Dietary nutrient levels regulate protein and carbohydrate intake,gluconeogenic/glycolytic flux and blood trehalose level in the insect<Emphasis Type="Italic"> Manduca sexta</Emphasis> L.

This study examined the effects of dietary casein and sucrose levels on nutrient intake, and distinguished the effects of carbohydrate and protein consumption on growth, fat content, pyruvate metabolism and blood trehalose level of 5th instar Manduca sexta larvae. Growth increased with increasing casein consumption but was unaffected by carbohydrate intake. Fat content also increased with carbohydrate consumption, but on carbohydrate-free diets fat content increased with increased protein consumption. Blood trehalose level and pyruvate metabolism were examined by nuclear magnetic resonance spectroscopy analysis of blood following administration of (3-(13)C)pyruvate. On diets containing sucrose, blood trehalose increased with increasing carbohydrate intake, and on most diets trehalose was synthesized entirely from dietary sucrose. Pyruvate cycling, indicated by the alanine C2/C3 (13)C enrichment ratio, increased with carbohydrate consumption reflecting increased glycolysis, and pyruvate decarboxylation exceeded carboxylation on all sucrose diets. Larvae that consumed <75 mg/day sucrose were gluconeogenic, based on the [2 (trehalose C6)(Glx C3/C2)]/alanine C2] (13)C enrichment ratio. On carbohydrate-free diets, blood trehalose levels were low and maintained entirely by gluconeogenesis. Blood trehalose level increased with increasing protein intake. Pyruvate cycling was very low, although many insects displayed higher levels of pyruvate decarboxylation than carboxylation. All gluconeogenic larvae displayed alanine (13)C enrichment ratios <0.35 and had blood trehalose levels <50 mM.     

Nicotine moderates the effects of macronutrient balance on nutrient intake by parasitized <Emphasis Type="Italic">Manduca sexta</Emphasis> L

Effects of dietary nicotine and macronutrient ratio on M. sexta larvae were examined. Larvae were fed a carbohydrate-biased, protein-biased or diet having equal amounts of casein and sucrose, with and without nicotine. Without nicotine, larvae displayed compensatory feeding on the low protein diet, but despite consuming more, grew least on this diet. Nicotine at 0.5% had no effect on nutrient consumption. Nicotine at 1.0 and 2.0% reduced overall consumption and thereby also reduced nicotine consumption. Larvae parasitized by C congregata displayed reduced nutrient intake and growth on all diets. Parasitized larvae responded to 1% nicotine similarly to unparasitized larvae. At 0.5% nicotine, they displayed reduced consumption on all diets, possibly due to altered chemoreceptor sensitivity to nicotine. When offered a choice of two diets having different macronutrient ratios, one with and the other without 0.1% nicotine, all larvae preferred the diet lacking nicotine and failed to regulate nutrient intake such that the nutrient intake target, a ratio of nutrients supporting optimal growth, was achieved. Parasitized larvae consumed less nicotine on a fresh weight basis than unparasitized insects, suggesting that the feeding response of parasitized larvae to nicotine minimizes the exposure of nicotine to developing parasites.     

Interactions of dietary protein and carbohydrate determine blood sugar level and regulate nutrient selection in the insect Manduca sexta L

The non-homeostatic regulation of blood sugar concentration in the insect Manduca sexta L. was affected by nutritional status. Larvae maintained on diets lacking sucrose displayed low concentrations of trehalose, the blood sugar of insects, which varied from 5 to 15 mM with increasing dietary casein level between 12.5 and 75 g/l. These insects were glucogenic, as demonstrated by the selective 13C enrichment of trehalose synthesized from [3-13C]alanine, and de novo synthesis was the sole source of blood sugar. The distribution of 13C in glutamine established that following transamination of the 13C substituted substrate, [3-13C]pyruvate carboxylation rather than decarboxylation was the principal pathway of Pyr metabolism. The mean blood trehalose level was higher in insects maintained on diets with sucrose. At the lowest dietary casein level blood trehalose was approximately 50 mM, and declined to 20 mM at the highest casein level. Gluconeogenesis was detected in insects maintained on sucrose-free diets at the higher protein levels examined, but [3-13C]pyruvate decarboxylation and TCA cycle metabolism was the principal fate of [3-13C]alanine following transamination, and dietary carbohydrate was the principal source of glucose for trehalose synthesis. Feeding studies established a relationship between nutritional status, blood sugar level and dietary self-selection. Insects preconditioned by feeding on diets without sucrose had low blood sugar levels regardless of dietary casein level, and when subsequently given a choice between a sucrose diet or a casein diet, selected the former. Larvae preconditioned on a diet containing sucrose and the lowest level of casein had high blood sugar levels and subsequently selected the casein diet. Larvae maintained on the sucrose diet with the highest casein level had low blood sugar and self-selected the sucrose diet. When preconditioned on diets with sucrose and intermediate levels of casein, insects selected more equally between the sucrose and the casein diets. It is concluded that blood sugar level may be intimately involved in dietary self-selection by M. sexta larvae, and that in the absence of dietary carbohydrate, gluconeogenesis provides sufficient blood sugar to ensure that larvae choose a diet or diets that produce an optimal intake of dietary protein and carbohydrate.     

Parasitism of an insect Manduca sexta L. alters feeding behaviour and nutrient utilization to influence developmental success of a parasitoid

The effects of macronutrient balance on nutrient intake and utilization were examined in Manduca sexta larvae parasitized by Cotesia congregata. Insects fed an artificial diet having constant total macronutrient, but with varied ratios of protein and carbohydrate, with altered diet consumption in response to excesses and deficiencies of the individual macronutrients. Bivariate plots of protein and carbohydrate consumption for non-parasitized larvae demonstrated a curvilinear relationship between points of nutrient intake for the various diets, and the larvae grew best on carbohydrate-biased diets. The relationship was linear for parasitized larvae with the growth uniform across diets. On protein-biased diets, the larvae regulated the nitrogen content, containing similar amounts of nitrogen regardless of consumption. Efficiency of nitrogen conversion in non-parasitized larvae was greatest on carbohydrate-biased diets, while nitrogen conversion by parasitized larvae was greatest with intermediate nutrient ratios. Accounting for carbohydrate consumption, the lipid content decreased as dietary carbohydrate increased, but parasitized larvae contained significantly less lipid. The total biomass of parasites developing in individual host larvae was positively correlated with host protein consumption, but the individual parasites were similar in size. Parasitism influences host nutrient consumption in a manner that achieves uniform host growth under diverse nutritional regimes, thereby constraining blood nutrient concentrations within limits suitable for parasite growth and development.     

Parasitism enhances the induction of glucogenesis by the insect, Manduca sexta L

Metabolic alterations that accompany parasitism of invertebrate animals can play an important role in parasite development. Employing 13C NMR, this study examined pyruvate cycling from (2-(13)C)pyruvate in the lepidopteran insect Manduca sexta, and the effects of parasitism by the hymenopteran Cotesia congregata on the gluconeogenic formation of trehalose, the haemolymph or blood sugar of insects. Larvae were maintained on a semi-synthetic sucrose-free diet, or on the same diet with sucrose at 8.5 g/l. Pyruvate cycling was evident from the 13C enrichment in C3 of alanine, derived following carboxylation to oxaloacetate, and was similar in parasitized and normal insects regardless of diet. Trehalose was formed following de novo synthesis of glucose, and net synthesis was estimated from the 13C distribution in trehalose and alanine. The 13C-enrichment ratio [2trehalose C6/alanine C3] is an indicator of the level of gluconeogenesis relative to glycolysis, both enrichments were derived from (2-(13)C)pyruvate in the same manner. The ratio was greater than unity in all insects, regardless of diet, but was significantly greater in parasitized larvae, demonstrating an enhanced level of gluconeogenesis. This was confirmed by analysis of the 13C distribution in trehalose and glutamine derived from (3-(13)C)alanine. Despite enhanced de novo trehalose formation in parasitized insects, the haemolymph sugar level was similar to that of normal larvae. Because haemolymph trehalose regulates dietary carbohydrate intake, but not gluconeogenesis, the results suggest that accelerated induction of gluconeogenesis is an adaptive response to parasitism that provides increased carbohydrate for parasite growth and simultaneously maintains nutrient intake.     

Aberrant nutritional regulation of carbohydrate synthesis by parasitized Manduca sexta L

The present studies confirm that storage carbohydrate synthesis from [1-(13)C]glucose is elevated in Manduca sexta parasitized by Cotesia congregata, despite a decrease in the rate of metabolism of the labeled substrate. Further, the results demonstrate that a similar pattern of carbohydrate synthesis and glucose metabolism was induced in normal larvae by administration of the glycolytic inhibitor, iodoacetate. (13)C enrichment of C6 of trehalose and glycogen demonstrated randomization of the C1 label at the triose phosphate step of the glycolytic/gluconeogenic pathway and suggested that gluconeogenesis, that is, de novo carbohydrate formation, contributed to the synthesis of carbohydrate in both normal and parasitized insects. Accounting for differences in the (13)C enrichment in C1 of trehalose and glycogen due to direct labeling from [1-(13)C]glucose, the mean C6/C1 labeling ratios in trehalose and glycogen of parasitized larvae and insects treated with iodoacetate were greater than the mean ratio observed in normal larvae, suggesting a greater contribution of gluconeogenesis to trehalose labeling in parasitized insects. This conclusion was confirmed by additional investigations on the metabolism of [3-(13)C]alanine by normal and parasitized insects. The pattern of (13)C enrichment in hemolymph trehalose observed in normal larvae maintained on a low carbohydrate diet indicated a large contribution of gluconeogenesis, while gluconeogenesis contributed very little to trehalose labeling in normal insects maintained on a high carbohydrate diet. Parasitized insects maintained on a high or a low carbohydrate diet displayed a significantly greater contribution of gluconeogenesis to trehalose labeling than was observed in normal larvae maintained on the same diets. In conclusion, these investigations indicate that regulation over the utilization of dietary glucose for trehalose and glycogen synthesis as well as the dietary regulation of de novo carbohydrate synthesis were altered by parasitism.     

Pyruvate cycling and implications for regulation of gluconeogenesis in the insect, Manduca sexta L

Pyruvate cycling was examined in the insect Manduca sexta L. (2-(13)C)pyruvate was injected into 5th instar larvae maintained on a semisynthetic high sucrose, low sucrose, or sucrose-free diet. Pyruvate cycling and gluconeogenesis were determined from the distribution of (13)C in blood metabolites, including trehalose, the blood sugar of insects, and alanine. Pyruvate cycling was evident from the (13)C enrichment of alanine C3, synthesized by transamination of pyruvate following carboxylation to oxaloacetate and cycling through phosphoenolpyruvate. Based on the relative (13)C enrichments of alanine C2 and C3, insects maintained on the high sucrose diet displayed higher levels of cycling than insects on the other diets. Insects on all the diets, when subsequently starved, displayed low levels of cycling. Gluconeogenesis was evident in insects on sucrose-free or low sucrose diets from the selective (13)C enrichment in trehalose. The level of gluconeogenesis relative to glycolysis was indicated by the (13)C enrichment of trehalose C6 and alanine C3, both enrichments metabolically derived in the same manner. Insects starved after maintenance on the sucrose-free or low sucrose diets remained glucogenic. Insects on the high sucrose diet were not glucogenic, and subsequent starvation did not induce gluconeogenesis. The results indicate that pyruvate kinase plays a critical role in regulating the gluconeogenic/glycolytic balance, and that inhibition of pyruvate kinase is a principal regulatory event during induction of de novo trehalose synthesis. Gluconeogenesis failed to maintain homeostatic levels of blood trehalose, supporting the conclusion that blood sugar level may be important for mediating nutrient intake. Possible factors involved in the regulation of gluconeogenesis in insects are discussed.     

Blood sugar formation due to abnormally elevated gluconeogenesis: aberrant regulation in a parasitized insect, Manduca sexta Linnaeus.

Alterations of carbohydrate metabolism associated with parasitism were examined in an insect, Manduca sexta L. In insect larvae maintained on a low carbohydrate diet gluconeogenesis from [3-13C]alanine was established from the fractional 13C enrichment in trehalose, a disaccharide of glucose and the blood sugar of insects and other invertebrates. After transamination of the isotopically substituted substrate to [3-13C]pyruvate, the latter was carboxylated to oxaloacetate ultimately leading to de novo glucose synthesis and trehalose formation. Trehalose was selectively enriched with 13C at C1 and C6 followed by C2 and C5. 13C enrichment of blood sugar in insects parasitized by Cotesia congregata (Say) was significantly greater than was observed in normal animals. The relative contributions of pyruvate carboxylation and decarboxylation to trehalose labeling were determined from the 13C distribution in glutamine, synthesized as a byproduct of the tricarboxylic acid cycle. The relative contribution of carboxylation was significantly greater in parasitized larvae than in normal insects providing additional evidence of elevated gluconeogenesis due to parasitism. Despite the increased gluconeogenesis in parasitized insects the level of blood sugar was the same in all animals. Because de novo glucose synthesis does not normally maintain blood sugar level in insects maintained under these dietary conditions the findings suggest an aberrant regulation over gluconeogenesis. The 13C labeling in trehalose was nearly symmetric in all insects but the mean C1/C6 13C ratio was higher in parasitized animals suggesting a lower activity of the pentose phosphate pathway that brings about a redistribution of 13C in trehalose following de novo glucose synthesis. Additional studies with insects maintained on a high carbohydrate diet and administered [1,2-13C2]glucose confirmed a decreased level of pentose cycling during parasitism consistent with a lower level of lipogenesis. It is suggested, however, that the pentose pathway may facilitate the synthesis of trehalose from dietary carbohydrate by directing hexose phosphate cycled through the pathway to the production of energy.     

An in vivoC NMR study on the effect of dietary sugar on pyruvate cycling during glucogenesis from (2-C)pyruvate in Manduca sexta L.

Pyruvate cycling from (2-¹³C)pyruvate was detected in vivo in intact 5th instar Manduca sexta larvae by application of NMR spectroscopy. Cycling was evident from the enrichment of C3 in alanine following transamination of recycled pyruvate in larvae maintained on casein-based diets with or without sucrose. This metabolism is assumed to principally occur in the fat body. Analysis of ¹³C enriched metabolites released into the hemolymph indicated that isotopic dilution of recycled pyruvate was sufficiently great that further metabolism of the recycled metabolite did not occur to any significant extent under these dietary conditions. The C3/C2 ¹³C-enrichment ratio of alanine, therefore, accurately reflected the relative degree of pyruvate cycling and indicated that the rate of cycling was approximately three-fold lower in larvae maintained on diets lacking sucrose. Moreover, based on the distribution of ¹³C in trehalose, these larvae displayed significantly greater rates of gluconeogenesis. Enrichment of C1, C2, C5 and C6 were principally due to carboxylation of the isotopically substituted substrate catalyzed by pyruvate carboxylase and, therefore, reflected net carbohydrate synthesis. Trehalose C3 and C4 enrichments were principally due to pyruvate dehydrogenase-catalyzed decarboxylation and reflected incorporation of label following metabolism through the TCA cycle. Pentose cycling following glucogenesis significantly affected the ¹³C distribution in trehalose in insects on both diets, and the relative intensity of trehalose C6 was, therefore, used for comparing the rates of gluconeogenesis and pyruvate cycling. Based on the ¹³C enrichment of trehalose C6 relative to C3 of alanine the mean rate of pyruvate cycling relative to the rate of gluconeogenesis was approximately 60% in larvae on the diet lacking sucrose, while the rate of pyruvate cycling in larvae maintained on the diet supplemented with sucrose was greater than the gluconeogenic flux. The results were consistent with the conclusion that pyruvate kinase likely plays an important role in regulating gluconeogenesis in M. sexta larvae.     

Feeding behaviour and nutrient selection in an insect <Emphasis Type="Italic">Manduca sexta</Emphasis> L. and alterations induced by parasitism

Manduca sexta L. larvae exhibit broad food acceptance with regard to nutrient content during the first 3 days of the last stadium. Larvae fed diets with a constant combined level of casein and sucrose, but variable ratios, display a linear relationship between protein and carbohydrate intake. Larvae grow best on a diet with equal nutrients, but will consume an excess of one nutrient in order to obtain an adequate amount of the other, as nutrient ratio shifts. Parasitized larvae feed similarly, but the nutrient ratio does not affect growth. Unparasitized larvae regulate intake of protein and carbohydrate when offered choices of protein-biased and carbohydrate-biased diets having combined nutrient levels of 120 g/l, but with variable ratios. Larvae normally consume equal amounts of nutrients, regardless of ratio, and grow similarly. As combined nutrient level is reduced in one diet, larvae abandon regulation and feed randomly. Parasitized larvae offered choice diets with 120 g/l combined nutrients do not regulate nutrient intake. Consumption of nutrients varies widely, but growth is unaffected. Larvae offered choices of diets having equal amounts of casein and sucrose but variable fat (corn oil), fail to regulate fat intake, although both unparasitized and parasitized larvae prefer a diet containing higher fat.     

中红侧沟茧蜂寄生对寄主粘虫血淋巴糖类、脂类和蛋白质含量的影响

为揭示寄生蜂寄生对其寄主的生理调控机制, 室内对中红侧沟茧蜂Microplitis mediator寄生与未被寄生寄主粘虫Mythimna separata幼虫血淋巴中糖类、脂类和蛋白含量变化进行了测定。结果显示: 在滞育与非滞育条件下, 被寄生的粘虫血淋巴中糖原浓度均比未被寄生的粘虫高。滞育条件下寄生后12 d差异显著（P<0.05）, 被寄生粘虫糖原含量为7.93 μg/mL, 未被寄生粘虫糖原含量为4.70 μg/mL; 非滞育条件下寄生后6 d差异显著（P<0.05）, 被寄生粘虫糖原含量为14.35 μg/mL, 未被寄生粘虫糖原含量为5.47 μg/mL。海藻糖含量测定结果显示, 在滞育条件下寄生蜂对被寄生粘虫无明显影响, 而非滞育条件下影响效果差异显著（P<0.05）, 寄生后4 d被寄生粘虫海藻糖含量为46.82 μg/mL, 未被寄生粘虫含量为26.72 μg/mL。在滞育与非滞育两种条件下, 寄生与未被寄生寄主脂类和蛋白含量没有显著性差异。结果说明: 寄生蜂的存在使寄主血淋巴中的糖原含量增高; 非滞育条件是影响被寄生粘虫海藻糖含量变化主要因素; 粘虫对中红侧沟茧蜂的寄生表现相当强的适应性和忍受力。     

Effects of natural and synthetic neuroactive substances on the growth and feeding of cabbage looper, Trichoplusia ni

In this study we investigated the effects of two naturally occurring beta-carboline alkaloids and two synthetic tricyclic antidepressants on the growth and food consumption of fifth instar larvae of the cabbage looper, Trichoplusia ni Hübner (Lepidoptera: Noctuidae). In artificial diets at high concentrations (3,000 ppm), harmane, amitriptyline, and imipramine reduce growth and feeding; harmane reduced feeding consistently at a lower concentration (200 ppm). In animals other than insects, beta-carboline alkaloids inhibit monoamine oxidase (MAO) activity and thus affect rates of disposition of serotonin and other monoamine neurotransmitters. Because brain serotonin levels are associated with variation in rates of carbohydrate and protein intake in insects, the effects of beta-carboline alkaloid ingestion on dietary self-selection behavior were examined. Choosing between diets lacking carbohydrate but containing protein and diets lacking protein but containing carbohydrate, larvae consumed a greater proportion of diet containing protein but lacking carbohydrate in the presence of harmane than in its absence. These results are consistent with beta-carboline alkaloid-mediated persistence of serotonin in the brain due to MAO inhibition. Alternatively, these results could reflect alkaloid-mediated peripheral inhibition of sucrose taste receptors influencing ingestive behaviors. That beta-carboline alkaloid ingestion is associated with changes in feeding behavior is consistent with a possible defensive role for these compounds in plant foliage.     

Correlation between concentration of hemolymph nutrients and amount of fat body consumed in lightly and heavily parasitized hosts (Pseudaletia separata)

Two states of parasitization in the Pseudaletia separata-Cotesia kariyai system were examined: one that was lightly parasitized and one that was heavily parasitized. We predicted that the consumption of fat body and hemolymph nutrients depends on the number of parasitoid larvae in the host. Lightly parasitized hosts (average clutch size+/-S.E.: 42.5+/-16.2, N=15) and heavily parasitized hosts (average clutch size+/-S.E.: 230.2+/-8.8, N=15) were prepared artificially. Eight days after parasitization, perivisceral fat body was depleted in the heavily parasitized host, although peripheral fat body was not yet consumed, but by day 10 most of the peripheral fat body was consumed. In lightly parasitized hosts, perivisceral fat body was not consumed by day 10. The parasitoid larvae deplete the perivisceral fat body first and then consume the peripheral fat body in the heavily parasitized host. The amount of trehalose, the major carbohydrate in the hemolymph, was related to the number of parasitoid larvae developing in the host. In a heavily parasitized host, trehalose concentrations remained low. However, in lightly parasitized hosts, the amount of trehalose increased 8 days after parasitization and then decreased by day 10. Protein and total lipid concentrations in the hemolymph of the heavily parasitized host were significantly lower than in lightly parasitized host on day 10, suggesting that the large number of parasitoid larvae depleted the fat body and hemolymph nutrients by day 10. High concentrations of total lipid on day 8 and 10 in lightly parasitized hosts and on day 8 in heavily parasitized host are likely to be attributed to the teratocytes.     

Glucogenic blood sugar formation in an insect Manduca sexta L.: asymmetric synthesis of trehalose from 13C enriched pyruvate

Gluconeogenesis and blood sugar formation were examined in Manduca sexta, fed carbohydrate- and fat-free diets with varying levels of casein. De novo carbohydrate synthesis was examined by nuclear magnetic resonance spectroscopy of the 13C enrichment in blood trehalose and alanine derived from (2-(13)C)pyruvate and (2,3-(13)C(2))pyruvate administered to 5th instar larvae. Gluconeogenic flux and blood trehalose concentration were positively correlated with protein consumption. On all diets, the 13C distribution in trehalose was asymmetric, with C6 more highly enriched than C1. The C6/C1 13C enrichment ratio, however, decreased with increased protein consumption and gluconeogenic flux. Although the asymmetric 13C enrichment pattern in trehalose is consistent with pentose cycling via the pentose phosphate pathway following de novo synthesis, experiments employing [2,3-(13)C(2)]pyruvate demonstrated that pentose cycling is not detected in insects under these nutritional conditions. Analysis of the multiplet NMR signal structure in trehalose due to spin-spin coupling between adjacent 13C enriched carbons showed the absence of uncoupling expected by pentose phosphate pathway activity. Here we suggest that the asymmetric 13C distribution in trehalose results from a disequilibrium of the triose phosphate isomerase-catalyzed reaction.     

Dietary self‐selection and rules of compromise by fifth‐instar Vanessa cardui

We examined dietary self‐selection and rules of compromise for protein (P) and digestible carbohydrate (C) intake by fifth‐instar Vanessa cardui L. (Lepidoptera: Nymphalidae: Nymphalini). We presented six fat‐free diet pairs to larvae in a choice trial to determine the ‘intake target’. In addition, we fed larvae seven fat‐free single diets differing in dietary nutrient ratio in no‐choice trials to determine the rules of compromise they exhibit when constrained to a singular, sub‐optimal dietary source. In choice trials, caterpillars regulated nutrient intake to a ratio of 1 protein to 1.09 carbohydrate (1P:1.09C), exhibiting tighter regulation of protein than of carbohydrate. Furthermore, larvae from different diet pair treatments did not differ in pupal mass or stadium duration. In no‐choice experiments, larvae reduced consumption on increasingly protein‐biased diets and increased consumption on increasingly carbohydrate‐biased diets, relative to a 1P:1C ratio diet. Differences in carbohydrate consumption were much greater between no‐choice treatments than differences in protein consumption. Dietary nutrient ratio affected pupal mass when accounting for initial larval mass. Pupal mass decreased as nutrient ratio was shifted off of 1P:1C, but to a greater extent when the ratio was skewed toward carbohydrate. Stadium duration increased as nutrient ratio diverged from 1P:1C, being more pronounced when shifted toward carbohydrate than toward protein. Regulation to near 1P:1C is consistent with results found for other Lepidoptera, and the rule of compromise exhibited by V. cardui is consistent with that expected for a generalist herbivore.     

Nutrient balance in medfly, Ceratitis capitata, larval diets affects the ability of the developing insect to incorporate lipid and protein reserves

The Mediterranean fruit fly [Ceratitis capitata Wiedemann (Diptera: Tephritidae)], or medfly, is mass produced in many facilities throughout the world to supply sterile flies for sterile insect technique programs. Production of sterile males requires large amounts of larval and adult diets. Larval diets comprise the largest economic burdens in the mass production of sterile flies, and are one of the main areas where production costs could be reduced without affecting quality and efficacy. The present study investigated the effect of manipulating diet constituents on larval development and performance. Medfly larvae were reared on diets differing in the proportions of brewer's yeast and sucrose. We studied the effect of such diets on the ability of pupating larvae to accumulate protein and lipids, and on other developmental indicators. Except for diets with a very low proportion of brewer's yeast (e.g., 4%), pupation and adult emergence rates were in general high and satisfactory. The ability of pupating larvae to accumulate lipid reserves and proteins was significantly affected by the sucrose and yeast in the diet, and by the proportion of protein to carbohydrates (P/C). In contrast to previous nutritional studies conducted with other insects, low P/C in medfly larval diets (with excess dietary carbohydrates) resulted in pupating medfly larvae having a relatively reduced load of lipids; medfly larvae protein contents in these diets were, as expected, relatively low. Similarly, high P/C ratios in the diet produced larvae with high protein and lipid contents. Differences with other insects may be due to differential post‐ingestion regulation where a high dietary carbohydrate diet reduces the lipogenic activity of the larvae, and induces a shift from lipid to glucose oxidation. Larvae reared on low P/C diets spent more time foraging in the diet than larvae maintained on a high P/C diet, suggesting a compensatory mechanism to complement nutrient intake. The results suggest that the content of brewer's yeast, the most expensive diet component, could be fine‐tuned without apparently affecting fly quality.     

Blood sugar formation from dietary carbohydrate is facilitated by the pentose phosphate pathway in an insect Manduca sexta Linnaeus

Dietary carbohydrate, the principal energy source for insects, also determines the level of the blood sugar trehalose. This disaccharide, a byproduct of glycolysis, occurs at highly variable concentrations that play a key role in regulating feeding behavior and growth. Little is known of how developing insects partition the metabolism of dietary carbohydrate to meet the needs for blood trehalose, ribose sugars and NADPH, as well as energy production. This study examined the effects of varying dietary sucrose levels between 3.4 and 34 g/l in an artificial diet on growth rate, depot fat content and blood sugar formation from (13)C-enriched glucose in Manduca sexta. (2-(13)C)Glucose or (1,2-(13)C(2))glucose were administered to larvae by injection and after 6 h blood was analyzed by nuclear magnetic resonance spectroscopy. [2-(13)C]Trehalose was the principal product of [2-(13)C]glucose, but trehalose was also (13)C-enriched at C1 and C3, demonstrating activity of the pentose phosphate pathway. The trehalose C1/C2 (13)C-enrichment ratio, a measure of the substrate cycled through the pentose pathway, significantly increased with increasing dietary sugar, and reached a mean of 0.22 at the highest level. Blood trehalose concentration increased from approximately 38 mM at the lowest dietary carbohydrate level to 75 mM at the highest. Moreover, blood trehalose, growth rate and depot fat all increased in precisely the same way in relation to the level of pentose cycling. Based on the multiplet (13)C-NMR signal structure of trehalose synthesized from [1,2-(13)C(2)]glucose by insects maintained on a high carbohydrate diet, it was established that the formation of trehalose from glucose phosphate derived directly from the administered substrate, with no involvement of the pentose pathway, was greater than that from glucose phosphate metabolized through the pentose pathway prior to trehalose synthesis. On the other hand, glucose phosphate first metabolized through the pentose pathway contributed more to pyruvate formation than did glucose phosphate formed from the labeled substrate metabolized directly to pyruvate via glycolysis; this finding based on the multiplet (13)C-NMR signal structure in alanine derived from pyruvate. The results suggest that as dietary carbohydrate increases blood sugar synthesis from glucose phosphate derived directly from dietary sugar is facilitated by the pentose pathway which provides an increasing amount of substrate to pyruvate formation.     

Interactions between parasitized and unparasitized conspecifics: parasitoids modulate competitive dynamics

Parasitism influences many aspects of a host's behavior and physiology. Therefore, parasitism is also likely to influence the competitive ability of the host. Field populations of phytophagous insects are often a mix of parasitized and unparasitized conspecifics and the inclusion of parasitism in their competitive dynamics may alter expected outcomes. We investigated the influence of parasitism by the hymenopteran parasitoid Phanerotoma franklini Gahan on the competitive interactions among larvae of its host Acrobasis vaccinii Riley. We found that parasitized larvae were poorer competitors and required less food to complete development compared to unparasitized larvae. To examine the influence of parasitism on the competitive dynamics of this system, we constructed an individual-based model parameterized with our laboratory data. The model examined the role of resource availability and parasitism rate on larval survival. The model suggests that parasitized larvae (and, hence parasitoids) experience higher levels of mortality from competition than unparasitized larvae. Further, the model also suggests that the decreased consumption of resources by parasitized larvae results in a decline in the occurrence of competition as the parasitism rate increases. We suggest that these observations may be general to many parasitoid-host systems.     

Compensation by locusts for changes in dietary nutrients: behavioural mechanisms

ABSTRACT. The detailed behavioural mechanisms underlying an instance of compensation for changes in dietary nutrients are described for the first time in an insect. Nymphs of Locusta migratoria L. were given one of four artificial diets on the third day of the fifth instar, and their feeding patterns recorded in detail for 12 h. The diets represented combinations of two protein and two digestible carbohydrate levels (28% and 14% dry weight) presented in an otherwise complete nutrient mix. At the nutrient levels used, locusts regulated their intake of food with respect to protein but not digestible carbohydrate in the diet. They ate more of the lower protein diets by eating the same sized meals more frequently than insects fed on the higher protein diets. Compensation was not complete over the 12 h observation period: insects on the lower-protein diets ingested and absorbed 72% as much nitrogen as those insects fed on the higher-protein diets. Possible physiological mechanisms underlying the behavioural mechanisms are discussed.     

Larval nutrition affects lipid storage and growth, but not protein or carbohydrate storage in newly eclosed adults of the grasshopper Schistocerca americana

Nitrogen availability from dietary protein can have profound effects on the physiology and evolutionary ecology of insect herbivores. While many studies consider the effects of nutrition on consumption and gross body composition of protein and other important nutrients, few consider partitioning to storage for future use. I used chemically defined artificial diets to quantitatively manipulate the amount of dietary carbohydrates and proteins available to growing larvae of the grasshopper Schistocerca americana to determine how larval nutrient availability affects growth and all three classes of stored nutrients (proteins, lipids, and carbohydrates) carried over from larval feeding into adulthood. Larvae on poor diets increased consumption, but could not compensate for diet quality, eclosing small and containing no significant nutrient stores at adulthood. Individuals fed intermediate to high nutrient content diets as larvae were significantly larger and contained a significantly greater proportion of lipid stores at adult eclosion, but not protein or carbohydrate stores than individuals fed low nutrient content diets. This suggests that larvally derived lipid stores may be more important to adult fitness than carbohydrate or protein stores. This result is contrary to previous studies performed on the role of larval nutrition and allocation to protein stores, and this difference is likely due to variation in the relative availability of protein in adult diets across species.