Chitin Synthesis

Formed of proteins, glycoproteins, and chitin microfibrils in a proteoglycan matrix, the peritrophic matrix (PM) separates the food from the midgut epithelium in most but not all insects. A PM occurs in two forms. A type I PM is delaminated from the entire midgut epithelium and, in some cases, may only be formed in response to feeding and the type of meal ingested. A type II PM is produced by a specialized region of the anterior midgut called the cardia and forms a continuous sleeve (or sleeves) that is always present. As it is positioned between food and midgut epithelium, the PM plays key roles in the intestinal biology of the insect. The PM may protect the midgut epithelium from mechanical damage and insult from pathogens and toxins; it must act as a semipermeable membrane regulating passage of molecules between the different midgut compartments; and it may separate the midgut lumen into different, physiologically significant compartments.

The two-spotted spider mite Tetranychus urticae Koch is one of the economically most important pests in a wide range of outdoor and protected crops worldwide. Its control has been and still is largely based on the use of insecticides and acaricides. However, due to its short life cycle, abundant progeny and arrhenotokous reproduction, it is able to develop resistance to these compounds very rapidly. As a consequence, it has the dubious reputation to be the"most resistant species" in terms of the total number of pesticides to which populations have become resistant, and its control has become problematic in many areas worldwide. Insecticide and acaricide resistance has also been reported in the ectoparasite Sarcoptes scabiei, the causative organism of scabies, and other economically important Acari, such as the Southern cattle tick Rhipicephalus microplus, one of the biggest arthropod threats to livestock, and the parasitic mite Varroa destructor, a major economic burden for beekeepers worldwide. Although resistance research in Acari has not kept pace with that in insects, a number of studies on the molecular mechanisms responsible for the resistant phenotype has been conducted recently. In this review, state-of-the-art information on T. urticae resistance, supplemented with data on other important Acari has been brought together. Considerable attention is given to the underlying resistance mechanisms that have been elucidated at the molecular level. The incidence of bifenazate resistance in T. urticae is expanded as an insecticide resistance evolutionary paradigm in arthropods. Copyright © 2010 Elsevier Ltd. All rights reserved.

Trehalose is a non-reducing disaccharide comprising two glucose molecules. It is present in high concentration as the main haemolymph (blood) sugar in insects. The synthesis of trehalose in the fat body (an organ analogous in function to a combination of liver and adipose tissue in vertebrates) is stimulated by neuropeptides (hypertrehalosaemic hormones), released from the corpora cardiaca, a neurohaemal organ associated with the brain. The peptides cause a decrease in the content of fructose 2,6-biphosphate in fat body cells. Fructose 2,6-biphosphate, acting synergistically with AMP, is a potent activator of the glycolytic enzyme 6-phosphofructokinase-1 and a strong inhibitor of the gluconeogenic enzyme fructose 1,6-biphosphatase. This indicates that fructose 2,6-biphosphate is a key metabolic signal in the regulation of trehalose synthesis in insects. Trehalose is hydrolysed by trehalase (E.C. 3.2.1.28). The activity of this enzyme is regulated in flight muscle, but the mechanism by which this is achieved is unknown. Trehalase from locust flight muscle is a glycoprotein bound to membranes of the microsomal fraction. The enzyme can be activated by detergents in vitro and by short flight intervals in vivo, which indicates that changes in the membrane environment modulate trehalase activity under physiological conditions.