暖化對農作物害蟲族群成長、組成及拓殖的影響 — 以大豆蚜為例

Abstract

了解暖化對農業系統的影響，是與糧食安全相關的重要議題。暖化對農作物害蟲的影響，會改變作物在暖化下的表現，然而我們仍不清楚1) 暖化會如何改變害蟲族群的大小及組成，進而影響其拓殖？2) 暖化會如何影響三營養階的農業系統 (作物-害蟲-天敵)？本研究利用大豆、大豆蚜、七星瓢蟲探討此議題，因為三者分別為重要的作物、害蟲與生物防治物種。本研究首先進行暖化實驗，檢測暖化對蚜蟲族群數量與組成(有翅型比率)的影響。我們自三個樣區採集大豆蚜，累代飼育，之後隨機挑選無翅型成蚜感染大豆植株，並根據本世紀末2-4oC的暖化預測 (IPCC 2013)，進行控制組(24.5°C)、增溫2°C與增溫4oC的暖化處理。其次的拓殖實驗則利用暖化實驗的結果，檢測暖化下不同的族群組成(結構)將如何影響蚜蟲族群的拓殖與天敵(七星瓢蟲)的生物防治效果。最後則利用數學模擬，檢測根據拓殖實驗結果所提出的假說機制。暖化實驗顯示，最適溫度區間4oC的暖化會改變大豆蚜的族群組成(減少有翅型大豆蚜)，但不影響大豆蚜族群的總數量(有翅型+無翅型)。拓殖實驗發現，天敵的出現無法控制模擬暖化前(高有翅蚜比率)的蚜蟲族群拓殖，但對模擬暖化後(低有翅蚜比率)的蚜蟲族群拓殖有顯著的抑制效果。數學模擬可產生與拓殖實驗類似的交互作用(有翅蚜比率x 天敵處理)。以上的研究結果顯示，未來發生在亞熱帶地區、最適溫度區間的暖化，可能不影響大豆蚜初期的族群總數量，但卻可能改變其族群組成結構(降低有翅型比率)，進而促進天敵控制的成效(減緩蚜蟲族群爆發)。

Understanding warming impact on agricultural system is important because it is related to food security. While it has been known that the response of crop pests to warming (e.g. higher population under warming) is a critical mechanism for the change in crop production under warming (e.g. reduced crop yield), some knowledge gaps remain. For example, it is unclear if warming will affect not only the population size but also the population composition of pests (e.g. alate vs. apterous), consequently changing pest colonization. In addition, warming impact on a tri-trophic system (crop, pest, predator) is less explored. To help fill up the knowledge gaps, this study examined soybeans, soybean aphids, and ladybugs because they represent an important crop, pest, and biocontrol agent, respectively. I first investigated how an experimental warming of 2-4 oC (predicted future warming) would affect aphid population growth and alate proportion in a laboratory. The results showed that warming suppressed alate production (from about 5 to 1 %) without affecting the total aphid population (alate and apterous). Based on the warming experiment results, I then manipulated the initial alate proportion accordingly (warming vs. ambient) in a field colonizatoin experiment, and observed how fast aphids would colonize soybeans in open-top chambers under the presence and absence of a ladybird beetle (major predator and biocontrol agent of aphids). I found interactive effects of alate proportion and predation. Specifically, when the initial proportion of alate aphids was high (5 %; ambient-temperature scenario), aphid outbreaks happened earlier in the presence of a predator. In contrast, when the initial proportion of alate aphids was low (1%; warming scenario), aphid outbreaks happened later in the presence of a predator. Modelings could replicate similar results, suggesting that warming impact on pest population composition (alate proportion) can interact with predator treatment (biocontrol). These results suggest that warming around optimal temperature of soybean aphids may not increase their initial population size but limit their colonization potential by changing population composition (alate proportion), consequently making aphids more susceptible to predation. In other words, biocontrol using predators could become more promising in limiting pest outbreaks and protecting crop yield under future climate change.