唐德宗興元、貞元之際發生了罕見的巨大蝗災，東盡海濱，西逾河隴，造成北方農業經濟崩潰、民戶流散，以及廣泛的饑饉死亡。安史之亂後尚未復元的唐帝國經此重創，更為衰弱，允為中唐另一大變局。興貞蝗災原因固然多端（動員、戰爭與乾旱），然建中元年（780）「兩稅法」的推行恐是一大關鍵。安史亂後徵稅已極繁苛，而德宗初行兩稅法，「已重於舊」。尤其兩稅法以州縣「都數」為財稅汲取之準則（總額制），變相鼓勵各級官僚非法誅求，也是對「均攤逃戶」的默認。兩稅法施行後，稅賦「比大曆之數再倍」，於是以粟麥旱作為主的華北農業脆弱帶的百姓棄地逃亡，「一室空而四鄰亦盡」，形成重稅∕棄地∕攤逃循環。土地拋荒造成蝗蝻大量滋生，最終演成蟲蝗遍地、糧產崩潰的結果。蝗災之後，德宗與宰臣李泌創置中和節，令二月初一祭勾芒（草木神），祈年穀，其命義主要出於《禮記．月令》農事與自然結合的敬天順時、涵育長養思維，一方面反映興貞饑災帶給德宗極大的震動與創傷，另一方面則顯示李泌匡輔訓導用意之深長。本文以糧價峰值及戶口減耗推估蝗旱造成的破壞，且就華北農業及蝗蟲生物特性提出理論性說明。作者認為，興貞大災對華北農業脆弱帶的深重打擊及所促發的民戶流亡潮，可以從災荒史角度補充解釋安史亂後南北經濟重心易位的加速，以及何以往後唐宋帝國對南方稻作與江淮漕運如此深沉依賴。

In 784-5, a devastating locust infestation swept through northern China, causing widespread famine and the death or displacement of millions. Striking the north before the Empire had fully recovered from the An Lu-shang Rebellion (755-763), this locust infestation proved a critical moment for mid-Tang China. While many possible causes have been suggested for the infestation, from mobilization and military conflict to drought, there is another possible cause that has not received due attention: the two-tax law (兩稅法), promulgated in 780. After the Rebellion, taxes were drastically increased in rate and complexity. Tang De-zong’s new tax exacerbated this trend. Especially burdensome was the fixed quota allotted for each local administration. As opportunities for tax appropriation by officials multiplied, households further had to shoulder more of the burden when their neighbors left the land. Shortly after 780, as the tax burden grew exponentially, more and more peasants fled the land, especially in the millet-and wheat-growing north. Land abandonment meant a higher tax burden for those who stayed on the land, which in turn forced them to flee, as well. With vast tracts of land untilled, locusts swarmed and farming collapsed. After the locust plague subsided, De-zong instituted the Zhong-he Festival. Offerings were made to a vegetation deity in hopes of a good harvest. The idea that such blessings may come from showing respect for nature was based on the teachings in Li ji “Yue ling.” This festival reveals how profoundly shaken De-zong was and suggests prime minister Li Mi’s tactful guiding hand. This paper explores the causes of the locust infestation of 784-5, which might shed some light on the characteristics of locusts behavior and the agricultural fragility of north China. Furthermore, it argues that this locust infestation and its severe impact on the agricultural productivity and demography of northern China was a significant moment in the southward shift of the economic center of the empire.