小地老虎及粘虫幼虫对灭幼脲表现自然耐药力的体壁结构和生化基础

小地老虎Agrotis ypsilon rottem.幼虫对灭幼脲具有一定的自然耐药力。本文以粘虫Mythimna separata(Walker)作为敏感性虫种与之进行比较,实验结果表明,灭幼脲对两种试虫的室内毒力相差4倍左右,引起差异的原因,在体壁结构方面主要在于:(1)小地老虎幼虫的表皮层较粘虫的厚4.2倍左右;(2)上表皮不是匀质结构,依靠少数蜡道与体表沟通;(3)几丁质片层内的孔道数较少,仅及粘虫的1/4。由此构成了表皮对疏水性的灭幼脲表现抗穿透的性能。小地老虎幼虫体壁还含有较强的生化防卫体系,灭幼脲对多功能氧化酶、芳基酰胺酶有明显激活效应,这两种酶都是灭幼脲的降解酶。由此认为,小地老虎幼虫对灭幼脲所表现的自然耐药力,是由体壁的抗穿透性能以及由灭幼脲所激活的适应酶所造成。

ULTRASTRUCTURAL AND BIOCHEMICAL STUDIES OF INTEGUMENTSIN RELATION TO NATURAL TOLERANCE TO DIFLUBENZURININ BLACK CUTWORM AND ARMYWORM

Diflubenzuron (Dimilin, TH-6040) is one of the effective growth regulators used as the insecticide for controlling the early instar larvae of armyworm -Mythimna separatee (Walker); but the black cutworm Agrotis ypsilon Rottem. exhibits considerably higher-natural tolerance to this toxicant. The results of comparative toxicity tests in the laboratory and ultrastructural and bioehemieol studies of the integuments of both species, are summarized as follows:1. The toxicity tests showed that the LD50 for topical application of diflubenzu-ron to the cutworm larvae in the fifth and sixth instars were 69,59 and 136.23 ug per larva respectively. They were approximately 4.9 and 3.9 times greater than those for the armyworm larvae in the same instars. The median effective doses or ED50 for the cutworm larvae in the fifth and sixth instars were 24.67 and 103.21 ug per larve respectively, which were 3.3 and 2,9 times higher than the ED100 for the armywarm larvae in the same developmental stages. The action of intoxication was comparatively slow in cutworm larvae and death oecurrs chiefly in the prepupal and pupal stages.2. The causes for higher natural tolerance to diflubenzuron of the black cutworm as revealed by ultrastructural investigation and biochemical studies can be outlined as follows:a. The larva in the sixth instar has a very thick cuticular layer measuring 32.45. urn which is four times thicker than that of the armyworm in the same instar and there are much less pore canals within the endoeuticle as compared with that of the latter. Instead, the number of chitinous lamellae within the endoouticle is five times larger than that of the axmyworm. These features in cutieular structure presumably contribute to building up an effective barrier against the penetration of the hydrophobic diflubenzu-ron to reach, the epidermal cells.b. The topical application of diflubenzuron at LD50 level to the larvae in sixth in-star also enhanced the activities of mixed-function oxidase (MFO) and arylamidase to 10 and 2.7 times respectively, while in the armyworm in the same instar there was no MFO activity detected and the arylamidase activity was inhibited and reduced to 26-29%. Since both enzymes have detoxieating effects to diflubenzuron these biochemical differences seem to be important as the physiological basis for the higher natural tolerance in the black cutworm.c. The process of chitin synthesis was completely inhibited by diflubenzuroh in poisoned larvae when applied at the early stage (within eight hours after the fifth molt) of the sixth intar. After the injection of diflubenzuron at LD50 level, most of the larvae showed retarded growth and the chitin content of the cuticle remained inchanged at the original level (18-20%), while in the untreated normal larvae it would increase toa maximum of 50% within four day's growth period. The poisoned larvae would diebefore or during the prepupal or pupal stage, mainly due to the failure of molting. Presumably this is the main toxie of diflubenzuron to the different larval stages of some susceptible insect species.The problems of diflubenzuron resistance in relation to the cross resistanec to orga-nophosphorus, carbamate, and pyrethroid insecticides and negative cross resistance to ju-venoids are briefly reviewed and discussed in consideration of application of diflubenzuron in the field for controlling insect pests.