Azadirachtin larvae

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). 

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. 

Toosendanin (39) from the Chinese species Melia toosen-danan displays activity as strong as azadirachtin against larvae of Spodoptera litura and has better activity than azadirachtin against the yellow stemborer, Scirpophaga incertulas, and the Asiatic comborer, Ostrinia furnacalis (Fig. 25.11) (Towers et al., 1989). 

Certain other meliaceous seed extracts were equally active to azadirachtin in bioassays (Mikolajczak and Reed, 1987). TTiose of Aglaia cordata were more potent than extracts of neem toward larvae of Spodoptera frugiperda (Mikolajczak et al., 1989). A limonoid compound from Carapa procera had antifeedant activity comparable to that of azadirachtin, but much weaker insecticidal properties. This compound, methyl 3 3-isobutyryloxy-l-oxomeliac-8(30)-enate (42), also was found in Khaya senegalensis and K. nyasica (Mikolajczak et al., 1988). 

Azadirachtin, applied topically to final-instar larvae of lepidopteran insects, affects oogenesis and reproductive maturation in subsequent female moths (74). Moths obtained from such treated larvae developed fewer mature oocytes, possibly as a result of interference of azadirachtin with vitellogenin synthesis and/or its uptake by developing oocytes. Larval treatments also cause decreased viability of emerging larvae from affected eggs. 

Chitin synthesis in wing pads of 1-day-old fifth-instar larvae of Locusta is stimulated in vitro by 20-hydroxyecdysone and makisterone A 88), This chitin biosynthesis system requires integrity of the microtubular system and the chitin-synthetase complex. Chitin synthesis can be inhibited in this test system by azadirachtin as well as by colcemid, a microtubular poison, or by diflubenzuron, a chitin synthetase inhibitor. It is also of interest to note that RH5849 stimulates chitin synthesis in wing discs of Plodia interpunctella 89). 

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