Survival Rates of Culex tritaeniorhynchus (Diptera, Culicidae) Larvae in Fallow Rice Fields before Summer Cultivation | 休閑田におけるコガタアカイエカ幼虫の生存率と死亡要因

Survival rates and mortality factors were studied with larvae of Culex tritaeniorhynchus, a vector mosquito of Japanese encephalitis (JE) virus, by field experiments in fallow rice fields before summer cultivation in Nagasaki, Japan. Adult emergence rates were very low in experimental quadrats with a natural complex of predators including aquatic insects and fishes, the average being 0.02. Higher the predator density, lower the emergence rate. Emergence rates increased notably in enemy-free cages where partial drying and unsuitable quality of water were main mortality factors. The mortality rate due to predation was considered to be density-independent and less influenced by the yearly fluctuation of weather conditions than the mortality rate due to drying or unsuitable quality of water. It was also considered that chemical control in spring or early summer is ineffective to suppress the peak abundance in midsummer and consequently to prevent the JE epidemic. Chemical control in this early season may even be followed by population explosion in midsummer through the elimination of predators. It is desirable that control in this early season depends on methods highly specific to mosquitoes including tritaeniorhynchus such as the application of mass-produced parasites to breeding places. Adult control by light traps operated at animal houses may be most practical at the present time of Japan.

長崎では,コガタアカイエカの越冬からさめた雌成虫は3月下旬に出現し,それ以後,6月末の田植までの期間は,主に休閑中の湿田から発生する.そこで,休閑田における本種幼虫の生存率と死亡要因を明らかにするために野外実験を行った.捕食性天敵が自然状態で存在する枠内に放された幼虫が成虫として羽化する率はきわめて低く,平均約2%であった.枠内の天敵密度が高いほど羽化率は低かった.天敵を除去した網内での生存率は顕著に高く,ここでは水の干上がりと水質不良が主な死亡要因であった.捕食による死亡率は密度非依存的で,また干上がりや水質不良による死亡率に比べて気象条件の年次的変動に影響されにくいと考えられた.殺虫剤による休閑田の幼虫駆除は盛夏の成虫個体数,ひいては日本脳炎患者数を減少させる効果に乏しいのみでなく,天敵を殺してしまうことにより本種が増殖しやすい条件をつくりだす危険もある.従って,この季節の駆除は,天敵に及ぼす影響のごく小さい方法に限定した方がよい.休閑田の幼虫に対する大量生産した寄生虫の散布,畜舎に集まる成虫のライト・トラップによる捕殺などは好ましい方法であろう.沖縄の条件は本土とはことなるので,上の考えをそのまま適用することはできない.