Insects, developmental effects

Miura, T. and R.M. Takahashi. 1974. Insect developmental inhibitors. Effects of candidate mosquito control agents on nontarget aquatic organisms. Environ. Entomol. 3 631-636. 

Terpenoids are structurally based on the isoprenoid (C5) unit and include monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids, steroids and carotenoids. These compounds can be further modified to generate greater structural complexity. Thus the saponins are surface active amphiphiles deriving from the glycosylation of steroid (C27) or triterpenoid (C30) entities. Plant triterpenoids with very specific biochemical effects include those that mimic the effects of mammalian steroid hormones or of insect developmental hormones. 

Chemicals, known as semiochemicals, are compounds produced by plants that play a pivotal role in their selection by insects as hosts due to their ability to exert behavioural or developmental effects. They may be released into the atmosphere and used as cues by herbivorous insects to locate their hosts or they may be involved in the modulation of insect feeding. Alternatively, they may protect the plant by attracting insects that prey on or parasitize herbivorous pests. 

Signaling pheromones are animal-produced, interindividual chemicals that modulate behavior in conspecifics. Like visual and auditory signals, they have comparatively rapid effects exchange of signals takes seconds or minutes. (Priming pheromones [Ch. 8], hy comparison, trigger slower endocrine or developmental processes.) The pheromone concept, originally based on insects (Karlson and Luscher, 1959), has been debated for vertebrates, notably mammals (e.g. Beauchamp etal., 1976 Johnston, 2001). Often it is better to use the term body odors to avoid particular assumptions. Now the term pheromones is widely used for vertebrates, without any particularly narrow definition implied. 

C. M. Williams ( 1) prepared the first active extract from the cecropia moth and showed that this extract prevented adult development when applied to insect pupae. Treated pupae molted into morphogenetic monsters and died. Treatment of other stages produced no ill effects. From these studies it became clear that during the transformation of the immature insect into the adult (during the pupal stage) the juvenile hormones must be absent. This short developmental period is completely deranged when supplied with JH. 

For freshwater invertebrates, frequently used species are the pelagic crustacean Daphnia magna and the sediment-dwelling insect Chironomus sp., while marine crustacean test methods have used copepods and mysids. Molluscs and echinoderms are also important invertebrate species for developmental and reproductive effects assessment (EC 2003). Reproductive and developmental inhibition may be caused by both endocrine and nonendocrine modes of action however, based on current knowledge, PNEC assessments should be based on impaired fitness parameters (e.g., reduced rates of fertility, development, or fecundity) and not on molecular or biochemical changes (Ingersoll et al. 1999 Hutchinson 2002 Barata et al. 2004). 

Other neurohormones of the AKH/RPCH family also stimulate the synthesis of cytohemes a+b in the fat body of B. discoidalis. At equivalent doses, Pea-CAH-I has as much activity as Bld-HrTH, but Pea-CAH-II and Lom-AKH-I are inactive (Figure 3). Therefore, the ability to stimulate developmentally-related heme synthesis in tile fat body appears to be a property of many of the AKH/HGH peptides. The inability of Pea-CAH-II and Lom-AKH-I to stimitiate heme synthesis in B. discoidalis does not indicate that these peptides do not have this effect in their host insect. Rather, these two peptides may ffer so much in structure from Bld-HrTH that they fail to interact adequately with the fat body receptors of B, discoidalis to promote a response, but could be effective at higher doses. 

Many xenobiolics affect development in insects, and it is possible that by combining pi ant-mediated effects and chemical manipulation, such developmental... 

Our present meagre understanding of normal endocrine processes in invertebrates makes the assessment of chemical endocrine disruption in the field difficult (LeBlanc 1999). Steroid roles differ between species and sometimes sexes, and their influence may vary at different developmental stages. In most studies of invertebrates, endocrine disruption appears to involve androgenization rather than oestrogenic effects (see Box 7.2). Arthropods (crustaceans and insects), annelids and molluscs use ecdysteroids, terpenoids and vertebratelike sex steroids for endocrine control. For example, the ecdysteroid ecdysone is naturally converted to 20-hydroxyecdysone (Fig. 7.10), which induces moulting (ecdysis) in both insect larvae and crustaceans. 

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