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Population Density, Spatial Distribution and Damage Evolution of Neochetina eichhorniae Warner and N. bruchi Hustache on Eichhornia crassipes (Mart.) Solms - Laubach
於台南縣德元埤田間選定慈善宮 (TK)、新鳳橋 (SF) 及四號池(FP) 等三試驗區釋放象鼻蟲防治布袋蓮，TK及SF試驗區分別於1996/12/08及1997/01/06 ~ 07各釋放一次象鼻蟲，於FP則於1997/05/24釋放一次及1998/01/09 ~ 03/04釋放六次共釋放七次。德元埤於1997年7月及2000年7月進行放水、清池及燒毀乾枯布袋蓮之管理措施，造成三試驗區布內袋蓮族群減少或完全清除，其中釋放於慈善宮試驗區之象鼻蟲仍能於布袋蓮殘株存活達4年以上並隨布袋蓮再次繁殖及增殖族群，證實象鼻蟲在該試驗區已建立其族群，並且放水及清除部分布袋蓮不會造成族群之滅絕。
田間象鼻蟲成蟲於布袋蓮葉片上取食造成之食痕密度與成蟲密度及幼蟲密度之相關係數低，但食痕密度對隧道密度、蟲孔密度及象鼻蟲族群之總密度 (成蟲、幼蟲、隧道及蟲孔) 相關係數則高。經回歸分析以連續20母株及其分生株與側芽為一取樣單位下，獲得 (象鼻蟲族群之總密度 = 85.95 + 0.0404食痕密度 (R2 = 0.903, p = 0.0001)。因此田間象鼻蟲族群之總密度得以由食痕密度評估之。不同取樣單位下象鼻蟲成蟲、幼蟲及食痕均傾向聚集分布，但隧道隨取樣單位不同而傾向聚集或隨機分布，蟲孔隨取樣單位不同而傾向聚集或均勻分布。
三試驗區象鼻蟲密度以慈善宮試驗區最高 (每0.25平方公尺3.37隻成蟲及5.21隻成蟲)，四號池次之 (每0.25平方公尺0.52隻成蟲4.38隻幼蟲)，新鳳橋近於0 (每0.25平方公尺0.01隻成蟲，但幼蟲、隧道及蟲孔密度均為0)。該三試驗區中象鼻蟲族群密度較高之慈善宮試驗區，於布袋蓮植株上成蟲取食之食痕密度與幼蟲造成之隧道及蟲孔密度顯著較高，每株布袋蓮無性繁殖之分生株及側芽密度顯著較低，顯示於較高族群密度下象鼻蟲成蟲對布袋蓮直接嚙食造成之食痕及幼蟲取食葉柄內組織造成隧道與蟲孔，顯著減少布袋蓮分生株及側芽密度，抑制布袋蓮新生之分生株及側芽之無性繁殖，降低布袋蓮生長勢。總之，利用食痕密度評估象鼻蟲之總密度檢定結果，可簡化今後田間之取樣技術及方法。象鼻蟲族群為害布袋蓮族群造成高密度之食痕、隧道及蟲孔密度，顯著降低布袋蓮之生長勢。
This research selected three experimental areas (TK, SF, FP)among Der-Yuan irrigation lake in Tainan county to observe the effects of releasing weevils into these areas as biocontrol agent of waterhyacinth. Weevils were released only once into TK and SF area on 12/08/1996 and on 01/06/1997 separately, so were weevils only once into FP area on 05/24/1997 and six times (about once a week) from 01/09 to 03/04/1998. However, these three areas were so badly destroyed by farm irrigative unplug and burning waterhyacinth in July of 1997and of 2000 that the waterhyacinth populations were either eliminated or dwindled in these areas. Nevertheless, we found the weevils in TK area still had successfully survived for 4 years and established its population with the increase of waterhyacinth. It indicates that weevils wouldn't be exterminated by irrigative unplugging and had established its own population.
Through this research we find the density of scars has low correlation coefficient with the density of adults as well as with that of larvae. But the correlation coefficients between scar density and young larvae canal density, matural larvae tunnel density, and the total density of weevils (including adults, larvae, canals and tunnels) are high. So using 20 plants (including runners and lateral buds), we can find the relationship below by regression analyses: total density of weevils = 85.95 + 0.0404 * scar density (R2 = 0.903, p = 0.001). This equation can help us to measure the total density of weevils population by scar density. By different methods of sample unit, the amounts of adults, larvae and scars tend to be aggregation distribution, and the canals tend to be aggregation or random distribution, and tunnels to be aggregation or uniform distribution.
The density of weevils area is highest(3.37 adults, 5.21 larvae / 0.25 m2)in TK, second in FP(0.52 adults, 4.38 larvae / 0.25 m2), and near zero in SF(0.01 adults, 0 larvae, 0 canal and 0 tunnels / 0.25m2). Among three high weevil population density tested areas, the scar density of adults, canal density and tunnel density of larvae are significantly higher, and the density of runner as well as of lateral bud are significantly lower in the areas. It shows that the direct nibble scars of adults, the tunnels, and the tunnel of larvae decrease the densities of runner as well as of lateral bud, therefore suppress the growth of waterhyacinth. In conclusion, using the scar density to evaluate the total densities of weevils can simplify the techniques and methods of sampling. The weevil population damaging waterhyacinth population caused the high scar density, canals and tunnels density and lowered the growth of weevils.
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