Caterpillars' compensatory feeding response to diluted nutrients leads to toxic allelochemical dose

Many herbivores increase their consumption rate as dietary nutrient concentration declines. This compensatory response can mitigate the fitness-lowering impact of reduced food quality, but little is known about its costs. In this study we tested the hypothesis that one cost to a faster consumption rate can be the ingestion of a toxic dose of an allelochemical occurring in the food. We fed velvetbean caterpillars a diet with progressively diluted nutrient levels but containing the same concentration (% fresh mass, fm) of caffeine, a methylxanthine alkaloid. Larvae compensated for the reduced nutrient level, with those fed the most diluted diet increasing their biomass-relative consumption rate (fm) 2.6-fold over larvae fed the undiluted diet. Consequently, their rate of caffeine ingestion increased to a pharmacologically effective dose, interfering with food utilization, slowing growth, reducing subsequent feeding and lowering survival. These results suggest that greater allelochemical ingestion can be one cost of an increased consumption rate, although additional studies with other allelochemicals and species are necessary to more broadly evaluate whether insects can adaptively balance their intake of nutrients and allelochemicals through adjustments in consumption rate. In addition, these results highlight the importance of measuring consumption rates of allelochemicals and other ingested biocides, not just their dietary concentration, when assessing efficacy against herbivores.     

Complete nutrient content of four species of commercially available feeder insects fed enhanced diets during growth

Commercially raised feeder insects used to feed captive insectivores are a good source of many nutrients but are deficient in several key nutrients. Current methods used to supplement insects include dusting and gut‐loading. Here, we report on the nutrient composition of four species of commercially raised feeder insects fed a special diet to enhance their nutrient content. Crickets, mealworms, superworms, and waxworms were analyzed for moisture, crude protein, fat, ash, acid detergent fiber, total dietary fiber, minerals, amino acids, fatty acids, vitamins, taurine, carotenoids, inositol, and cholesterol. All four species contained enhanced levels of vitamin E and omega 3 fatty acids when compared to previously published data for these species. Crickets, superworms, and mealworms contained β‐carotene although using standard conversion factors only crickets and superworms would likely contain sufficient vitamin A activity for most species of insectivores. Waxworms did not contain any detectable β‐carotene but did contain zeaxanthin which they likely converted from dietary β‐carotene. All four species contained significant amounts of both inositol and cholesterol. Like previous reports all insects were a poor source of calcium and only superworms contained vitamin D above the limit of detection. When compared to the nutrient requirements as established by the NRC for growing rats or poultry, these species were good sources of most other nutrients although the high fat and low moisture content of both waxworms and superworms means when corrected for energy density these two species were deficient in more nutrients than crickets or mealworms. These data show the value of modifying the diet of commercially available insects as they are growing to enhance their nutrient content. They also suggest that for most insectivores properly supplemented lower fat insects such as crickets, or smaller mealworms should form the bulk of the diet. Zoo Biol. 34:554–564, 2015. © 2015 The Authors. Zoo Biology published by Wiley Periodicals, Inc.     

Role of superoxide dismutase in the protection and tolerance to the prooxidant allelochemical quercetin in Papilio polyxenes,Spodoptera eridania,and Trichoplusia ni

Larvae of the black swallowtail butterfly, Papilio polyxenes, the southern armyworm, Spodoptera eridania, and the cabbage looper, Trichoplusia ni, have different feeding habits and dietary breadth, which contributes to differences in their exposure and tolerance to dietary prooxidant allelochemicals. The antioxidant enzyme activities of larvae of these insects have been previously determined, with the levels being P. polyxenes > S. eridania > T. ni. The relative activities of these antioxidant enzymes are consistent with the relative exposure of these insects to prooxidants. This suggests that the antioxidant enzymes may play a role in the defense against allelochemical toxicity in these insects. Dietary diethlydithiocarbamate (DETC), a copper chelating agent and superoxide dismutase (SOD) inhibitor, was shown to inhibit SOD in all three insects. Toxicological studies were conducted using four diets for each insect. The standard diets for each insect were supplemented with either control (solvent), quercetin (a prooxidant), DETC, or DETC plus quercetin. Nontoxic doses of each compound for each insect were used. Inhibition of SOD in P. polyxenes and S. eridania dramatically increased quercetin-induced toxicity as measured by relative growth and consumption rates in these species. DETC had no effect on quercetin toxicity in T. ni. These results elucidate the important role of SOD in the prooxidant allelochemical defense of insects.     

Food intake, conversion efficiency, and feeding behaviour of tobacco hornworm caterpillars given artificial diet of varying nutrient and water content

ABSTRACT Fifth stadium tobacco hornworm caterpillars, Manduca sexta (L.), given artificial diet diluted to varying extents with either cellulose or water compensated for the food's reduced nutrient content by eating more of it. This compensation was, however, in most cases not sufficient to maintain normal growth rates. When the water content of the diet was reduced, the insects ate less than the usual fresh weight of food but maintained their intake of nutrients. Nevertheless, growth rate was impaired. The insects were better able to compensate for dilution of their food with water than with cellulose. The efficiency of conversion of ingested food (ECI) was decreased when the diet was adulterated with cellulose. At moderate dilution (50% nutrient) this was due mostly to decreased approximate digestibility (AD), but at greater dilution (25% and 10% nutrient content) the efficiency of conversion of digested food (ECD) was decreased. ECI was maintained when the water content of the diet was increased to give 50% nutrient concentration, but was decreased when water content was changed more radically (200%, 25% and 10% nutrient diets). This was due mostly to increased metabolic costs (decreased ECD) in all cases. The retention time of food in the gut was progressively decreased (i.e. speed of passage was increased) as nutrients were replaced by cellulose. By contrast, dilution of the diet with water resulted in only slight changes in retention time, except at extreme dilution (10% nutrient content) when retention time was reduced. Compensation of food intake was achieved by spending more (or less) time eating. Video analysis of feeding behaviour showed that there were significant changes in the length of feeding bouts and of interfeed gaps when caterpillars fed on diets of altered composition. For diets diluted with cellulose, changes in bout length and bout frequency contributed substantially to the increased time spent feeding on the adulterated food. For diets diluted with water, however, almost all of the compensatory change in behaviour was due to increased bout length, with bout frequency affected only slightly. This suggests that volumetric feedback contributes principally to the termination of feeding bouts in caterpillars, while nutrient flow may affect both the initiation and termination of feeding.     

Linking larval nutrition to adult reproductive traits in the European corn borer Ostrinia nubilalis

Throughout an organism's lifetime, resources are strategically allocated to many different functions, including reproduction. Reproduction can be costly for both sexes; females produce nutrient‐rich eggs, whereas males of many species produce large and complex ejaculates. In capital breeding insects, nutrients are mainly acquired during the larval period, yet allocation decisions impact the reproductive fitness of adults. The present study examines the effect of larval dietary nitrogen on both male and female reproductive traits in the European corn borer moth Ostrinia nubilalis Hübner, whose adults do not feed and whose males transfer a large, nitrogen‐rich spermatophore. One day post‐eclosion, O. nubilalis adults reared on one of three different diets (3.0%, 1.6%, or 1.1% nitrogen) are mated and two experiments are undertaken: one to measure nitrogen and carbon content of male ejaculates, and the other to determine female fecundity and fertility. Although male larval diet does not alter the percentage nitrogen content of adult somatic tissue, males reared on the higher nitrogen diet (3.0%) produce spermatophores with increased nitrogen relative to somatic nitrogen. Furthermore, females raised on the 3.0% nitrogen diet receive spermatophores with lower carbon : nitrogen ratios and thus more nitrogen. Overall, females lay more eggs as their larval dietary nitrogen increases, although they lay fewer eggs when their mates are raised on the higher (3.0%) nitrogen diet. This suggests that O. nubilalis females may use male‐derived nitrogen not to supplement egg production, but rather for somatic maintenance. Overall, the present study furthers our understanding of how larval diet can affect adult fitness in Lepidoptera.     

Food consumption, utilization and detoxification enzyme activity of larvae of three polyphagous noctuid moth species when fed the botanical insecticide rotenone

We assessed the effects of the isoflavonoid rotenone, an insecticidal allelochemical occurring in various legumes, on larval performance of three polyphagous noctuid species: the corn earworm (CEW), the fall armyworm (FAW) and the southern armyworm (SAW). As rotenone concentration was increased up to 1% fresh mass in an artificial diet, neither mortality (for all three species) nor food consumption (for SAW and FAW) was significantly affected, but developmental time of the latter two species was prolonged. In contrast, for CEW developmental time was shortened and food consumption declined, especially at the two highest rotenone concentrations. Final biomass of all three species declined as dietary rotenone increased.Analysis of covariance (ANCOVA) was used to evaluate larval food utilization analogous to, but without the potential statistical problems of, the traditional ratio-based indices approximate digestibility (AD) and efficiency of conversion of digested food (ECD). Frass output, statistically adjusted to account for differences in consumption, increased with rotenone concentration for all three species, suggesting that, when ingested, this allelochemical interfered with the digestion and/or absorption of ingested food, especially at the higher concentrations tested.Significant statistical interactions in the ANCOVA necessitated the use of utilization plots to examine the linear regressions between biomass gain and absorption at each rotenone concentration. At the two highest concentrations, rotenone tended to reduce the amount of food absorbed by the larvae, as well as their ability to convert the absorbed food to biomass.When piperonyl butoxide, an inhibitor of the polysubstrate monooxygenase (PSMO) system known to metabolize rotenone, was added to the diet along with 0.5 or 1% rotenone, mortality increased significantly only for FAW, approaching or exceeding 50%. The only apparent effects of dietary rotenone on PSMO (aldrin epoxidase) activity (adjusted by ANCOVA for the covariate, crude homogenate protein) were for CEW, where activity decreased in all rotenone treatments compared to the control, and for SAW, where activity at the 3 lowest rotenone concentrations (0.001–0.1% fm) declined about 50% below that of larvae on the control diet, whereas at 0.5 and 1% rotenone, it increased approximately 2-fold over that on the control diet.     

Phytochemical defences and performance of specialist and generalist herbivores: a meta-analysis

1. Phytochemical coevolution theory, a long-standing paradigm in plant–insect interactions, predicts that specialist herbivores are less negatively affected by the allelochemicals of their host plants than are generalist herbivores. Although this theory is prevalent in plant–insect science, it is not always supported by empirical studies measuring the performance of specialist and generalist insects in response to allelochemicals. 2. The present study aimed to investigate: (i) whether there a difference between specialist and generalist performance in response to allelochemicals and (ii) whether the effect of allelochemicals on specialists and generalists depend upon allelochemical class or insect order. 3. A meta-analysis was conducted incorporating 76 effect sizes drawn from studies that directly compared the performance of specialist and generalist insects in response to treatment and control diets. Most of the effect sizes were related to the performance metric growth, the insect order Lepidoptera, and the allelochemical class nitrogen-containing compounds. 4. As predicted by phytochemical coevolution theory, specialist insects responded less negatively to allelochemicals of their hosts than generalist insects in terms of growth. There were no significant differences in terms of fecundity or survival, or among allelochemical classes or insect orders. 5. These results support the prediction of phytochemical coevolution theory that specialist insects respond less negatively to allelochemicals of their hosts than generalists, although only in terms of growth.     

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.     

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.     

Disparate determinants of summer and winter diet selection of a generalist herbivore,Ochotona princeps

The North American pika, Ochotona princeps, is a generalist herbivore that simultaneously selects two distinct diets: one consumed immediately (summer diet), the other harvested, transported, and stored for later consumption (winter diet). I investigated factors influencing diet selection at two sites on the West Knoll of Niwot Ridge, Boulder County, Colorado during 1991 and 1992. The composition of summer and winter diets differed significantly from each other as well as from the relative abundance of food items in the environment. Thus, pikas were not foraging randomly for either diet. To explore winter and summer diet selection, I tested two existing hypotheses: (1) that plant morphology restricts the winter diet breadth to plants that are easily harvested and large, and thereby maximizes the amount collected per foraging effort, or (2) to compensate for nutrients lost during storage, pikas bias their winter diet with high-nutrient species. I also tested the hypothesis that plant secondary compounds may be higher in the winter diet either because they function as preservatives or because pikas delay consumption of these species until the toxins degrade. For individual dietary items, I measured energy, nitrogen, water, fiber, total phenolic, condensed tannin, and astringency contents. There was little evidence to suggest that morphology excluded plants from the winter diet. Plant size was not a good predictor of abundance in the winter diet. Even after harvesting costs had been experimentally removed, cushion plants were still not included in the winter diet. There was weak support for an effect of nutrients on winter diet selection; in three of four cases, the winter diet was significantly lower in water and higher in total energy content as predicted by the nutrient compensation hypothesis. However, other nutrients exhibited no consistent pattern. Nutrients were not reliable predictors of the winter diet in multiple regression analyses. There was strong support for the hypothesis of manipulation of secondary compounds. The winter diet was significantly higher in total phenolics and astringency. Total phenolics were consistent predictors of the winter diet in multiple regression analyses. The winter diets of six additional pika populations contained plant species high in secondary compounds. The results suggest that pikas preferentially select plants with high levels of secondary compounds for their winter diet, possibly because the presence of such compounds promotes preservation of the cache. This behavior may also enable the exploitation of an otherwise unusable food resource, i.e., toxic plants.     

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.     

Dietary generalists and nutritional specialists: Feeding strategies of adult female blue monkeys (Cercopithecus mitis) in the Kakamega Forest,Kenya

Generalist primates eat many food types and shift their diet with changes in food availability. Variation in foods eaten may not, however, match variation in nutrient intake. We examined dietary variation in a generalist‐feeder, the blue monkey (Cercopithecus mitis), to see how dietary food intake related to variation in available food and nutrient intake. We used 371 all‐day focal follows from 24 adult females (three groups) in a wild rainforest population to quantify daily diet over 9 months. We measured food availability using vegetation surveys and phenology monitoring. We analyzed >700 food and fecal samples for macronutrient content. Subjects included 445 food items (species‐specific plant parts and insect morphotypes) in their diet. Variation in fruit consumption (percentage of diet and total kcal) tracked variation in availability, suggesting fruit was a preferred food type. Fruits also constituted the majority of the diet (by calories) and some fruit species were eaten more than expected based on relative availability. In contrast, few species of young leaves were eaten more than expected. Also, subjects ate fewer young leaves (based on calories consumed) when fruit or young leaves were more available, suggesting that young leaves served as fallback foods. Despite the broad range of foods in the diet, group differences in fiber digestibility, and variation that reflected food availability, subjects and groups converged on similar nutrient intakes (grand mean ± SD: 637.1 ± 104.7 kcal overall energy intake, 293.3 ± 46.9 kcal nonstructural carbohydrate, 147.8 ± 72.4 kcal lipid, 107.8 ± 12.9 kcal available protein, and 88.1 ± 17.5 kcal structural carbohydrate; N = 24 subjects). Thus, blue monkeys appear to be food composition generalists and nutrient intake specialists, using flexible feeding strategies to regulate nutrient intake. Findings highlight the importance of simultaneously examining dietary composition at both levels of foods and nutrients to understand primate feeding ecology.     

Separate effects of macronutrient concentration and balance on plastic gut responses in locusts

It is well established that animal guts are phenotypically plastic, adjusting inter-alia to diet quality. However, the relative contributions due to the two principal dimensions of diet “quality”—nutrient concentration and nutrient balance—remain to be teased apart. We report an experiment using synthetic foods in which the balance and overall concentration (in relation to indigestible cellulose) of protein and digestible carbohydrate were varied orthogonally, and the effects on the dry mass of locust guts measured. There were three principal results: (1) larger guts were associated with dilute compared with concentrated diets, suggesting a compensatory response to ameliorate the impact of reduced diet quality; (2) there was, by contrast, an anti-compensatory response to nutrient imbalance, where larger guts were associated with surplus protein intake; (3) the experimental group given the food that contained low protein and low cellulose, the composition that predicted the smallest guts, showed a bimodal response in which half of the insects had guts that were larger than expected for their cellulose intake, suggesting that they were able to respond to a protein-related cue in the absence of significant dietary fibre. We discuss these results in relation to regulatory theory.     

Adaptations for avian frugivory: assimilation efficiency and gut transit time of Manacus vitellinus and Pipra mentalis

Summary I examined the digestive physiology of two avian frugivores, the golden-collared manakin, Manacus vitellinus, and the red-capped manakin, Pipra mentalis, to discover how these birds extract energy from fruit. Using 14 species of fruit in the natural diet of manakins, I examined the assimilation of nutrients from fruit pulp, fruit passage rates, seed passage rates, and gut morphology. Fruits in the manakins' diets had high water content (average, 84%) and low nutrient concentrations (3 kJ/g wet pulp; 17 kJ/g dry pulp; 1% nitrogen/g dry pulp). Manacus and Pipra did not differ in the average assimilation of energy in fruit pulp (63%), although it varied from 37 to 84% depending on fruit species. Assimilation of total nonstructural carbohydrates in the fruit pulp was very high (86–98%) in both species. Gut evacuation was rapid; maximum transit time of a labeled fruit was 30 min. Seeds passed through the gut faster (Manacus: 15 min; Pipra: 12 min) than the accompanying fruit epidermis (both spp: 22 min). Manakins regurgitated large seeds (>5 mm diameter) in 7 to 9 min. Rapid gut passage time, high assimilation of nonstructural carbohydrates, and the selective regurgitation and rapid elimination of bulky seeds enable manakins to process a large volume of food per day. By increasing rates of fruit intake and gut passage, manakins can effectively increase total nutrient uptake. These adaptations of manakins are requisite for harvesting sufficient nutrients from fruit, due to its low nutrient density, high water content, and bulky seeds.     

Herbivory and novel weapons: no evidence for enhanced competitive ability or allelopathy induction of <Emphasis Type="Italic">Centaurea diffusa</Emphasis> by biological controls

Biological control of weeds by arthropod herbivores is thought to work by reducing the competitive ability of the weed relative to the surrounding vegetation. However, the assumption that herbivory reduces plant competitive ability has not been tested in most biological control systems, and counter to expectation, recent research on the impact of biological control agents on invasive Centaurea species suggests that this genus may respond to herbivory by increased competitive ability through enhanced plant re-growth and/or by inducing increased production of phytotoxic allelochemicals. We examined the impact of two biological control agents of the invasive plant diffuse knapweed (C. diffusa) to see if feeding by either of these insects would enhance the plant’s competitive ability or allelochemical output. Sub-lethal herbivory by either of the biological control agents significantly reduced knapweed performance when the plant was grown in competition with either of two native species. Competition with knapweed significantly reduced the performance of both native species (Artemisia frigida and Bouteloua gracilis), and herbivory by one of the biocontrol agents resulted in a small but significant increase in both native species’ performance. Diffuse knapweed’s putative allelochemical 8-hydroxyquinoline was not detected in experimental or field collected soils from knapweed-infested sites. In contrast to other studies on the impacts of biological control on other Centaurea species, these data support the premise that biological control agents may reduce invading plant competitive ability. We find no evidence for diffuse knapweed allelopathy mediated by 8-hydroxyquinoline or enhanced allelopathy in response to herbivory by biological control agents.     

Interpreting phenotypic variation in plant allelochemistry: problems with the use of concentrations

Ecologists often use concentrations of defensive compounds as measures of plant allocation to defence and/or allelochemical production. I demonstrate that this practice may lead to erroneous conclusions because plants produce and allocate molecules (quantities) of compounds whereas concentrations reflect the distribution of these quantities in plant tissues and are, therefore, functions of plant biomass. As a tool for distinguishing between shifts in allelochemical production versus changes in plant biomass in determining allelochemical concentrations, I suggest using a technique known as graphical vector analysis (GVA) which has been developed for diagnosing nutrient limitations in forest stands, but has seldom been applied by researchers studying plant allelochemicals. I used data from several published studies to demonstrate how GVA can be applied to interpret ontogenetic and environmental effects on allelochemical levels and to compare the results obtained for different allelochemical types, plant species, treatments and experiments. These examples show that changes in plant biomass per se are an important source of variation in allelochemical concentrations and, therefore, concentration data can be easily misinterpreted if changes in absolute content and plant biomass are not considered simultaneously. Because studies reporting variation in allelochemical concentrations have been considered as tests for general theories of plant chemical defence, evidence in support of or against these theories should be re-examined using multivariate techniques such as analysis of covariance, allometric analysis and GVA. Received: 16 September 1998 / Accepted: 4 March 1999     

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.     

Resource allocation to testes in walnut flies and implications for reproductive strategy

Testes size often predicts the winner during episodes of sperm competition. However, little is known about the source of nutrients allocated to testes development, or testes plasticity under varying nutrient availability. Among many holometabolous insects, metabolic resources can derive from the larval or adult diet. Distinguishing the source of nutrients allocated to testes can shed light on life history factors (such as maternal influences) that shape the evolution of male reproductive strategies. Here we used an experimental approach to assess resource allocation to testes development in walnut flies (Rhagoletis juglandis) from differing nutritional backgrounds. We fed adult male walnut flies on sugar and yeast diets that contrasted with the larval diet in carbon and nitrogen stable isotope ratios. This design allowed us to assess the dietary source of testes carbon and nitrogen and its change over time. We found significant incorporation of adult dietary carbon into testes, implying that walnut flies are income breeders for carbon (relying more on adult resources). In contrast, we found little evidence that walnut flies incorporate adult dietary nitrogen into testes development. We discuss the implications of these allocation decisions for life history evolution in this species.     

The Role of Fruits and Insects in the Nutrition of Frugivorous Bats: Evaluating the Use of Stable Isotope Models1

We estimated the relative contribution of fruits and insects as sources of dietary protein in two species of Neotropical frugivorous bats (Artibeus jamaicensis and Sturnira lilium) using stable carbon and nitrogen isotope analyses. An insectivorous species (Pteronotus parnellii) was also included for comparison. We found constant patterns in stable carbon and nitrogen isotope composition in blood that separated the two species of frugivorous bats from the insectivorous bat. When we used these isotopic values (combined with those of dietary fruits and insects) to estimate the percent contribution of fruits and insects to the diet of the bats, we obtained different results, depending on assumptions and model adopted. We tested models using both 8“N and 8′³C results simultaneously and separately and further used diet‐tissue fractionation factors of 3%o for nitrogen and 1 and 3.5%o for carbon. We found that a carbon‐based model with a diet‐blood enrichment factor of 3.5%o produced the most parsimonious results. The model estimated that A. jamaicensis and S. lilium obtained most of their protein requirements from fruits, whereas P. parnellii fed mostly on insects. No sexual or seasonal variations in the diet of the two frugivorous species were detected. We found no evidence that the diet of sexually active females differed from that of nonsexually active females in the two species of frugivorous bats. We suggest that future studies better define isotopic fractionation between diet and tissues of bats using captive rearing and controlled diets.     

The effects of simultaneous variation in protein, digestible carbohydrate and tannic acid on the feeding behaviour of larval Locusta migratoria (L.) and Schistocerca gregaria (Forskal). I. Short-term studies

ABSTRACT The influence of simultaneously varying the levels in artificial diets of protein, digestible carbohydrate (14% or 28%) and tannic acid (absent or 10%) on the feeding behaviour of the oligophagous Locusta migratoria (L.) and the polyphagous Schistocerca gregaria (Forskal) (Acrididae) was investigated. Total consumption and detailed feeding behaviour were recorded over a 12 h period in choice and no-choice experiments. In addition, amounts eaten by Schistocerca of the 14% protein, 14% carbohydrate diet with and without tannic acid were measured at regular intervals throughout the fifth stadium, and insect growth over this period was recorded. There were no interactive effects of nutrient levels and tannic acid, despite the fact that both species compensated for dilution of dietary protein by increasing consumption. Only male Locusta compensated for dilution of dietary carbohydrates, and this compensation was much less marked than for protein. Tannic acid did influence feeding as a main effect, however. It caused an increase in amounts eaten by Schistocerca in both choice and no-choice experiments. This increased consumption was due to an increase in the number of meals taken. A shorter latency period before and a longer duration of the first meal by naive insects suggested a phagostimulatory rather than a post-ingestive effect of tannic acid. The stimulatory effect was only apparent for the first 24 h of continuous exposure, but this temporary enhancement none the less resulted in the insects being heavier at adult ecdysis. Stadium duration was also somewhat reduced. In a no-choice situation, no effect of tannic acid on the feeding behaviour of Locusta was observed. When given a choice, however, this species took significantly more meals on the tannic acid-free diet, these being of similar average size to meals taken on the tannic acid diet. Significantly more insects took their first meals on the tannic acid-free diet in the choice test, indicating a deterrent effect of tannic acid in Locusta.