Nutrient utilization by caterpillars of the generalist beet armyworm, Spodoptera exigua

Abstract.  Beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), caterpillars are destructive crop pests responsible for considerable annual economic losses. These generalist herbivores are challenged with a diversity of dietary choices that can impact their survival, development and fecundity. In the present study, dietary choices of S. exigua caterpillars, based on the protein to digestible carbohydrate (P : C) ratio of the food, and the impact of nutritionally unbalanced foods on caterpillar performance are assessed. In choice experiments, individual third-instar caterpillars are offered simultaneously a P-biased and a C-biased food until pupation. Caterpillars feed nonrandomly and select a slightly P-biased diet (22P : 20C). In no-choice experiments, second instar caterpillars are reared until pupation on diets ranging in P : C ratio from extremely P- to extremely C-biased. High mortality and delayed development are observed on the C-rich, P-poor diets, highlighting the potential deleterious effects of excess carbohydrates and the importance of protein for growth and development. Diet-dependent differences in pupal weight or pupal lipid reserves are not observed. This contrasts with closely-related Spodoptera species where pupal mass and lipid stores increase on C-rich, P-poor diets. On the extremely P-biased diet, performance is similar to that of individuals reared on the self-selected diet, suggesting that these caterpillars may efficiently be deaminating excess amino acids to generate carbon skeletons, which are shunted into lipid biosynthesis. Spodoptera exigua caterpillars exhibit flexible and efficient pre-ingestive nutrient intake regulation and post-ingestive utilization, allowing these generalist feeders to cope with the heterogeneous diets they may encounter.     

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.     

The glucogenic response of a parasitized insect Manduca sexta L. is partially mediated by differential nutrient intake

Induction of gluconeogenesis is accelerated in larvae of the insect Manduca sexta L. parasitized by Cotesia congregata (Say), maintaining the concentration of the blood sugar trehalose, an important nutrient for parasite development. Investigation has demonstrated that when host larvae are offered a choice of diets with varying levels of sucrose and casein, parasitized insects consume a different balance of these nutrients, principally due to a decrease in protein consumption. The result is metabolic homeostasis, with normal unparasitized and parasitized larvae exhibiting similar levels of gluconeogenesis and blood sugar level. In the present study, normal unparasitized and parasitized larvae were maintained on individual chemically defined diets having the balance of protein and carbohydrate consumed by each when offered a dietary choice. Total dietary nutrient, the sum of carbohydrate and protein, was provided at six levels, composed of three pairs of diets. Each diet pair consisting of diets having equivalent overall nutrient ratios of 2:1 and 1:1 casein/sucrose. Host growth and diet consumption were significantly affected by dietary nutrient level and the magnitude of these effects was influenced by parasitism. Due to the effects of dietary nutrient level on diet consumption, none of the unparasitized and parasitized larvae within any of the three diet pairs consumed protein and carbohydrate at the levels predicted by the earlier choice experiments. Among insects on all of the diets, however, two groups of unparasitized and parasitized larvae consumed the expected levels of protein and carbohydrate. In each case, gluconeogenesis, as measured by 13C nuclear magnetic resonance spectroscopy (NMR) analysis of pyruvate cycling and trehalose synthesis from [2-13C]pyruvate, was evident in unparasitized and parasitized insects, confirming the conclusions of the earlier experiments. Generally, all larvae that consumed less than approximately 250 mg of sucrose over the 3-day feeding period, were gluconeogenic, regardless of diet. Differential carbohydrate consumption, therefore, was an important factor in inducing gluconeogenesis in both unparasitized and parasitized insects. The selective 13C enrichment in trehalose displayed by non-gluconeogenic larvae on some diets demonstrated trehalose formation from [2]pyruvate. The absence of net carbohydrate synthesis in these insects was likely due to an elevation of glycolysis. There was no significant effect of diet consumption or parasitism on blood trehalose level. Parasitized larvae displayed higher levels of gluconeogenesis than did unparasitized insects, a finding consistent with the conclusion that blood sugar is rapidly sequestered by developing parasites. The parasite burden, the total number of parasites developing within host larvae, as well as the number of parasites emerging from host larvae to complete development, was significantly less at the lowest dietary nutrient level, but was otherwise similar at all dietary nutrient levels. Moreover, the number of parasites that emerged increased with increasing diet consumption as reflected by host final weight.     

Temperature‐by‐nutrient interactions affecting growth rate in an insect ectotherm

Temperature and nutrition are two prominent environmental variables influencing juvenile growth rate in ectotherms. These two factors interact in complex ways. Here, we present a comprehensive analysis of the interactive effects of temperature and nutrition on various components of fitness (growth rate, survival), food intake, and level of energy storage in an insect herbivore, caterpillars of Spodoptera exigua Hübner (Lepidoptera: Noctuidae). In a factorial experimental design, final‐instar caterpillars (i.e., fifth instars) were individually reared at one of three constant temperatures (18, 26, and 34 °C), in which they received one of six diets differing in their ratio of protein and digestible carbohydrate [P:C mixture, expressed as the percentage of diet by dry mass: protein 42%:carbohydrate 0% (42:0), 35:7, 28:14, 21:21, 14:28, and 7:35]. Within the range of test temperatures, larval growth rate increased with rising temperature and was strongly affected by P:C mixture, reaching a maximum on moderate P:C diets at each temperature and falling at very high and low P:C mixtures. There was a significant temperature*diet interaction, such that the difference in growth rates between temperatures was greatest on moderate P:C diets and least on the most extreme diets (42:0 and 7:35). Food intake rate patterns followed a similar trend to growth rate. Rapidly growing animals at high ambient temperature suffered high mortality across all dietary P:C mixtures, but to a greater extent on the extremely unbalanced diets. This suggests that there are developmental and physiological costs associated with fast growth at high temperature, as indicated by high rate of pupation failure and reduced lipid storage efficiency. Our study shows how temperature and nutrition interplay to mediate phenotypic variations in growth rates and energy utilization in an insect ectotherm.     

The effects of nutritional imbalance on compensatory feeding for cellulose-mediated dietary dilution in a generalist caterpillar

Abstract. The interactive effects of macronutrient balance [protein (P) : carbohydrate (C) ratio] and dietary dilution by cellulose on nutritional regulation and performance were investigated in the generalist caterpillar Spodoptera littoralis (Boisduval). Caterpillars were reared through the final stadium on one of 20 foods varying factorially in macronutrient content (P + C%: 42, 33.6. 25.2 or 16.8%) and P : C ratio (5 : 1, 2 : 1, 1 : 1, 1 : 2 or 1 : 5). The animals compensate by eating more of diluted foods, but suffer reduced nutrient intake in proportion to the degree of dilution. Increase in food intake with dilution is greater on balanced than imbalanced foods and this is reflected in greater reduction of dry pupal mass with dilution in the latter. Whereas dilution results in a reduction in the amount of whichever macronutrient is in excess in the food, by contrast, the ability to compensate for the deficient macronutrient in the food is unaffected by nutrient imbalance. Excess protein intake due to nutritional imbalance (diets with high P : C ratios) results in a regulatory decrease in the efficiency of retention of ingested nitrogen relative to restricted protein intake on oppositely imbalanced foods (low P : C ratios). By contrast, decreased protein intake due to dietary dilution is associated with a non‐regulatory reduction in the efficiency of retention, irrespective of P : C ratio. Dilution is similarly associated with reduced utilization efficiency of ingested carbohydrate. The ecological implications of these results are discussed.     

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.     

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.     

Three hundred and fifty generations of extreme food specialisation: testing predictions of nutritional ecology

We used a strain of diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), that had been reared for approximately 350 generations in a precisely characterised environment to test hypotheses regarding the influence of nutritional heterogeneity on the evolution of nutrient regulatory responses. Caterpillars were maintained with ad libitum access to a diet that emulated that of an extreme nutritional specialist, comprising a homogeneous food of fixed nutrient composition. We measured performance (survival, development rate, and pupal mass), as well as the protein and carbohydrate intake of individual caterpillars confined to one of a range of single foods differing in their protein, carbohydrate, and water content. In a separate experiment, we measured the amount and balance of protein and carbohydrate self-selected by caterpillars presented with nutritionally complementary foods. Results showed a close fit with three of four predictions about the nutritional responses of 'nutrient specialist' feeders: (1) survival, development rate, and pupal mass were highest for animals given diets with the protein:carbohydrate composition of the ancestral culture diet, and dropped off sharply with higher and lower protein:carbohydrate balance, (2) caterpillars coped poorly with dietary dilution by water, irrespective of the macronutrient balance, and (3) the self-selected intake point corresponded with the macronutrient balance that gave peak performance (i.e., that of the ancestral culture diet). The fourth prediction, that caterpillars would be disinclined to over-ingest nutrients on imbalanced diets, was at best weakly met. We hypothesise that the evolution and maintenance of the specialist strategy might, paradoxically, require some degree of environmental heterogeneity.     

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.     

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.     

Sex‐specific effects of protein and carbohydrate intake on reproduction but not lifespan in Drosophila melanogaster

Modest dietary restriction extends lifespan (LS) in a diverse range of taxa and typically has a larger effect in females than males. Traditionally, this has been attributed to a stronger trade‐off between LS and reproduction in females than in males that is mediated by the intake of calories. Recent studies, however, suggest that it is the intake of specific nutrients that extends LS and mediates this trade‐off. Here, we used the geometric framework (GF) to examine the sex‐specific effects of protein (P) and carbohydrate (C) intake on LS and reproduction in Drosophila melanogaster. We found that LS was maximized at a high intake of C and a low intake of P in both sexes, whereas nutrient intake had divergent effects on reproduction. Male offspring production rate and LS were maximized at the same intake of nutrients, whereas female egg production rate was maximized at a high intake of diets with a P:C ratio of 1:2. This resulted in larger differences in nutrient‐dependent optima for LS and reproduction in females than in males, as well as an optimal intake of nutrients for lifetime reproduction that differed between the sexes. Under dietary choice, the sexes followed similar feeding trajectories regulated around a P:C ratio of 1:4. Consequently, neither sex reached their nutritional optimum for lifetime reproduction, suggesting intralocus sexual conflict over nutrient optimization. Our study shows clear sex differences in the nutritional requirements of reproduction in D. melanogaster and joins the growing list of studies challenging the role of caloric restriction in extending LS.     

Dietary choice for a balanced nutrient intake increases the mean and reduces the variance in the reproductive performance of male and female cockroaches

Sexual selection may cause dietary requirements for reproduction to diverge across the sexes and promote the evolution of different foraging strategies in males and females. However, our understanding of how the sexes regulate their nutrition and the effects that this has on sex‐specific fitness is limited. We quantified how protein (P) and carbohydrate (C) intakes affect reproductive traits in male (pheromone expression) and female (clutch size and gestation time) cockroaches (Nauphoeta cinerea). We then determined how the sexes regulate their intake of nutrients when restricted to a single diet and when given dietary choice and how this affected expression of these important reproductive traits. Pheromone levels that improve male attractiveness, female clutch size and gestation time all peaked at a high daily intake of P:C in a 1:8 ratio. This is surprising because female insects typically require more P than males to maximize reproduction. The relatively low P requirement of females may reflect the action of cockroach endosymbionts that help recycle stored nitrogen for protein synthesis. When constrained to a single diet, both sexes prioritized regulating their daily intake of P over C, although this prioritization was stronger in females than males. When given the choice between diets, both sexes actively regulated their intake of nutrients at a 1:4.8 P:C ratio. The P:C ratio did not overlap exactly with the intake of nutrients that optimized reproductive trait expression. Despite this, cockroaches of both sexes that were given dietary choice generally improved the mean and reduced the variance in all reproductive traits we measured relative to animals fed a single diet from the diet choice pair. This pattern was not as strong when compared to the single best diet in our geometric array, suggesting that the relationship between nutrient balancing and reproduction is complex in this species.     

Nutrients, not caloric restriction, extend lifespan in Queensland fruit flies (Bactrocera tryoni)

Caloric restriction (CR) has been widely accepted as a mechanism explaining increased lifespan (LS) in organisms subjected to dietary restriction (DR), but recent studies investigating the role of nutrients have challenged the role of CR in extending longevity. Fuelling this debate is the difficulty in experimentally disentangling CR and nutrient effects due to compensatory feeding (CF) behaviour. We quantified CF by measuring the volume of solution imbibed and determined how calories and nutrients influenced LS and fecundity in unmated females of the Queensland fruit fly, Bactocera tryoni (Diptera: Tephritidae). We restricted flies to one of 28 diets varying in carbohydrate:protein (C:P) ratios and concentrations. On imbalanced diets, flies overcame dietary dilutions, consuming similar caloric intakes for most dilutions. The response surface for LS revealed that increasing C:P ratio while keeping calories constant extended LS, with the maximum LS along C:P ratio of 21:1. In general, LS was reduced as caloric intake decreased. Lifetime egg production was maximized at a C:P ratio of 3:1. When given a choice of separate sucrose and yeast solutions, each at one of five concentrations (yielding 25 choice treatments), flies regulated their nutrient intake to match C:P ratio of 3:1. Our results (i) demonstrate that CF can overcome dietary dilutions; (ii) reveal difficulties with methods presenting fixed amounts of liquid diet; (iii) illustrate the need to measure intake to account for CF in DR studies and (iv) highlight nutrients rather than CR as a dominant influence on LS.     

The development of a synthetic diet for investigating the effects of macronutrients on the development of Plodia interpunctella

The use of chemically defined artificial diets has allowed researchers to examine questions within nutritional ecology about how macronutrients affect life‐history traits and resource‐based trade‐offs. Using a chemically defined diet, it is possible to manipulate both the total nutritional content and the ratio of macronutrients (i.e., proteins, carbohydrates, or lipids) within the diet. Studies using the geometric framework have made use of these diets to examine lifespan, fecundity, and immune responses. Here, we develop an artificial diet suitable for rearing lepidopteran larvae. We created diets with three proportions of non‐nutritive material (30, 50, and 70% indigestible cellulose) relative to protein and carbohydrate macronutrients, and compared these to standard wheat bran laboratory diet. We then examined the effects of variable nutrient content on lifespan and development time in Plodia interpunctella Hübner (Lepidoptera: Pyralidae). The artificial diets supported development (almost) as well as bran‐based laboratory diets. Total nutrient content affected development time: females that fed on the diet with the highest nutrient content took the longest time to reach eclosion. We also found evidence to support dietary restriction, with larvae receiving the fewest nutrients having the longest lifespan as adults. These findings are indicative of the usefulness of this diet as a tool to further investigate the effects of nutrient content and macronutrient imbalance on resource‐based trade‐offs and life‐history traits.     

Discriminating tastes: self‐selection of macronutrients in two populations of grasshoppers

Abstract The capacity to self‐select an optimal balance of macronutrients (protein and carbohydrate) is studied in two populations of Melanoplus sanguinipes F. (Orthoptera: Acrididae). One population derives from the subarctic (interior of Alaska) and the other from the temperate zone (Idaho, U.S.A.). Over the duration of the fourth and fifth stadia, Alaskan grasshoppers consistently self‐select a diet centred on a 0.90 ratio of protein : carbohydrate, whereas protein and carbohydrate intake by the Idaho grasshoppers is contingent on the particular food choices presented to them. When restricted to imbalanced diets, the Alaskan grasshoppers develop more rapidly than the Idaho grasshoppers, regardless of diet composition. The Idaho grasshoppers also have a greater amount of lipid than the Alaskan grasshoppers across all diets. Performance measures (body mass, survival, developmental times) are more sensitive to dietary imbalances in the Alaskan grasshoppers than in the Idaho grasshoppers. When fed diets with low, but balanced, proportions of protein and carbohydrate, grasshoppers of both populations are able to increase consumption to compensate for the low concentration of nutrients. The results suggest that demographic responses of insects to changes in host plant quality, such as may result from climate change, may differ among populations within a species.     

Protein-leverage in mice: the geometry of macronutrient balancing and consequences for fat deposition

Objective: The Protein‐Leverage Hypothesis proposes that humans regulate their intake of macronutrients and that protein intake is prioritized over fat and carbohydrate intake, causing excess energy ingestion when diets contain low %protein. Here we test in a model animal, the mouse: (i) the extent to which intakes of protein and carbohydrate are regulated; (ii) if protein intake has priority over carbohydrates so that unbalanced foods low in %protein leads to increased energy intake; and (iii) how such variations in energy intake are converted into growth and storage. Methods and Procedures: We fed mice one of five isocaloric foods having different protein to carbohydrate composition, or a combination of two of these foods (N = 15). Nutrient intake and corresponding growth in lean body mass and lipid mass were measured. Data were analyzed using a geometric approach for analyzing intake of multiple nutrients. Results: (i) Mice fed different combinations of complementary foods regulated their intake of protein and carbohydrate toward a relatively well‐defined intake target. (ii) When mice were offered diets with fixed protein to carbohydrate ratio, they regulated the intake of protein more strongly than carbohydrate. This protein‐leverage resulted in higher energy consumption when diets had lower %protein and led to increased lipid storage in mice fed the diet containing the lowest %protein. Discussion: Although the protein‐leverage in mice was less than what has been proposed for humans, energy intakes were clearly higher on diets containing low %protein. This result indicates that tight protein regulation can be responsible for excess energy ingestion and higher fat deposition when the diet contains low %protein.     

Influence of dietary nutritional composition on caterpillar salivary enzyme activity

Caterpillars are faced with nutritional challenges when feeding on plants. In addition to harmful secondary metabolites and protein- and water-limitations, tissues may be carbohydrate-rich which may attenuate optimal caterpillar performance. Therefore, caterpillars have multiple strategies to cope with surplus carbohydrates. In this study, we raise the possibility of a pre-ingestive mechanism to metabolically deal with excess dietary sugars. Many Noctuid caterpillars secrete the labial salivary enzyme glucose oxidase (GOX), which oxidizes glucose to hydrogen peroxide and gluconate, a nutritionally unavailable carbohydrate to the insect. Beet armyworm, Spodoptera exigua, larvae were restricted to diets varying in protein to digestible carbohydrate (P:C) ratio (42p:21c; 33p:30c; 21p:42c) and total nutrient concentration (42% and 63%). High mortality and longer developmental time were observed when caterpillars were reared on the C-biased, P-poor diet (21p:42c). As the carbohydrate content of the diet increased, caterpillars egested excess glucose and a diet-dependent difference in assimilated carbohydrates and pupal biomass was not observed, even though caterpillars restricted to the C-biased diet (21p:42c) accumulated greater pupal lipid reserves. Larval labial salivary GOX activity was also diet-dependent and gluconate, the product of GOX activity, was detected in the frass. Unexpectedly, GOX activity was strongly and positively correlated with dietary protein content.     

How female caterpillars accumulate their nutrient reserves

Female Lepidoptera are often heavier than males. We examined the importance of consumption and post-ingestive processing as mechanisms for female Heliothis virescens larvae to meet the protein and carbohydrate requirements. In experiments in which caterpillars had a choice of diets, enabling them to select an appropriate protein and carbohydrate intake, females caterpillars ate more carbohydrate than males, but only on the heavily carbohydrate biased treatment. Overall, the sexes were not distinguished according to the selective feeding behavior, but females accumulated more protein and carbohydrate over the whole instar than the males did. Additionally, when given no choice, females ate more than males and accumulated more protein provided the diet contained a high proportion of protein. If they were reared on a high carbohydrate diet, there were no differences between the sexes. Our results indicate that female H. virescens larvae accumulate protein by regulating both intake and post-ingestive processing on high protein foods. In the field, late instar H. virescens feed on anthers, which are protein-rich and have the highest amino acid content relative to other cotton floral tissues.     

Effects of dietary protein and carbohydrate on life‐history traits and body protein and fat contents of the black soldier fly Hermetia illucens

We investigate how the black soldier fly Hermetia illucens L. (Diptera: Stratiomyidae) responds to dietary protein (P) and carbohydrate (C) contents and the P:C ratio in terms of both immature and adult life‐history traits, as well as effects on larval body composition. Nine chicken‐feed based diets varying in their P:C ratio are formulated. We test three protein concentrations (10%, 17% and 24%) and three carbohydrate concentrations (35%, 45% and 55%) and their combinations. All nine diets support the complete development and reproduction of this species. Survival is high on all diets. Development time, larval yield, larval crude fat and egg yield are more influenced by P and C contents than by the P:C ratio. Low contents result in a shorter development time. Larval yield is higher on diets with higher C‐contents. Pupal development is faster on a low dietary P‐content for all three C‐contents. Egg yield only increases when P‐content increases, although it also varies with the P:C ratio. Larval crude protein content is similar on all nine diets but increases when C‐content is low (10%) in P10 and P17. Larval crude fat content is high at P24‐diets irrespective of C‐content. We conclude that a high macronutrient content combined with a low P:C ratio positively affects H. illucens performance. The diet P17:C55 supports the highest larval and adult performance and results in a high larval body protein content and an intermediate crude fat content.     

Different diets and amino acid supplementation do not affect the voluntary consumption of ethanol by rats

The effects of four different diets (control diet: 19.5% protein, 60.5% carbohydrate, 10% fat; diet I: 65% protein, 10% carbohydrate, 10% fat; diet II: 5% protein, 76% carbohydrate, 10% fat; diet III: 20% protein, 69% carbohydrate, 1% fat; diet IV: 69% protein, 15% carbohydrate, 1% fat) and supplementation with 3 amino acids (tryptophan: 150 mg/kg/d; arginine: 400 mg/kg/d; taurine: 380 mg/kg/d) on the voluntary consumption of ethanol were investigated in rats using the 2 bottle method. First, rats received the control diet and diets I, II, III and IV for 20 days with a choice of ethanol for the last 6 days only. Ethanol consumption was similar in all dietary groups. Second, rats received the control diet for 8 days followed by diets I, II and IV for another 8 days. Ethanol was offered throughout both periods. The switch to the special diets did not affect ethanol consumption. Third, rats received a control diet with arginine, tryptophan or taurine added to the drinking fluids for 16 days with a choice of ethanol for the last 5 days; thereafter supplementation stopped but the ethanol choice remained. No difference in the voluntary intake of ethanol was noted but ethanol consumption fell after cessation of arginine supplementation. In conclusion, diets differing greatly in their composition or supplementation with these 3 amino acids did not affect the voluntary choice of ethanol by rats in a significant manner.