform a split plot in time analysis. It revealed that Pyriproxyfen was inferior to Fipronil and Spinosad in controlling RIFA and the time trend of declination of relative mound density differed significantly among insecticide treatments. The validity of this analysis was doubtful because the data of Taoyuan experiment failed to meet the Huynh-Feldt assumptions. Adding REPEATED statement to Proc GLM produced virtually the same results.
Thirdly, the log-transformed relative mound density of each plot was separately fitted to a quadratic function of time, expressed in weeks after application of insecticides. Taking initial mound density as the covariate, covariance analyses were then used to investigate the effects of treatment groups on the plot-specific evolution over time. Only the Fipronil treatment in Taoyuan experiment showed significant effect in controlling RIFA. These results are invalid because the assumptions of covariance analysis were violated.
Finally, we invoked Proc MIXED to fit a general linear mixed model to each of the two data sets. The results showed that all these insecticides were effective in both experiments. In Taoyuan experiment, the pre-treatment mound density gave no significant influence on the control rates of the insecticide. And quadratic time trend of the log-transformed relative mound density was detected in both the Fipronil and Spinosad treatments, while only linear time effect was significant in the Pyriproxyfen treatment. In Chiayi experiment, the pre-treatment mound density showed significant influence on the control rates of the insecticides. And all the three insecticides showed the same time trend in declining mound density.
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