Insect development

Resistance to DDT has been developed in many insect species. Although there are some cases of metabolic resistance (e.g., strains high in DDT dehydrochlorinase activity), particular interest has been focused on kdr and super kdr mechanisms based upon aberrant forms of the sodium channel—the principal target for DDT. There are many examples of insects developing resistance to dieldrin. The best-known mechanism is the production of mutant forms of the target site (GABA receptor), which are insensitive to the insecticide. 

The complexity of the new insecticidal chemicals brings many other problems. Synthetic organic chemicals are not effective against all pests. There is a marked selectiveness in action even between closely related species of insects. Some insects have already developed resistance to some of the newer materials. The idea of insects developing resistance to certain chemicals is not new. The over-all principle is well established in a few cases. The early development of flies resistant to DDT, a chemical which had been highly and universally effective for fly control, came as a surprise. Other cases of resistance to DDT are being indicated, and at least one kind of mite has developed resistance against another of the newer chemicals—parathion. 

This kind of impact is complicated by the possible influence of these chemicals on the process of evolution itself When a chemical comes into widespread use, especially an insecticide, it is only a matter of time before the target insects develop resistance. That is, those individual insects with natural resistance (as a result of natural variation and mutation) survive successive assaults of a given chemical and reproduce broods with similar immunity, which come to be common in the whole population. In a short period of time (because insects reproduce quickly), the species becomes resistant altogether (Table 4.6). ... 

Walton B, Buchanan M. 1980. Teratogenic effects of fuel oils on insects developing in contaminated substrates. Second Chemical Congress of the North American Continent San Francisco, California. 

Plants that produce "specific toxins may be plagued by Insects that develop a tolerance to these toxins in much the same way as Insects develop tolerance to synthetic insecticides. Two examples from this chapter are the tobacco hornworm and the boll weevil which have developed a high tolerance to nicotine and gossypol, respectively. Some occurrence in the distant past may have placed sufficiently high selection pressure on these Insects that they developed tolerance to these compounds. Alternatively, the same effect could have occurred by a low selection pressure applied over a very long period time. Other plants protect themselves by employing general" toxins. 

The pyrethrins are valuable insecticidal components of pyrethrum flowers, Chrysanthemum cinerariaefolium (= Tanacetum cinerariifolium) (Compositae/Asteraceae). The flowers are harvested just before they are fully expanded, and usually processed to an extract. Pyrethrum cultivation is conducted in East Africa, especially Kenya, and more recently in Ecuador and Australia. The natural pyrethrins are used as a constituent of insect sprays for household use and as post-harvest insecticides, having a rapid action on the nervous system of insects, whilst being biodegradable and non-toxic to mammals, though they are toxic to fish and amphibians. This biodegradation, initiated by air and light, means few insects develop resistance to the pyrethrins, but it does limit the lifetime of the insecticide under normal conditions to just a few hours. 

Higley, L. G. and Haskell, N. H. (2001). Insect development and forensic entomology, in Forensic Entomology the Utility of Arthropods in Legal Investigations (J. J. Byrd and J. L. Castner, Eds.). Boca Raton, FL CRC Press, 287-302. 

A thorough knowledge of the succession occurring on buried carrion is required if it is to be employed in PMI estimation. As with surface carrion, a predictable succession may be used to estimate PMI, and in cases where dipteran larvae are still present, the use of temperature dependent developmental data may be applied. The use of such data obviously requires consideration of season and temperature. VanLaerhoven and Anderson (1999) determined soil temperature to be a better predictor of internal temperature of buried carrion than ambient temperature and suggested use of soil temperature for estimation of insect development. 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

The negative implications for pest control by precocenes themselves are clear, but it remains to be seen whether the expansion of their spectrum of activity is limited merely by the chemical structural features of precocenes or, more problematically, by the hormonal mechanisms which control insect development. In either case the JH antagonist approach to the control of larval insect pests presents a major challenge to chemical and physiological research. 

Synthetic analogues of this compound, such as the trienes, are also effective at arresting insect development, providing that the double bond geometry is controlled. The Z,E,E geometrical isomer of the triene is over twice as active as the , ,E-isomer, and over 50 times as active as the ,Z,Z- or Z,E,Z-isomers. 

Figure 3. Dependence of oxidase activity on insect development as indicated by conversion of (Z)-11-tetradecen-l-ol to (Z)-ll-tetradecenal in 30 min. by excised glands of H. subflexa. H. yirescens and H. zea. Bars indicate the activity in intact glands at each time relative to that in glands of insects of the same species 48 h after adult emergence (100%) (n - 10, each species).

Non-cyclic sesquiterpenes include the volatiles a- and (3-farnesene (which have alarm pheromone activity) and pleasant odorants from Citrus sinsensis (orange) (Rutaceae), namely a- and [i-sinensal (mandarin peel odour) and nerolidol from orange flower oil (oil of neroli). The epoxide JH III is produced by Cyperus iria (Cyperaceae) and acts critically on insect development. 

The steroids known to play major regulatory roles in insect development and metamorphosis all fall into the class of polyhydroxylated ketosteroids called ecdysones (19-22). With the exception of Makisterone A (a C2 ecdysteroid identified from the milkweed bug Oncopeltus fasciatus) the known insect ecdysteroids constitute a group of eight or nine steroids that differ from one another... 

In view of their critical importance in regulating the growth and development of insects, the enzymes should continue to be viewed as potentially valuable targets around which to develop new pest control agents. This possibility is likely to become more realistic as the enzymes are further characterized and the full extent of their many roles in insect development are more fully understood. 

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