Ecdysis

Insect Growth Regulators. These compounds (40—45), unlike most conventional insecticides, interfere with biochemical processes that are unique to arthropods eg, molting, ecdysis, and formation of the chitinous exoskeleton. Therefore, they are selective insecticides with very low mammalian toxicity. 

All nematodes undergo four post-embryonic moults, characterized by the synthesis of a new extracellular proteinaceous cuticle from the predom-inandy syncytial hypodermis. Five separate cuticles are synthesized by all nematodes by a process referred to as moulting or ecdysis. 

Ecdysis is the final stage and results in the shedding of the old cuticle. Immediately prior to ecdysis the pharynx begins twitching and gland secretions, predominantly composed of proteases, are released to aid removal of the old cuticle. Finally the pharynx lining is replaced and the animal then pushes with its head to break out of the old cuticle. 

Gamble, H.R., Purcell, J.P. and Fetterer, R.H. (1989) Purification of a 44 kilodalton protease which mediates the ecdysis of infective Haemonchus contortus larvae. Molecular and Biochemical Parasitology 33, 49-58. 

Daly, H.R., B.T. Hart, and I.C. Campbell. 1992. Copper toxicity to Paratya australiensis IV. Relationship with ecdysis. Environ. Toxicol. Chem. 11 881-883. 

Weis, J.S. 1976. Effects of mercury, cadmium, and lead salts on regeneration and ecdysis in the fiddler crab, Uca pugilator. U.S. Natl. Mar. Fish. Serv., Fish. Bull. 74 464-467. 

MOLLUSCS Clam, Anodonta cygnea Larvae exposed for 24 days showed molt rate as in controls, but high mortality immediately after ecdysis reduced growth of survivors 16... 

The growth-inhibitory activity of azadirachtin fed in artificial diet to three species of agricultural pests, gossypiella, H. zea, and frugiperda, was compared to the activity of a number of limonoids isolated from plants in the Meliaceae and the Rutaceae (Table VI). After azadirachtin, the most active limonoid was cedrelone (Figure 13). Cedrelone was unique among the compounds tested in Table VI since it was the only limonoid, besides azadirachtin, to cause an inhibition in ecdysis (LC50 = 150 ppm) when fed to pink bollworm larvae (54). 

Table VII. Ecdysis-and Growth-Inhibitory Activities of Some Neem...

EI50 values are the concentrations resulting in 50% ecdysis inhibition. 

Another limonoid isolated from neem seeds and determined to be as potent as azadirachtin as an ecdysis inhibitor has been identified as 3-deacetylazadirachtinol (Figure 15) (57). Both compounds were lethal to 50% of the treated H. virescens larvae (EI5Q) at 0.8 ppm in artificial diet (Table VII). Structurally, there are two differences between the compounds. In 3-deacetylazadirachtinol, the C-ll-O-C-13 ether linkage of azadirachtin is reductively cleaved at the 11 position and the acetoxyl group at C-3 is hydrolyzed to a hydroxyl group. 

Figure 15. 3-Deacetylazadirachtinol, a Potent Insect Ecdysis Inhibitor Isolated from the Seeds of Azadirachta indica (57)...

The tobacco hornworms are observed over a seven-day period for signs of toxicity, reduced feeding, relative weic t gain (compared to controls) and ability to shed exuvia (ecdysis). 

Larvae of Inoplnatus emerge as major sequestrators of cardenolides primarily because their hemolymph, which is present in a relatively large volume, effectively sequesters high concentrations of polar cardenolides (26). Cardenolide excretion largely reflects loss of these steroids as components of the larval exuviae, the concentration of these compounds becoming relatively stable after pupal ecdysis. These steroids are ubiquitously distributed in the adult moth, having been derived primarily from the rich cardenolide pool in the larval blood. 

Isolation of Phytoecdysones as Insect Ecdysis Inhibitors and Feeding Deterrents... 

Figure 2. A molting cycle failure of the silkworm, Bombyx mori, caused by ingestion of the crude methanol extract of Ajuga remota root. The insect underwent normal apolysis, but failed to complete ecdysis. Thus, it could not remove its head capsule or its trunk exuviae. Magnification X11.

Insect growth regulation involving such processes as water balance or ecdysis (moulting) are processes that are insect specific but are also fundamental to insect survival. Another is chitin biosynthesis, as chitin is a major component of the insect cuticle. 

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