The introduction of the emerald ash borer (EAB), an invasive beetle, has caused widespread destruction of ash trees in North America. To combat this, scientists have proposed the use of biological control agents, specifically parasitic wasps from Asia that naturally prey on EAB. This approach is considered promising for three main reasons. First, parasitic wasps are highly host-specific, meaning they only attack EAB and do not harm other native insects, thus preserving ecological balance. Second, unlike chemical pesticides, which require repeated applications and can contaminate soil and water, parasitic wasps establish self-sustaining populations that continuously control EAB without ongoing human intervention. Third, field trials in Michigan and Maryland have shown significant reductions in ash tree mortality within five years of wasp release, suggesting that this method is effective in real-world conditions. Therefore, proponents argue that biological control is a safe, sustainable, and successful strategy for managing EAB infestations.
🎧 听力讲座
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The lecture challenges the reading's optimism about biological control using parasitic wasps. While the wasps are indeed host-specific, the professor points out that this specificity is a double-edged sword: because the wasps rely solely on EAB for survival, they cannot control EAB populations below a certain threshold, as the wasps die off when prey becomes scarce, leaving enough EAB to rebound and continue damaging trees. Furthermore, the claim of self-sustaining control is misleading; in practice, the wasps require repeated releases because their establishment rates are low, especially in colder climates where they fail to survive winters. Even in successful trials, the professor notes that reductions in ash mortality were only observed in isolated areas, not in broader forest landscapes where EAB spreads rapidly. Additionally, the wasps do not affect EAB larvae that are already deep inside large trees, so mature ash trees still die, undermining the long-term effectiveness. Thus, the lecture argues that biological control is not a silver bullet and may only provide partial, temporary relief.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
