是誰惹的禍?──毛毛蟲大反撲

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1998 / 10月

文‧張靜茹 圖‧張良綱


電視畫面上不斷飛出蝴蝶繽紛招展的身影,提醒人們別忽視了台灣各地即將消失的蝴蝶谷。當保育蝴蝶的聲音持續不斷,與蝴蝶同一家族的飛蛾卻一向少為人關注。直到今夏,台灣許多校園、苗圃、行道樹,因為成千成萬的毛毛蟲呼朋引伴,啃光枝繁葉茂的大樹,引起好幾百人皮膚紅腫發癢,釀成一場場災難,飛蛾才頻頻上報。

飛蛾與蝴蝶的幼蟲,被統稱為「毛毛蟲」。毛毛小蟲大反撲,是為什麼?


「墾丁里文化巷遭燈蛾幼蟲肆虐,鎮公所前往消毒滅蟲無效……」

「台東市徐州街、廣東路等地,發生不知名黑色毛毛蟲啃食空地上蓖麻樹葉,每天清晨、傍晚集體翻進圍牆,從門縫、窗戶爬進屋內,數量之多,令人毛骨悚然……」

毛毛蟲侵入屋內?人們「消毒無效」,只覺「毛骨悚然」?報紙從各地傳出的毛毛蟲新聞,活像電影裡突變的巨蟲,扭動著軀體,張牙舞爪,準備對人類發動一波波突襲。

惡客盈門

今年夏天,從台東市區、南部屏東到中部南投、北部新竹,毛毛蟲如火燎原,數以萬計的在各地啃食林木、破壞景觀。

住在南投埔里的老先生巫坤波,種著一分多地的翠綠「竹柏」。當他六月份出國旅遊回來,竹柏園裡迎接他的是一棵棵奄奄一息的禿幹,爬滿千千萬萬的蟲子。一根根火柴棒似的黃帶枝尺蛾幼蟲,蠕動著圓圓的頭兒,滋味十足的拼命啃食所剩無幾的綠葉。許多已搜尋不到嫩葉的蟲兒,就掛在長長的絲上飄晃,欲借風兒盪到他處,有的乾脆拱起身子,辛苦的爬出田埂,進了人家庭院。

放暑假前,靠海邊的新竹南寮國小,五、六棵超過二樓高的茄冬樹,突遭烏桕黃毒蛾幼蟲秋風掃落葉般的啃光樹葉,如今被修剪得只剩一樓高的茄冬,重新勃發出尺長的綠芽,工友執長竿往樹梢一掃,掉下幾十隻毛毛蟲,圍觀者立刻從樹蔭下跳開,大夥對黃毒蛾身上的刺毛引發全校五百多位師生皮膚紅腫、奇癢無比的記憶猶新,學校為此還停課兩天。同時,黃毒蛾幼蟲也在一縣之隔的苗栗竹南國小發飆,大嚼樹齡年過半百的茄冬、槭樹。

在地球上百萬種昆蟲裡,飛蛾與蝴蝶組成的家族,因為翅膀上覆有顏色五彩繽紛的鱗片,被稱為「鱗翅目」,它們的數量與種類可觀,在構成昆蟲三十幾個目中名列第二。

好竹出歹筍?

翩翩蝶、蛾,生下毛毛小蟲,人們慣常將這些身體長著密密細毛、緩緩起伏挪動的小蟲稱為毛毛蟲。其實鱗翅目幼蟲並非都裹著細毛,有的身上長滿肉刺,有的光滑無凸瘤,外型非常多樣。

民國六、七○年代前成長的人,多數都有與毛毛蟲「第一類接觸」的記憶,但當綠地漸由人們生活中消失,都會裡逐日難得一見毛毛蟲,不少蝴蝶、蛾類還被生物學界建議列入保育名單;毛毛蟲為何突然在各地爆發,多到爬進人家屋裡,好似惡客盈門?

根據生態學上的推測,影響昆蟲族群變動的因素,除了天敵、寄主植物等「生物因子」,氣候、土壤、人類污染等「環境因子」往往才是蟲子大發生的主因。近年來的氣候異常遂成為人們懷疑的首要對象。

「其實每年都有蟲子,」新竹南寮國小輔導主任劉芳遠說,每年七、八月學生放暑假後,校園裡幾棵大樹就是毛毛蟲的天下,但去年冬天明顯的高溫,讓蟲兒提早報到,數量又比往年多,才造成學生過敏的「蟲害」。

同時在屏東墾丁社區一片兩百多坪的空地上,上百棵蓖麻也遭燈蛾幼蟲將綠葉一掃而空。林試所恆春分所助理莊鈴木也推測,暖冬讓綠樹提早開花結果,加上五、六月南部雨量充足,植物搶著長,可能是毛毛蟲數量爆發的「輔助因子」。

天氣暖化讓蟲口暴增?大部份蛾類研究人員的看法卻是「純屬缺乏研究的臆測之詞」,因為如果天氣造成毛毛蟲增加,其他種類的昆蟲應該也會跟進。

六年來時常走訪各地、進行林木病蟲害防治的林試所研究員趙榮台說,「今年林試所接到的案子確實多一點,」過去平均一個月三、四件,今年兩個月已發生十幾件,危害林木的除了蛾類幼蟲,肇事者還有浮塵子、粉蝨等其他生物。

其實毛毛蟲變少了?

目前台灣並未有機構或研究者針對溫室效應、雨量變化、植物生長與毛毛蟲族群變動等進行長期監測,因此缺乏真正的數據證實毛毛蟲的盛況與天氣有關。

有人更提出不同看法,認為今年毛毛蟲明顯少了。常理來看,熱帶地區的昆蟲較多,人們自然認為溫度高,蟲兒就旺,「但昆蟲是經過長久時間適應高溫,與溫室效應造成短期內溫度升高是否同一回事?」近來出版台灣《昆蟲入門》與《昆蟲圖鑑》的生態攝影者張永仁說,許多毛毛蟲以蛹過冬,往往需要凍過、冷過,才「心甘情願」化蛹而出。他的經驗是,台灣中海拔的毛毛蟲帶回平地飼養,多半養不活。

顯然,快速的冷冬、暖冬,該冷不冷,該暖不暖,都違反自然。「氣候暖化沒有提供昆蟲更好的繁殖機會,」張永仁甚至感覺到,今年夏季颱風次數減少,雨水不足,影響了毛毛蟲蛻變,「許多觀察、拍攝昆蟲的人都覺得今年毛毛蟲少了。」

近來台灣賞蝶團體的調查就發現,過去每年四月至十月可以持續出現上千萬隻蝴蝶的美濃黃蝶翠谷,今年卻只有十萬隻左右。原有的八十種蝴蝶,今年也驟降為五十種。生物學者初步歸納指出,南部乾、雨季不分明正是原因之一。

一物剋一物

野外工作者的反應,為何與新聞報導大相逕庭?

仔細看看這幾次的毛毛蟲事件,都發生在離住家不遠的校園、行道樹、建築空地、苗圃,毛毛蟲有機會爬進住家、教室,人與蟲「正面衝突」,加上新聞渲染,才讓人覺得毛毛蟲倏地多得好似要「佔領」寶島了。

毛毛蟲大發生的戲碼,在自然界其實從未停止上演。鱗翅目幼蟲絕大多數以植物葉片、果實、花苞為食,它們生存的最大任務彷彿就是埋頭苦吃,若出現天敵減少、花木比往年華發等等「生物因子」的變動,毛毛蟲族群很快就會失控。

植病、昆蟲系師生的經驗豐富,「每回出去採集,都會看到毛毛蟲爬滿樹梢,」台大昆蟲系教授楊平世說。不需就教專家,「這種場面過去多得很,」在今夏茄冬樹上爬滿毛毛蟲的竹南國小校園,工友邱素靜指著樹說,自己小時候看多了。

毛毛蟲爆發,也常在一夕間平復。民國五十年代在屏東南州唸書的農委會保育科技正方國運描述,當時教室是成排矮房子,傍著成排大樹,常常毛毛蟲一下出現一大群,大樹很快被啃光,接著,一大群鳥兒來了,嘰嘰喳喳的飽餐一頓。

大自然的舞臺不停上演著一消一長、互替互換的循環。趙榮台形容發生在校園的「蟲變」,是一場昆蟲與植物的戰爭,蟲兒大舉進攻,大樹一時不敵,但旗開得勝的毛毛蟲不會得意太久。植物並非弱者,它們利用化學、物理方式保護自己,毛毛蟲也有各種天敵,從寄生的病毒、菌類、寄生蜂,到直接大快朵頤蟲兒的鳥類,甚至毛毛蟲寅吃卯糧,一夕間吃光樹葉、果實,蟲數即刻劇降,族群得到自我調節。

除了極少數蛾類,如黃帶枝尺蛾毛毛蟲啃完葉片可能繼續啃食樹皮,絕大部份毛毛蟲是不會將自己的「衣食父母」趕盡殺絕的,大樹自有回春之時,往往第二年、甚至個把月,又大抽枝條,欣欣向榮。

單一樹種的危機

不時接到學校詢問「毛毛蟲吃光樹葉該怎麼辦」的林試所生物系助理范義彬說,前幾年台北縣、市與嘉義等多所國小校園,也曾遭榕舞毒蛾、荳莢野螟幼蟲啃食榕樹、水黃皮等青青校樹。但今年范義彬去電詢問,得到的回答都是:蟲兒數量已不復去年盛況。

如此看來,校園裡往往像個小食物鏈,毛毛蟲可以得到適時控制。但毛毛蟲事件,是否都可以等閒視之?

林試所林木疫情中心負責人張瑞璋指出,都市綠地、校園裡的樹種,常常是從幾株母樹大量培育出來的,樹樹同出一脈,生長與抵抗力退化,讓蟲兒有可趁之機。

以竹南國小為例,過去校園接公園,今天左邊開二十米大道切掉一點校地,右邊蓋房子又刮走一點草皮,學生多了,校園卻縮水,草地又全鋪上PU地板,幾棵大樹困在小小的泥土框框裡,樹根透不過氣,苟延殘喘,抗蟲無力。因此校園裡最普遍的茄冬樹,往往就是黃毒蛾大量繁殖的樂園。

小小毛蟲大爆發,人為因素脫不了關係。特別是南投竹柏園遭黃帶枝尺蛾大舉入侵的例子,竹柏是台灣原生植物羅漢松的一種,台灣羅漢松多達七種,但在林務人員的記憶中,為何從未聽聞黃帶枝尺蛾大量現身?

奇妙的是,多數昆蟲與白蟻拒吃竹柏,因為畏懼它身上特殊的代謝物質,唯獨黃帶枝尺蛾一物剋一物。但在天然林裡,螳螂捕蟬,黃雀在後,有白殭菌、寄生蠅虎視眈眈地候著黃帶枝尺蛾,幼蟲又只愛寄居在至少兩公尺高的樹上,此時樹已夠健壯,幾隻「小毛蟲」根本不構成威脅。沿著台北木柵指南宮步道,零星站立著高大的羅漢松,幾隻黃帶枝尺蛾幼蟲蠕行於枝葉間,與大樹彼此相生相剋,和平共存。

在台灣,羅漢松與黃帶枝尺蛾並未嚴重失衡;在琉球,黃帶枝尺蛾卻是林務人員的心腹大患。過去琉球廣由外地引進羅漢松,作為神社材料,黃帶枝尺蛾幼蟲順天應人,把握良機,洗劫大片的羅漢松純林,琉球因此曾與台灣合作,計畫進口黃帶枝尺蛾天敵,中、日還為此開過多次跨國會議。可惜計畫尚未成功,如今台灣已成「疫區」。近來,台灣農民視驅蟲的竹柏為優良景觀樹種,大量人工栽種,千萬年的平衡抵不過幾年的造景,苗圃終於成為第一道被黃帶枝尺蛾瓦解的防線。

全民「造」蟲

同樣的,當全民造林業務蒸蒸日上,也是昆蟲學者最憂心忡忡之時。

因應全球二氧化碳增加的趨勢,政府推動大量造林,但短期內在人為開發過的環境造純林,代表的是未來土地上的生產者只有一種,無法孕育生物的複雜度,如此創造出的是殘缺不全的食物鏈。

同一種林木遭受蟲害的例子俯拾皆是。今天都市裡到處是樟樹,就到處是樟白介殼蟲,忙壞了「樹醫生」四處救火;大量種銀合歡,等於便宜了木蝨;日據時代廣種泡桐,光復後,全省泡桐由北而南同時遭簇葉病肆虐。在人為推波助瀾下,過去毛毛蟲如蝗蟲過境也有前例。

十多年來,黑角舞蛾幼蟲在台灣釀成的災害,足以讓今年各地的毛毛蟲相形見絀。

在中部沿海,黑角舞蛾猛烈「攻擊」海岸防風林,木麻黃一棵棵失去綠意,防風、綠化功能盡失,母蛾靠著善飛的本領,四處延伸觸角,已危害中部的荔枝、龍眼果園。

林木疫情中心

一九○三年,英國生物學家史溫侯在台灣首次記錄到黑角舞蛾,一九二八年,苗栗就出現黑角舞蛾危害人工相思樹林,因此一度被稱為「相思樹毒蛾」。

接著,日據時期開始大量種植的木麻黃,垂垂老矣,也成為黑角舞蛾的宿主;更有人懷疑黑角舞蛾是由外地被引進台灣的「外來種」,才會對防風林造成強大殺傷力。

比較起來,校園裡的毛毛蟲事件在學界看來「小事一樁」,因為學界更擔心外來昆蟲的災害。舉世聞名的外來種昆蟲成為生態大敵,也是飛蛾毛毛蟲惹出來的。

十九世紀以來,歐洲舞毒蛾成為美國林業人員的最痛。歐洲移民為了生產蠶絲,將歐洲舞毒蛾引進北美,最後舞毒蛾由實驗室被釋出,四通八達的交通將之帶往各州,橫掃北美大陸,危害林木,破壞景觀,舞毒蛾身上的毒毛,更成為可怕的過敏源。一九八一年舞毒蛾危害最烈時,曾經盤據六百萬公頃森林,百年來美國試過噴灑各種藥劑、發動學童捕抓等等對策,至今仍無法掌握舞毒蛾大發生的頻率。

美國的憂心可從他們關心台灣黑角舞蛾的情形看出來,由於擔心黑角舞蛾可能隨著台中港船隻「偷渡」,步舞毒蛾之後「攻佔」美國,美國農業部已連續四年派人來台灣觀察、並協助撲殺黑角舞蛾。

台灣並引以為戒,成立「林木疫情中心」,接受各地對林木疫情的通報,以即時因應,「如此也才可能更有深度的看整體林業體系的問題,將傷害減到最低,」林木疫情中心負責人張瑞璋說。

誰在培養「駭客」?

事實上,相較於短期收穫的農業,鱗翅目幼蟲對林業造成的危害,只是小巫見大巫。

蛾類幼蟲中專吃豆類的豆莢野螟,早被公認為世界性分佈的「大害蟲」;僅一種專吃葉菜類的小菜蛾,就讓農試所投資二十幾年的心力,試圖找出最有效的防治對策。

但毛毛蟲真有這麼可怕嗎?從整體昆蟲世界看來,蝶、蛾幼蟲並沒有特別的「攻擊性」,比之蚊子、蒼蠅等繁殖力高、飛行力強的雙翅目昆蟲,大部份蛾與蝶一年才一代,其中也只有少數種類喜愛團體活動、可能群起「危害」作物。

特別是蝴蝶,對環境的要求嚴格,需要一定的飛翔空間與適當的陽光才願求愛、繁殖,雌蝶又不將卵產在一處,卵數又少得可以數出來,絕非「駭客」型昆蟲。

毛毛蟲之為禍,追根究柢,還是人為的農林業生產方式在推波助瀾。成甲成頃的農作、林木,看來雖整齊畫一,悅人耳目,卻是毛毛蟲的天堂。例如紋白蝶,最愛逗留在高麗菜、白菜田;熱帶地區的柑橘類果園,也提供了鳳蝶幼蟲充足的糧食。蛾類當中,確實又有一些繁殖快,對環境不甚挑剔的,在清一色的農地裡,不發也難。

現代的農林生產原則下,要求作物百分之百收成,少數的蛾、蝶「害蟲」,長期受到大力防堵,遭農藥毒殺,但抗藥性也越強,往往一波未平一波又起,人類與蟲的爭鬥少能高奏凱歌,只能與之達成「恐怖平衡」,不斷噴藥控制。

針對舞毒蛾在美國的「激情上演」,《科技反撲》一書指出,噴灑農藥只有威脅到生物多樣性,卻無法消滅舞毒蛾,舞毒蛾如今「危機與平靜歲月交互上演」,隔幾年就危害一次。

難道人與蟲之間的問題真的無解?以「昆蟲的社會行為」作為研究主題的趙榮台建議,人們不如換個角度看待毛毛蟲,才可能接納它的「缺點」,它也不再是人類難以消滅的大敵。

除了死蟲,沒有一隻好蟲?

在食物鏈裡,毛毛蟲的存在,有其意義。常在宜蘭福山採集飛蛾的范義彬,看過麻雀捕食木麻黃大敵──黑角舞蛾,「麻雀將蛾的翅膀拉掉,吃它胖胖的肚子,」范義彬的形容聽來殘忍,卻是蛾類在生態裡不可或缺的角色。許多研究青蛙、蝙蝠、獼猴的生物界同行,常常來請教范義彬,這些動物常吃的蛾類是什麼蛾?

趙榮台解釋毛毛蟲的「正面」角色:小蟲兒吃花木,其他生物再吃毛毛蟲,它們是金字塔上最基層的生物,扮演食物轉換者,是較大型動物的蛋白質來源。據估計,鱗翅目昆蟲佔肉食性野生動物食物來源的百分之一左右。如果蝶、蛾一夜間消失,將導致許多動物難以度日。

自然環境裡,毛毛蟲取食植物,也有助森林汰舊換新。《大美百科全書》寫的直接明白:「對生態系而言,蝶與蛾非常重要。因為幼蟲會大量攝食植物,成蟲訪花吸蜜達到花粉的傳播,同時它們也成為其他動物的食物。」有些植物如蘭花與牽牛花,便完全依賴特定幾種天蛾來傳粉。

人們眼中的「害蟲」,是生態體系的一份子。甚至有生態學者認為,歐洲舞毒蛾也並非全無是處,在美國,被開發過的土地上,強勢的橡樹壓抑了其他樹種的生長,舞毒蛾專吃橡樹葉子,等於讓其他林木有機會出頭,讓林地恢復生物歧異度。

最佳生態教材

排除利害關係,毛毛蟲的重要性益加凸顯。因此今天毛毛蟲的「大發生」,學界建議視個案處理。以農作物而言,收成期短,除非改變生產方式,否則目前仍需要適量噴藥;但庭園、森林作物短期出現蟲害,除了少數可能對林木造成致命的「蛀榦性」蝙蝠蛾、木蠹蛾,森林學者通常建議不要以噴藥方式對蟲兒大開殺戒。

久而久之,生長期長的大樹,常常與昆蟲達成雙方可容忍的「動態平衡」。就連貪食木麻黃的黑角舞蛾,幼蟲只有不到兩個月生命期,這段時間,木麻黃雖無力為人們防風、遮陽,但蟲子有長達九個月的卵塊期,木麻黃也就得到喘息。

至於南部恆春墾丁國家公園內,平均五年,玉帶鳳蝶即密密麻麻爬滿芸香科植物「過山香」上,互相推擠間,不斷有幼蟲由樹上紛紛墜落。有人問台大昆蟲系教授楊平世怎麼辦?他回答:許多人等不到機會看這樣的奇景,就當成最好的生態教材吧。

校園裡毛毛蟲偶發性的失控,若非心理上的恐懼,更是對學生的最好機會教育。張永仁認為,它讓人們知道:自然的力量會反過來壓制人對自然體系的不當干擾。

絕望的復仇者

只惜基於本能上的恐懼與不解毛毛蟲習性,多數學校反應劇烈。

毛毛蟲爬滿茄冬樹後,南寮國小試著以火焰槍火攻、以水管水攻,五棵茄冬樹甚至噴掉上百公升農藥。

學校過度反應,反讓毛毛蟲有反擊之機。烏桕黃毒蛾身上密佈刺棘般的細毛,噴藥固然能在短期內殺死毛毛蟲,但死前的掙扎讓刺毛脫離身體,在新竹的大風相助之下,就像「死前的最後一擊」,空氣中飄揚的毒毛驟然增加,進入孩子呼吸道,皮膚起風疹塊般紅癢難耐,全校半數師生因此就醫。

楊平世說,校園裡的大樹,只要平日多加修剪,就可以減少毛毛蟲大發生,遇上毒蛾,也不需刻意消毒,只需暫時與之隔離。人們只要多瞭解蟲兒,就能應付裕如,何需大動干戈?

毛毛蟲新聞接二連三,反映一般人對毛毛蟲的無知。台灣鱗翅目昆蟲將近一萬種,族繁不及備載,除了四百種蝴蝶,因為種類少、常被作為觀賞對象,研究也較多;至於蛾類,只有影響經濟收益的「害蟲」,才有機會成為被研究的對象。但由防治觀點對蛾類進行認識,造成蛾類生態研究極度缺乏,也反過來影響防治效果。

不只是蝴蝶王國

趙榮台說,台灣學術界對蛾類的知識不足,缺乏科學基礎,連最基本的蛾類辨識都有問題,一度把危害農作物的歐洲玉米螟誤認為亞洲玉米螟。

從農業觀點來看,更需要瞭解這些人類糧食的競爭者,「對其生活越瞭解,越容易逮到牠,」趙榮台說,掌握昆蟲生態、生活與族群的活動模式,可以針對其弱點加以預防。

昆蟲族群生態的研究,也可以使人們預測害蟲的密度變動和可能造成的損失,並預先決定施行何種防治,附帶減低了農藥的使用與對人體健康的衝擊。

小小台灣常被稱為「昆蟲王國」,但台灣對昆蟲生態的瞭解,往往只侷限在蝴蝶身上,大部份人說不出昆蟲王國的所以然來。

除非人們對周遭的其他生命,小如毛毛蟲都能多一點瞭解,否則,成群的毛毛蟲啃光大樹成為「驚悚」新聞,恐將一再出現報端。

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EN

The Caterpillars Strike Back-- But Who's to Blame?

Chang Chin-ju /photos courtesy of Vincent Chang /tr. by Robert Taylor

In a TV advert, a stream of colorful, flut-tering butterflies reminds people in Taiwan not to ignore the fact that butterfly valleys all over the island are at risk. But while many voices call on us to save Taiwan's butterflies, few seem concerned about their close cousins, the moths. It was not until this summer, when hordes of hungry caterpillars invaded many school campuses, plant nurseries and roadside trees, stripping branches bare of leaves and causing red itchy swellings on hundreds of people's skin, that moths finally made it into the news in a big way.

The larvae of butterflies and moths are all called caterpillars. But why should caterpillars "strike back" in this way?


"Tiger moth caterpillars are causing havoc in Wenhua Lane, Kenting. They have so far defeated all the town council's efforts to exterminate them. . . ."

"In areas of Taitung City such as Hsuchou Street and Kuangtung Road, unidentified black caterpillars are eating the leaves of castor-oil plants on a vacant lot. Every day at dawn and dusk they swarm over walls and crawl into neighboring houses in frightening numbers, coming in under doors and through windows. . . ."

Caterpillars invading houses? Caterpillars "defeating" efforts to eradicate them? Caterpillars in "frightening" numbers? In newspaper reports from different parts of Taiwan, wriggling caterpillars are described as if they were the mutant monsters of horror films, preparing to attack the human race in relentless waves with bared fangs and flailing claws.

Unwelcome guests

This summer, from Taitung City in the east and Pingtung County in the south to Nantou County in central Taiwan and Hsinchu County in the north, caterpillars spread like wildfire, leaving trees bare and ugly.

Old Mr. Wu Kun-po of Puli in Nantou County had planted over 1000 square meters of land with emerald-green Podocarpus formosensis trees. In June, he came back from a holiday trip overseas to find his plantation reduced to an expanse of bare, dying tree trunks covered in thousands of matchstick-like inchworms-caterpillars of the inchworm moth species Milionia basalis pryeri. The grubs' little round heads waved from side to side as they busily chomped their way through the last remaining leaves. Many, unable to find any more tender leaves to munch on, were hanging by long silken threads, waiting for the wind to swing them across to other trees; others arched their backs and wriggled off over the field embankments and into people's gardens.

Before the summer holidays, at Nanliao Elementary School on the coast of Hsinchu County, a grove of five or six bishopwood trees (Bischofia javanica) over two stories tall were rapidly stripped of their leaves by caterpillars of the two-spotted lymantriid moth (Euproctis bipunctapex), which left them as bare as if swept by autumn gales. Today, the bishopwood trees, which have been pruned back to a story tall, have sprouted foot-long new leaves. When a workman waves a long pole through the treetops, dozens of caterpillars come tumbling down. The onlookers immediately jump back out of the shade of the tree-they still have fresh in their memories how the spiny hairs on the caterpillars' bodies caused the skin of all the school's more than 500 pupils and teachers to swell up in an incredibly itchy red rash, so that the school had to be closed for two days. At the same time, two-spotted lymantriid moth caterpillars also went on the rampage at Chu-nan Elementary School in neighboring Miaoli County, ravaging the school's more than 50-year-old bishopwood and maple trees.

Among the world's million or more known insect species, the order formed by the moths and butterflies-called Lepidoptera (from Greek lepido-, "scaly" and ptera, "wings") for the often brightly-colored scales which cover their wings-accounts for a substantial proportion in terms of both populations and number of species. It is the second largest of the more than 30 insect orders.

Butterflies good, caterpillars bad?

The larvae of moths and butterflies are called caterpillars. Their name in Chinese-maomaochong-means "hairy grubs," and in fact the English word caterpillar also comes from Old North French cate pelose, meaning "hairy cat." But although many caterpillars do have a dense covering of fine hairs on their bodies, some are covered with fleshy spines, while others have completely smooth skins. Lepidoptera larvae, which amble along at a leisurely pace with undulating bodies, come in a tremendous variety of shapes and sizes.

Most people who grew up before the 1970s or '80s have memories of "close encounters of the first kind" with caterpillars. But as green spaces have gradually disappeared from people's lives, opportunities to see caterpillars in the cities have grown fewer and fewer, and biologists have even recommended that many butterfly and moth species be placed on the protected list. So why did the numbers of caterpillars in many places explode to such levels that they invaded people's homes?

In ecological terms, changes in insect populations are affected both by "biological factors," such as their natural enemies and host plants, and by "environmental factors," such as weather, soil and man-made pollution. The abnormal weather of recent years has gradually become the main suspect.

"Actually, we get caterpillars every year," says Liu Fang-yuan, head guidance counselor at Nanliao Elementary School. Every July and August, during the summer holidays, the several large trees in the school grounds become a caterpillar kingdom. But following unusually warm weather last winter, this summer the grubs reported for duty earlier than usual and in larger numbers, causing allergic reactions among the children. Only then were they perceived as a "pest."

At the same time, in the town of Kenting in Pingtung County, a hundred or more castor-oil plants (Ricinus communis) on a vacant lot several hundred square meters in area were stripped of all their leaves by tiger moth caterpillars (family Arctiidae). Chuang Ling-mu, a research assistant at the Hengchun research station of the Taiwan Forestry Research Institute (TFRI), says the warm winter caused trees to flower and fruit early, and the plentiful rainfall in May and June boosted plant growth. He speculates that these may have been "contributory factors" in the explosion in caterpillar populations.

Is climatic warming behind the sudden growth in insect populations? Most heterocerists (moth researchers) take the view that in the absence of research, this is "pure conjecture," for if the weather causes caterpillar numbers to rocket, it should have the same effect on other insects too.

TFRI researcher Chao Jung-tai, who over the past six years has traveled all over Taiwan doing tree pest and disease control work, says: "There really has been some increase in the number of cases reported to the TFRI this year." In the past, reports averaged three to four a month, but this year there have been over ten in two months. Moth larvae have not been the only pests: they have also included leafhoppers, whitefly and others.

Less caterpillars, not more?

At present, there are no organizations or researchers in Taiwan doing long-term monitoring of relationships between the greenhouse effect, changes in rainfall, plant growth and changes in caterpillar populations. Hence there is no hard data to support the idea that the abundance of caterpillars is related to climatic factors.

Some people even argue that there has been a marked decline in the number of caterpillars this year. Generally speaking, insects are more abundant in the tropics than in cooler climes, and this naturally leads people to suppose that if temperatures rise, insect populations will increase. "But insects have adapted to high temperatures over long periods. That's not the same as rapid rises in temperature due to the greenhouse effect," says Chang Yung-jen, a nature photographer who recently published a two-volume introduction and field guide to Taiwan's insects. He notes that many caterpillars pupate during the winter, and they often need to be exposed to low or even sub-zero temperatures before they are "willing" to finish their pupation and emerge as adults. In his experience, when people bring caterpillars from Taiwan's medium elevations down to the plains to try to raise in captivity, the majority do not survive.

Evidently, short or warm winters and unseasonable cold or warm spells all upset the usual pattern of nature. "Climatic warming does not improve insects' chances of reproduction," says Chang Yung-jen. He even feels that the reduced rainfall this summer due to the small number of typhoons is affecting caterpillars' ability to pupate. "Many people who observe or photograph insects have the impression that caterpillar numbers are down this year," he says.

Recent surveys by butterfly watching groups in Taiwan have shown that this year in "Yellow Butterfly Valley" near Meinung in Kaohsiung County, where some 10 million butterflies usually appear over the course of the butterfly season from April to October, the numbers have fallen to only about 100,000, and of the 80 species normally found there, only 50 have been observed. Biologists' initial conclusion is that the lack of a clear distinction between the dry and rainy seasons in southern Taiwan this year is among the causes.

Waxing and waning

Why is the reaction of field workers so different from the impression given by media reports?

Looking more closely at the recent spate of caterpillar incidents, we find that they all happened in school grounds, roadside trees, vacant building lots, seed nurseries and other places close to residential buildings, so that the caterpillars had the opportunity to crawl into houses and classrooms and come into "face-to-face conflict" with human beings. This, and a degree of sensationalism in media reporting, gave the impression that caterpillars were rapidly growing so numerous that they were about to "occupy" the whole island.

In fact, out in the natural world, caterpillar swarms are far from unusual. Most Lepidoptera larvae feed on plants' leaves, fruit, or flower buds, and their main mission in life seems to be to get their heads down and eat for all they are worth. "Biological factors" such as a fall in the population of their natural enemies or unusually lush vegetation can quickly cause their numbers to get out of control.

Students and teachers in entomology and plant pathology departments have much experience of this. "Whenever we go out collecting specimens, we always see the treetops swarming with caterpillars," says Professor Yang Ping-shih of National Taiwan University's Department of Entomology. In fact, there's no need to ask the experts: "We used to see this kind of thing all the time," says Chiu Su-ching, a maintenance worker at Chu-nan Elementary School, where the bishopwood trees were infested with caterpillars this summer-as a child, she saw many such occurrences.

Furthermore, caterpillar population explosions usually end just as quickly as they begin. Fang Kuo-yun, a specialist in the Resources Conservation Division of the Council of Agriculture's Forestry Department, who went to school in Nanchou in Pingtung County in the 1960s, describes how at that time the classrooms were in rows of one-story buildings, alongside which were rows of tall trees. Often large swarms of caterpillars would suddenly appear and quickly eat all the leaves off the trees; but then big flocks of birds would arrive and feast on the caterpillars.

On nature's stage, the waxing and waning of different species and the supplanting of one by another is a scene which is constantly played out. Chao Jung-tai describes the schoolyard incidents as being part of a war between the insects and the plants. The insects mount a massive attack, which the trees are at first unable to fend off. But despite their initial victory, the caterpillars rarely have the last laugh. The trees are not defenseless weaklings: they protect themselves by chemical and physical means, and the caterpillars have all sorts of natural enemies too, from viruses, fungi and parasitic wasps which attack them from the inside, to birds which eat them wholesale. The grubs are also the victims of their own appetites, for once they have eaten all the leaves and fruit off a tree, starvation acts as a regulating mechanism, and their numbers fall dramatically.

The caterpillars of a very small number of moth species, such as Milionia basalis pryeri, may go on to eat the bark of trees once they have finished the leaves. But the vast majority of caterpillars do not "bite the hand that feeds them" in this way. Generally speaking, the next spring-or even after a month or two-the tree will sprout new foliage and be restored to its former glory.

The dangers of monoculture

Fan Yi-pin of the TFRI's Division of Forest Biology, who often fields enquiries from schools as to what to do about caterpillars eating the leaves off their trees, recounts how a few years ago the banyan and pongamia (Pongamia pinnata) trees in the grounds of numerous elementary schools in Taipei City, Taipei County, Chiayi and elsewhere were infested with caterpillars of the clear-winged lymantriid moth (Perina nuda) and of Maruca testulalis, a pod borer. But this year when Fan telephoned schools to ask about the current situation, they all replied that there were fewer caterpillars than in previous years.

Thus it appears that within the small food chain of a school campus, caterpillar infestations will generally come under control sooner or later. But does this mean one can always simply sit back and wait for caterpillar attacks to pass?

Chang Jui-chang, who runs the TFRI's Center for Forestry Disease Control, points out that most trees in urban green areas and on school campuses have been propagated from a small number of parent trees, and their reduced vigor and immune resistance make them vulnerable to insect attack.

Chu-nan Elementary School is a good example. The school grounds used to adjoin a park, but today a 20-meter-wide main road has cut off part of the grounds to the left, while to the right some new houses have taken away part of the playground. The number of pupils has increased, but the school grounds have shrunk, and the areas where grass once grew are now all covered with polyurethane matting. The few large trees are ringed with concrete, depriving their roots of air. Hanging on to life by a thread, they have little ability to resist insect attacks. Hence the bishopwood trees, which are the commonest trees on school campuses, often become a breeding paradise for Milionia moths.

Human influences are certainly a factor in the occurrence of plagues of caterpillars. The swarm of inchworms in the podocarp plantation in Nantou County is a prime example. Podocarpus formosensis is a native Taiwanese podocarp, from the yellow-wood or Buddha's pine family (Podocarpaceae). Taiwan has seven native podocarps, but forestry experts do not recall ever hearing of them being attacked by such large numbers of Milionia basalis pryeri in the past.

Strangely, termites and the majority of other insects do not eat Podocarpus formosensis because of the special metabolites it produces; only M. basalis pryeri will touch it. But in natural forests, every creature has its natural enemies, and the insect fungus Beauveria bassiana and parasitic flies are ready and waiting for M. basalis pryeri. Furthermore, in the wild the inchworms prefer trees over two meters tall, and by the time trees have reached this size they are strong enough for a few caterpillars not to pose them any threat. Along the footpaths around Chihnan Temple in Taipei City's Mucha area, each of the scattered large podocarps has a few M. basalis pryeri living on it, but each is a match for the other and they coexist peacefully.

Plant a tree?

In Taiwan, there was formerly no serious imbalance in the ecological relationship between podocarps and Milionia basalis pryeri, but on Japan's Ryukyu Islands M. basalis pryeri has become one of the most serious pests forestry workers have to contend with. Podocarps were introduced into the Ryukyu Islands to provide wood for Shinto shrines, but this created a golden opportunity for M. basalis pryeri to plunder the large monocultural podocarp plantations. Because of this, a cooperative program was once set up between the Ryukyu Islands and Taiwan to introduce natural enemies of M. basalis pryeri as biocontrols, and several international conferences were held between the ROC and Japan. But the program failed to produce any tangible results, and the same problem has since emerged in Taiwan. In recent times, nursery operators in Taiwan have seized on Podocarpus formosensis as a visually attractive and supposedly insect-resistant tree species, and planted it in large numbers. But a few years of planting have upset the equilibrium established over thousands of years, and tree nurseries have become the places where Miliona basalis pryeri has breached the podocarp's defenses.

Similarly, current efforts to promote a "plant-a-tree" movement have entomologists very worried.

In response to the global trend of rising levels of atmospheric carbon dioxide, the government is encouraging large-scale reforestation. But rapid planting of single-species forests in previously cleared areas implies that in the future, each such area will contain only one producer species, thus limiting biodiversity and creating incomplete and fragmentary food chains.

Examples of monocultural woodland being attacked by insect pests can be found everywhere. In our cities today, camphor trees (Cinnamomum camphora) grow everywhere, and the camphor scale insect is everywhere too, so that tree surgeons are rushed off their feet treating outbreaks. Planting large numbers of white popinac trees (Leucaena glauca) is a gift to jumping plant lice (family Psyllidae). Under Japanese rule, paulownia (Paulownia x taiwaniana) was planted widely, but after Taiwan's return to Chinese rule, paulownia trees throughout the island were simultaneously stricken with fungal leaf curl disease. With unwitting human assistance, there have also been other cases in the past of insect larvae causing locust-like devastation.

The damage caused in Taiwan over the past decade or more by the casuarina moth (Lymantria xylina) puts this year's caterpillar swarms in the shade.

Along the coast of central Taiwan, casuarina moth larvae have vigorously "attacked" the coastal windbreak plantations, stripping the beefwood trees (Casuarina equisetifolia) of their green leaves so that they lose their function as windbreaks and their visual attractiveness. Furthermore, the female moths, which are strong flyers, have spread their wings to also threaten central Taiwan's lychee and longan orchards.

The Center for Forestry Disease Control

In 1903, the British naturalist Swinhoe first recorded the casuarina moth in Taiwan; by 1928 it was damaging Formosan koa (Acacia confusa) plantations in Miaoli County.

Next, as the beefwood trees which were planted in large numbers during the Japanese occupation began to grow old, they also became hosts for the casuarina moth. Some people even suspect that the casuarina moth was introduced from outside Taiwan, and this is why it is able to do such damage to beefwood plantations.

By comparison, experts regard the caterpillar swarms on school campuses as "minor incidents"; researchers are far more worried about the ecological threat from introduced insect species. All the well-known introduced insect pests are moths.

Since the 19th century, the European gypsy moth (Lymantria dispar) has been one of the biggest headaches for American foresters. European immigrants imported the moth into North America to produce silk, but eventually it escaped from the laboratories, and America's well-developed transport system carried it into every state. It swept across the North American continent, damaging and disfiguring forests, and the poisonous hairs on the caterpillars' bodies also came to be feared as a powerful allergen. In 1981, when the gypsy moth outbreak was at its height, it affected 6 million hectares of forest. Over the past century, various strategies have been tried in the USA, from spraying with all kinds of pesticides to sending out schoolchildren to catch the insects. But the incidence of outbreaks has never been brought under control.

American interest in Taiwan's casuarina moth situation shows how concerned they are by this pest. To prevent the moth entering the US as a "stowaway" on ships from Taichung Harbor, for the last four years the US Department of Agriculture has sent personnel to Taiwan to act as observers and to assist in exterminating casuarina moths.

Taking warning from other countries' experience, Taiwan has set up a Center for Forestry Disease Control to collate reports on forest pests and diseases from all over Taiwan in order to coordinate timely responses. "Only in this way can we get a more in-depth view of problems affecting the whole forestry industry, and reduce their harmful effects to a minimum," says TFRI researcher Chang Jui-chang, who is in charge of the center.

Who is encouraging the troublemakers?

In fact, the damage which Lepidoptera larvae cause to forestry pales into insignificance compared with the losses they cause in agriculture, with its much shorter cropping cycles.

The pod borer moth Maruca testulalis, whose larvae feed on pea and bean plants, has long been recognized as a worldwide major pest; and a single moth species, the diamond back moth (Plutella xylostella), whose larvae feed on cruciferous plants (including many leaf vegetables), has caused the Taiwan Agricultural Research Institute (TARI) to invest over two decades' effort into trying to find the most effective ways to control it.

But are caterpillars really so terrible? Compared with other members of the insect world, butterfly and moth larvae are not especially "aggressive," nor are they such rapid breeders and strong flyers as insects from the order Diptera, such as flies and mosquitoes. Most moths and butterflies produce only one new generation per year, and only a minority are likely to attack crops in swarms.

Butterflies in particular have very specific demands as to their environment. They will not breed unless they have adequate space in which to fly and the right amount of sunshine. The females produce only a small number of eggs, and do not lay them all in one place. They are far from being "plague" insects.

In fact, caterpillars' ability to cause damage is amplified by human methods of agricultural and forestry production. Hectare upon hectare of crops and forest plantations may look tidy and pleasing to the eye, but they are also caterpillar heaven. For instance, the Indian cabbage white butterfly (Pieris canidia) is at its most content in cabbage fields, while tropical citrus orchids provide plentiful food for the caterpillars of swallowtail butterflies (family Papilionidae). Many moths are faster breeders than butterflies and less particular about their environment, and amid vast expanses of farmland it would be remarkable if they did not proliferate.

Today's agricultural and forestry production methods aim for the highest possible yields. Over many years, great efforts have been made to control a small number of moth and butterfly "pests" by the use of insecticides. But the insects' resistance to the chemicals has grown, and often one outbreak has hardly receded when another arises. Humans have rarely been able to celebrate victories in their struggle with insects-they have only been able to achieve a "balance of power" by continuing to spray chemicals in order to keep the insects under control.

With reference to the history of the gypsy moth in America, in the book Why Things Bite Back: Technology and the Revenge of Unintended Consequences, author Edward Tenner notes that spraying with pesticides is a threat to biodiversity, but has not been effective in eradicating the gypsy moth. At present the US is going through "alternating periods of crisis and calm" with the moth, with a renewed crisis every few years.

Is there really no solution to the conflicts between humans and insects? Chao Jung-tai, whose research focuses on insect social behavior, suggests that if humans could look at caterpillars from another perspective and accept their "faults," they would no longer need to regard them as obdurate enemies.

The only good bug a dead bug?

Caterpillars have their own place in the food chain. Fan Yi-pin, who often collects moth specimens at Fushan in Ilan County, has watched sparrows eating that great enemy of beefwood trees, the casuarina moth. "The sparrows pull off the moths' wings and eat their plump abdomens." Fan's description may sound grisly, but it highlights the moths' indispensable ecological role. Biologist colleagues studying creatures such as frogs, bats or monkeys often ask him to identify the moths these animals feed on.

Chao Jung-tai explains caterpillars' "positive" role: the grubs eat plants, and other animals eat the grubs. They are nutrient converters at the very bottom of the food pyramid, and a source of protein for larger animals. It is estimated that Lepidoptera account for 1% of all wild carnivores' food sources. If butterflies and moths were to suddenly disappear, many animals would find it hard to survive.

By eating plant matter, caterpillars also assist the process of replacement and renewal in the natural environment. The Chinese-language edition of Encyclopedia Americana says plainly: "Ecologically, butterflies and moths are extremely important because their larvae consume large quantities of plant matter, the adult insects transfer pollen when they visit flowers to drink nectar, and they also serve as food for many other animals." Some plants, such as certain orchids and morning glories, rely entirely on specific hawk moth species (family Sphingidae) for their pollination.

The insects which humans regard as "pests" are part of the ecological system. For instance, some ecology researchers believe the gypsy moth is not entirely unbeneficial in the US: on previously cleared land, dominant oak species suppress the growth of other trees, but gypsy moth caterpillars eat oak leaves, thereby giving other trees more of a chance and helping woodland regain its diversity.

Object lessons in ecology

When one moves away from the narrow perspective of human economic interest, caterpillars' importance becomes very apparent. Hence experts suggest that caterpillar "swarms" should be dealt with flexibly according to the individual circumstances. Because of agricultural crops' short harvesting cycles, there is as yet little alternative to controlling pest outbreaks by the moderate use of pesticides, unless production methods can be changed; but when transient insect infestations occur in gardens or in woodland-except where "wood borers" such as ghost moths (family Hepialidae) or goat moths (family Cosiidae), which are potentially fatal to trees, are involved-forestry experts generally advise against attempting wholesale eradication with pesticides.

Trees, which have a long growing period, usually reach an eventual "dynamic equilibrium" with insects which both can tolerate. Even casuarina moth caterpillars, which eat beefwood leaves so voraciously, live for less than two months. Although the trees may not be able to function as windbreaks and shade givers during this period, they have plenty of time to recover during the nine months the insects spend as eggs.

In the Kenting National Park on the Hengchun Peninsula at Taiwan's southern tip, about once every five years dense swarms of larvae of the common mormon butterfly (Papilio polytes pasikrates) cover Clausena excavata trees (which belong to the family Rutaceae, the citrus family). There are so many caterpillars that they continually jostle each other off the trees. When NTU entomology professor Yang Ping-shih was asked what should be done about such swarms, he replied that many people never get the chance to see this remarkable sight-it can serve as an excellent aid to teaching ecology.

Occasional caterpillar population explosions in school grounds also present a superb educational opportunity for the children, if they can get over their fear. In Chang Yung-jen's view, such events demonstrate that the power of nature will thwart humans' misguided meddling in natural systems.

Desperate avengers

But sadly, due to instinctive fear and a lack of understanding of caterpillars' life cycles, the majority of schools overreact.

When caterpillars infested the five bishopwood trees at Nanliao Elementary School, the grubs were attacked with fire from flame wands and water from hosepipes, and even sprayed with over 100 liters of insecticide.

But this extreme reaction by the school gave the caterpillars an opportunity for revenge. Two-spotted lymantriid moth caterpillars' bodies are covered with fine, spiny hairs, and although the insecticide quickly killed the grubs, their death throes released the hairs from their bodies. The number of poisonous spines drifting in the air increased dramatically, and with the help of Hsinchu's strong winds they got into the children's respiratory tracts; the children also came out in an unbearably itchy red rash. Over half the school's staff and pupils had to seek medical attention.

Yang Ping-shih says that if large trees on school campuses are pruned regularly, the likelihood of caterpillar swarms is reduced, and if lymantriid moths do appear there is no need for spraying-all that is necessary is to keep people away from the trees for the time being. If people understood insects better, they could easily deal with them without needing to resort to drastic measures.

The stream of news reports about caterpillars reflects most people's ignorance about them. Taiwan has probably close to 10,000 species of Lepidoptera, many of which have not even been described. The relatively small number of butterflies among them-around 400 species-are considered aesthetically pleasing, so a fair amount of research has been done on them. But with moths it is a different story: only those which are economically significant "pests" are likely to be studied. However, studying moths purely from the perspective of controlling them has led to a dearth of research into their overall ecology, and this in turn impairs the effectiveness of control measures.

Not just a butterfly kingdom

Chao Jung-tai says that biologists in Taiwan lack sufficient knowledge of moths. Without adequate scientific knowledge, even basic identification of moth species is difficult. For instance, an outbreak of the crop-damaging European corn borer (Ostrinia nubilalis) was for a time misidentified as the Asian corn borer (Ostrinia farnacalis).

From an agricultural perspective, there is an even greater need to understand these creatures which compete with humans for food. "The more we know about how they live, the easier it is to deal with them," says Chao Jung-tai--if we understand insects' ecology, natural history and social behavior, we can devise control measures which target their weaknesses.

Studying the ecology of insect populations can also enable humans to predict changes in their density and destructive potential. This allows timely decisions to be made about the choice of control methods, thereby reducing pesticide use and its impact on human health.

Little Taiwan is often called an "insect kingdom," but people here's knowledge about insects is often limited to butterflies-- most know virtually nothing about the other citizens of that kingdom.

Unless people learn more about the other life forms all around them--including little caterpillars--then "sensational" reports about caterpillars defoliating trees are apt to continue to appear in our newspapers.

[Picture Caption]

How remarkable nature is. This summer saw a spate of newspaper reports o f swarms of caterpillars stripping trees bare of their foliage, but the moths which emerge from the chrysalises after the caterpillars pupate are important pollinators which help plants reproduce by transferring pollen between flowers. Pictured here is a hawk moth, which likes to suck nectar from flowers in the cool of dawn or dusk. (courtesy of Yen Sheng-hung)

A podocarp tree covered in inchworms. Caterpillars which gather togetherin such "insect armies" are vulnerable alone, so they seek safety in numbers and try to scare off predators with a show of combined military might. Pesticides sprayed by humans send them scuttling in all directions, but if they enter gardens or houses it is they who are accused of invading people's homes.

Caterpillars ot the two-spotted lymantriid moth. The allergic reactions for which over 500 elementary school pupils and teachers sought medical treatment this summer were caused by their spiny hairs, shown at right at 3000 times magnification. (right: courtesy of Chao Jung-tai)

Because of their subconscious fear of insects, many people's first reaction to caterpillars is to stomp them dead. When a swarm of caterpillarsat Nanliao Elementary School stripped trees bare and gave many children an itchy rash, the school authorities' initial strategy was to attack them with fire and water. But this only spread the allergen all around the schoolyard. (courtesy of Nanliao Elementary School).

Nature lives by its own rules. The pupae in the little white cocoons on this tree trunk have been parasitized by wasps whose larvae have already eaten away their bodies from the inside. The predations of caterpillars' natural enemies limit their numbers as effectively as any family planning campaign.

Large expanses of monocultural crops make easy pickings for insects. Tod ay's production methods depend on holding the "pests" at bay by spraying insecticides. But although humans may be able to maintain a "balance of power" in this way, the bugs can never be eradicated completely.

Do you remember childhood "close encounters" with caterpillars? Today many children take part in organized activities to bone up on ecological knowledge, but they have little opportunity to see moths in their everyday lives.

It may look like a hummingbird, but in fact it's a tiger moth. Many people think moths are only active at night, so those which come out in the day are easily mistaken for something else.

This hawk moth at rest looks like a jet fighter ready for takeoff. Are you interested in getting to know it better, observing it, and even keeping some records to serve as clues to understanding future events in the natural world?

 

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