幼蟲競爭對四紋豆象成蟲體重的影響及其作用機制

Abstract

四紋豆象幼蟲為內食性昆蟲，幼蟲在羽化之前無法自由離開寄主豆。前人研究顯示，隨著豆內的幼蟲數增加，資源的競爭程度增加，經常導致成蟲的平均體重顯著下降的現象。然而，本研究卻發現，當以小型綠豆為寄主，幼蟲期遭遇競爭的成蟲，其平均體重顯著較幼蟲期沒有競爭的成蟲為重。另以食物及空間相對較多的大型綠豆為寄主，卻沒有發現體重顯著變大的現象。顯示寄主大小是造成競爭後羽化體重變大的重要條件。本文旨在探討幼蟲競爭導致羽化體重增加的原因。首先透過模擬，檢驗體重上升的原因，是否完全源自較輕個體在競爭中被淘汰之故。結果顯示，模擬後獲得的體重分布，與實際的分布並不相符。顯示對體重的選汰不是導致體重上升的唯一原因。此外，具有競爭經歷的幼蟲,其發育期並無延長的現象。因此，延長發育期以增加取食量不是體重上升的原因。我們進一步假設，取食對手是造成羽化體重變大的原因。因此對三卵豆進行解剖。結果顯示，豆內幼蟲有互咬的行為，且在不完整的屍體上發現疑似被取食的痕跡，加上蟲屍不見的比例隨著時間上升。說明同類相食的現象存在於四紋豆象幼蟲，取食同類是獲得額外體重的可能來源。再者，經由測量幼蟲取食量的試驗，顯示三隻幼蟲處理下幼蟲取食量顯著大於一隻幼蟲處理者。此結果說明，優勢幼蟲藉由增加取食量，進而提高平均羽化體重的可能無法排除，其可能性猶待進一步的檢驗。此外，前人於四紋豆象South India品系之中，曾發現幼蟲能以取食所發出的震動抑制同種幼蟲的進食，進而決定幼蟲競爭的勝負。然而，本試驗結果顯示，所使用的4C6-4品系，同種幼蟲發出的取食震動，對其體重的效應並不顯著。

Larvae of the seed beetle Callosobruchus maculatus develop inside the seeds of their host plants, and are incapable of moving between seeds. Many studies reported a reduction of adult weight as larval density increases due to resource shortage. However, I observed a different trend that larval competition led to an increase of adult weight when the host size was small. The phenomenon disappeared when host size was large, indicating that host size was a crucial factor for the trend. In this study, I tried to answer how adult weight increases under larval competition. Since egg-to-adult development was not prolonged under larval competition, increased feeding period via longer development is not the cause of the greater adult weight. Furthermore, the proportion of large individuals of the observed distribution at high larval density was significantly higher than that of a simulated distribution where small individuals were eliminated by an imaginary selection. Thus, there must be other causes leading to greater adult weight besides selection on body size. I further hypothesized that cannibalism increases adult weight under larval competition. By dissecting beans with 3 larvae at regular intervals, I found interference behavior and the evidence of cannibalism in larvae of C. maculatus, the finding potentially supporting the cannibalism hypothesis because conspesifics could be extra food source when bean resource was limited. Nevertheless, dominant larvae could also gain more weight by raising total food consumption in response to competition. In this study, the total consumption at high larval density was significantly higher than that at low larval density, indicating the possibility of increased efficiency. Further investigation is required. Finally, a previous study demonstrated that South India strain of C. maculatus could emit feeding vibration at larval stage to affect conspesifics weight. But, no indication in my experiment showed feeding vibration of 4C6-4 strain affected adult weight.