不同生态型的喜旱莲子草对莲草直胸跳甲化蛹能力的影响

EFFECT OF MORPHOLOGICAL AND PHYSIOLOGICAL VARIATIONS IN THE ECOTYPES OF ALLIGATORWEED, ALTERNANTHERA PHILOXEROIDES ON THE PUPATION RATE OF ITS BIOCONTROL AGENT AGASICLES HYGROPHILA

喜旱莲子草 (Alternantheraphiloxeroides)是一种水陆两栖的外来杂草 ,我国 1986年从美国引进了该草的专食性天敌莲草直胸跳甲 (Agasicleshygrophila)进行生物防治。该天敌的化蛹能力受喜旱莲子草不同生态型的影响 ,从而影响到其生物防治的效果。从形态解剖的角度结合天敌和寄主植物的田间生理生态学研究了其机理。结果表明 :水生型喜旱莲子草无次生结构 ,而陆生型则具紧密的次生结构和丰富的晶体 ;不同生态型的喜旱莲子草茎秆形态和结构特征有显著差异 ,同一生态型草不同位置结构特征也存在显著差异。主成分分析和聚类分析结果表明当λ =3.5 6时 ,不同生态型不同节间位置可分为 3类 ,这 3类正好可解释田间喜旱莲子草受控制的情况。相关分析表明莲草直胸跳甲的化蛹率与茎秆节间长、茎秆外直径、髓腔成简单正相关 ,与皮层薄壁细胞厚度和密度、维管束厚度及髓腔薄壁细胞的厚度成简单负相关 ;用逐步回归建立了化蛹率与形态和解剖结构特性的模型 (y =1.5 332 -0 .2 4 83x3 - 0 .0 5 12x4- 1.6 349x5- 0 .95 77x6 R2 =0 .9999) ,表明茎秆直径大小不是影响莲草直胸跳甲化蛹能力的最重要因子 ,主要原因为维管柱的厚度 ,其次髓部薄壁细胞厚度、皮层密度和皮层厚度也不同程度影响了莲草直胸跳甲的化蛹能力。

Alternanthera philoxeroides, which originated in South America, is a noxious weed species in the warm temperate and subtropical regions of China, infesting rivers, lakes, ponds, and irrigation canals, as well as many terrestrial habitats. A host-specific flea beetle, Agasicles hygrophila, introduced from the USA in 1986 as a biological control agent, has become widely established in South China and has effectively controlled the weed in many water bodies. It has been observed, however, that the beetle's ability in controlling the weed in terrestrial habitats has been limited, apparently due to its poor and/or greatly varying pupation rates. This study analyses the mechanism underpinning the relationship between the beetle's pupation rate and the morphological and physiological characters of the three ecotypes of its host plant, viz. the weed growing in the water bodies (hydrophytes), that growing in wet soil with 15%-30% moisture content and the roots sometimes saturated with water (hygrophytes), and that growing in terrestrial soil with 5%-15% moisture content and the roots never saturated with water. Anatomical studies showed that hydrophytes do not have secondary structures in the stems, whereas stems of the hygrophytes and xerophytes have tight cortex cells and abundant crystals. The 7 morphological and anatomical characters of the alligatorweed analyzed in this study showed significant differences among the ecotypes of the weed as well as between the internodes of the same ecotype. Results of the principal component and clustering analysis placed these characters into three groups, which coincided with the distinctive levels of the beetle's pupation rate and the effect of controlling the weed observed in both potted plants and the field experiments. A stepwise regression model was established between the pupation rate of A. hygrophiia and the 7 morphological and anatomical characters of the alligatorweed stems and internodes: y= 1.533 2-0.248 3x 3-0.051 2x 4-1.634 9x 5-0.957 7x 6 (R 2=0.999 9). The pupation rate of the beetle was found to be in simple positive correlation with the lengths of the internodes and the external diameter of the stems and the diameter of the pith cavity of the alligatorweed. The pupation rate was found to be in simple negative correlation with the density and thickness of the cortex parenchyma, and the thickness of the vascular cylinder and the pith cavity parenchyma. It is concluded that the pupation rate and the biological control effect of the flea beetle on A. philoxeroides are affected primarily by the thickness of vascular cylinder, cortex parenchyma, pith cavity parenchyma and the density of cortex parenchyma of its host plant. This is a rather different explanation for the poor performance of the beetle on terrestrial alligatorweed from the observations of other researchers in the USA, Australia and China.