Metabolism of Stored Reserves in Insect Fat Body: Hormonal Signal Transduction Implicated in Glycogen Mobilization and Biosynthesis of the Lipophorin System*

Physiological Differences Between Seasonal Dimorphs of Agonoscena pistaciae (Hemiptera: Aphalaridae) Elicit Distinct Host Plant Responses, Informing Novel Pest Management Insights

We examined differences in the physiology and life history between dimorphs of the common pistachio psyllid, Agonoscena pistaciae (Burckhardt and Lauterer) (Hemiptera: Aphalaridae), and how they differ in elicitating host plant production of key metabolites and volatile compounds involved in the recruitment of herbivores and natural enemies. Summer morphs had higher activities of glutathione S-transferase, carboxylesterase, acetylcholinesterase, and cytochrome P450 monooxygenase, superoxide dismutase, catalase, peroxidase, phenoloxidase, and a higher total protein content compared to winter morphs, whereas the latter had higher amounts of lipid, carbohydrate, and glycogen. Winter morphs were heavier, with a higher chitin content and longer preoviposition period, but greater fecundity and longevity than summer morphs. A lower LC50 to thiamethoxam for winter morphs resulted in higher mortality following exposure to the recommended rate of this insecticide in a greenhouse trial. Feeding by winter morphs elicited more strongly the release of volatile compounds known to be attractive to other herbivores, whereas feeding by summer morphs elicited more strongly the release of volatiles implicated in the attraction of natural enemies. Feeding by psyllids increased the concentrations of nitrogenous compounds, carbohydrates, vitamins, and amino acids in plants, the winter morph eliciting larger changes and more improved host plant quality. We conclude that winter morphs are more vulnerable targets for chemical control in early spring, whereas management of summer morphs could rely more on conservation biological control.

Micronutrient Fertilization of Greenhouse Cucumbers Mitigates Pirimicarb Resistance in Aphis gossypii (Hemiptera: Aphididae)

The nutritional status of host plants can have direct impacts on herbivore physiology and insect-plant interactions. We investigated the effect of micronutrients, including manganese, iron, zinc, and copper, on cucumber plant physiology, and on the biology and physiology of a strain of Aphis gossypii Glover selected over 12 generations to be resistant to pirimicarb. The micronutrient treatment increased the activity of superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase in cucumber plants, and also increased levels of total phenolics, hydrogen peroxide, salicylic acid, and total chlorophyl, whereas malondialdehyde levels were unaffected. Pirimicarb-resistant cotton aphids that fed on micronutritient-amended cucumber plants expressed significantly decreased levels of acetylcholinesterase and detoxifying enzymes, specifically glutathione S-transferase, and carboxylesterase. Analysis of energy reserves in resistant A. gossypii fed on micronutritient-amended plants revealed decreases in the lipid and protein contents of aphids, whereas glycogen and carbohydrate contents showed no response. Resistant cotton aphids fed on micronutritient-amended plants showed significantly reduced fecundity, longevity, and reproductive periods, and a 1.7-fold reduction in pirimicarb LC50 compared with those fed on control plants. We conclude that micronutrient amendment negatively impacts the biological performance of insecticide-resistant cotton aphids, and diminishes their resistance to pirimicarb. Both direct effects on plant health, such as enhanced inducible defenses, and indirect effects on aphid fitness, such as reduced biological performance and detoxification abilities, were implicated. Therefore, optimization of micronutrient amendments could be a useful complement to other tactics for managing insecticide-resistant A. gossypii on cucumbers, and warrants exploration in other contexts.

Physiological responses of plants and mites to salicylic acid improve the efficacy of spirodiclofen for controlling Tetranychus urticae (Acari: Tetranychidae) on greenhouse tomatoes

Salicylic acid (SA) is a signaling molecule that can induce plant resistance to certain herbivores. Although the role of jasmonic acid in mediating mite-tomato plant interactions has been well studied, the role of salicylic acid has not. This study examined how the application of exogenous SA, via its effects on tomato plant physiology, alters the activity of mite digestive enzymes, mite energy reserves, and mite susceptibility to spirodiclofen. Enzymatic activity-including superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase-along with contents of total phenolic, hydrogen peroxide, and total chlorophyll significantly increased in plants 24 h after treatment with 2 mM of SA. In contrast, catalase activity significantly decreased in treated plants, and malondialdehyde content was unaffected. Mites fed on tomato plants treated with SA had significantly lower glutathione S-transferase, esterase, α-amylase, and aminopeptidase activities than those fed on control plants. Energy reserve analyses demonstrated a significant decrease in contents of lipid, protein, and glycogen in mites fed on SA-treated plants, whereas carbohydrate content significantly increased. The LC₅₀ of spirodiclofen was decreased 1.8-fold for Tetranychus urticae fed on SA-treated tomato plants compared to controls. Treatment of adult mites with 2 mM SA on leaf discs did not cause any direct mortality after 24 h. Finally, a greenhouse bioassay confirmed that spider mite mortality following exposure to spirodiclofen was significantly higher on SA plants than on control plants. Mortality of mites exposed to half of the recommended rate of spirodiclofen was similar to those exposed to the recommended rate when they were held on treated plants. These results have valuable implications for T. urticae management programs in tomato production.

Effects of sublethal concentrations of the chitin synthesis inhibitor, hexaflumuron, on the development and hemolymph physiology of the cutworm, Spodoptera litura

The effects of sublethal concentrations 0.1, 0.5, and 1.2 µg mL(-1)of the chitin synthesis inhibitor, hexaflumuron, on larval growth and development, the count and proportion of hemocytes, and carbohydrate content (trehalose and glyceride) in hemolymph were investigated in the cutworm, Spodoptera litura (Fabricious) (Lepidoptera: Noctuidae). When 3(rd) instar larvae were subjected to the sublethal concentrations, there were dose-dependent effects on larval weight and length of each instar larvae, percent pupation and the duration of development. Most of the larvae died during the molting process at all concentrations. Few individuals from 0.5 and 1.2 µg mL(-1)concentrations could develop to the 6(th) instar, while the pupae emerging from the 0.1 µg mL(-1)concentrations did not exceed 16% of the number of the initial larvae. In 5(th)instar S. litura, the total number of hemocytes was significantly increased at 24 hours post-treatment, whereas the proliferation of hemocytes was inhibited, plasmatocyte pseudopodia contracted, and granulocyte expanded at 96 hours post-treatment. The increases of plasmatocyte count and the decreases of granulocyte count were dose-dependent. The longer treatment time of the sublethal concentrations increased the content of total carbohydrate and trehalose in hematoplasma, and was dose-dependent in hemocytes. The content of glyceride in hemolymph was significantly higher at 24 hours post-treatment, but gradually returned to normal levels at 96 hours post-treatment as compared with the control. The results suggested that sublethal concentrations of hexaflumuron reduced S. litura larval survival and interfered with hemolymph physiological balances.

Boric acid-induced effects on protein profiles of Galleria mellonella hemolymph and fat body

The dietary effects of boric acid (BA) on the protein profiles of greater wax moth, Galleria mellonella (L.), were investigated in hemolymph and fat body of final instar (VIIth) and pupae. The insects were reared from first-instar larvae on an artificial diets containing 156, 620, 1250 or 2500 ppm of BA. We detected many undetermined protein fractions (6.5-260 kDa) in addition to well-defined protein fractions such as lipophorins and storage proteins in the tissues by using sodium dodecyl-sulphate polyacrylamide gradient gel electrophoresis. A marked quantitative change in the 45 kDa protein fraction of the hemolymph was observed in the VIIth instar larvae reared on 2500 ppm dietary BA.

Effects of copper on energy metabolism and larval development in the midge Chironomus riparius

When spiked in sediments, copper is known to reduce growth of Chironomus riparius larvae and the production of eggs by adult females. The aim of this work was to better understand the origin of these phenomena by studying the effects of copper using developmental and energetic biomarkers, such as changes in larval weight and age and changes in the levels of sugars and lipids. Four-day-old C. riparius larvae were exposed to nominal concentrations of copper of 0, 6.5, 12.5, 25 and 50 mg/kg of dry sediment (silica) in 0.6 l beakers. They were fed ad libitum and exposures were stopped at 7 and 9 days after the beginning of the tests. The larvae were weighed, sexed and aged. For each sex, the larvae belonging to the phases the most frequently found in the beakers were selected for dissection and measurement of energy reserves. The increase in the concentration of copper resulted in an increasing delay in larval growth in both sexes. Desynchronized development was observed, as shown by the increase in the number of individuals that remained in the third instar or early phases of the fourth instar, as well as by a reduction in age of males. Concerning energy reserves, the levels of sugars (glycogen, trehalose and glucose) in the dissected larvae remained almost constant among levels of exposure. In contrast, at the highest copper concentration (50 mg/kg), triglyceride levels suffered a slight reduction whereas the level of free glycerol significantly increased. It is concluded that selection of C. riparius larvae for both sex and age improves the relevance of some energy-yielding substrates as indicators of adverse physiological effects of copper.

New insights into adipokinetic hormone signaling

Flight activity of insects comprises one of the most intense biochemical processes known in nature, and therefore provides an attractive model system to study the hormonal regulation of metabolism during physical exercise. In long-distance flying insects, such as the migratory locust, both carbohydrate and lipid reserves are utilized as fuels for sustained flight activity. The mobilization of these energy stores in Locusta migratoria is mediated by three structurally related adipokinetic hormones (AKHs), which are all capable of stimulating the release of both carbohydrates and lipids from the fat body. To exert their effects intracellularly, these hormones induce a variety of signal transduction events, involving the activation of AKH receptors, GTP-binding proteins, cyclic AMP, inositol phosphates and Ca2+. In this review, we discuss recent advances in the research into AKH signaling. This not only includes the effects of the three AKHs on each of the signaling molecules, but also crosstalk between signaling cascades and the degradation rates of the hormones in the hemolymph. On the basis of the observed differences between the three AKHs, we have tried to construct a physiological model for their action in locusts, in order to answer a fundamental question in endocrinology: why do several structurally and functionally related peptide hormones co-exist in locusts (and animals in general), when apparently one single hormone would be sufficient to exert the desired effects? We suggest that the success of the migratory locust in performing long-distance flights is in part based on this neuropeptide multiplicity, with AKH-I being the strongest lipid-mobilizing hormone, AKH-II the most powerful carbohydrate mobilizer and AKH-III, a modulatory entity that predominantly serves to provide the animal with energy at rest.