Compensatory feeding response of the slug Sarasinula plebeia to dietary dilution

Summary Like many polyphagous herbivores, individuals of Sarasinula plebeia (Fischer) (Soleolifera: Veronicellidae) consume a variety of plant species that may differ in nutritional content. In this study we determined the ability of these slugs to compensate for such variation in diet composition. Dilution with water of an agar-based diet containing commercial guinea pig food or carrot root to obtain dry weights (dw) of 90, 70, 40 and 10% of diet fresh weight (fw), caused immature slugs to consume increasingly more fresh weight of food [as much as 4.7-(guinea pig) to 6.1-fold (carrot) more]. Dry weight consumption and body mass-relative dry weight consumption rate also increased at intermediate dilutions, buth with further dilution, dry weight intake declined despite the greater fresh weight consumption. At each dilution level, slugs fed the guinea pig diet consumed from ca. 5-to 6.4-fold more fresh weight than the carrotfed slugs. The former grew substantially, with their final biomass and body mass-relative growth rate varying curvilinearly with diet % dw. If these slugs had not fed more but instead maintained the same fresh weight consumption as slugs in the 90% dw tretments, without altering food utilization efficiencies, then their biomass gain in the 70, 40 and 10% dw treatments would have been only about 62, 43, and 21%, respectively, of the values actually attained. In contrast, carrot-fed slugs did not grow and were only able to maintain their initial biomass. For each diet, slug tissue water (% fw) was highest in the most diluted treatment but did not differ significantly among the other dilution levels. Approximate digestibility of the carrot diet was highest at intermediate dilution levels (ca. 75% of ingested food was digested and absorbed); for the guinea pig diet, this efficiency declined linearly from about 66% to 59% with increased dilution. For slugs that grew (i.e., those fed the guinea pig diet), effeciences of converting digested (29–52%) and ingested (18–33%) food to dry biomass were both curvilinearly related to diet % dw. Thus, S. plebeia, like many other herbivores, has the capacity to increase food consumption substantially inresponse to reduced dietary nutrient level, allowing the slugs to cope with variable nutrient content in their food plants.     

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.     

Compensatory increases in food consumption and utilization efficiencies by velvetbean caterpillars mitigate impact of diluted diets on growth

Fresh weight (fw) food consumption by caterpillars of Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae) increased almost 2-fold as the nutrients in an artificial diet were increasingly diluted with water (diets contained 65, 79, 86 or 89% fw water). Nonetheless, dry weight (dw) relative consumption rate (RCR) declined with diet dilution. The efficiency at which the consumed food is digested and assimilated (approximate digestibility, AD) increased on the 3 diluted diets, and the efficiency at which digested food is converted to biomass (ECD) increased on the 79 and 86% fw diets. As a consequence, dw gained and relative growth rate (RGR), which is the product of RCR x AD x ECD, on the 79% fw diet were similar to those on the 65% fw diet, but they declined on the more diluted diets. Relative nitrogen consumption rate also declined with dietary dilution, but this was compensated by an increase in nitrogen utilization efficiency such that the product of these, relative nitrogen accumulation rate, was similar on all four diets. Insect lipid content declined from 32% on the undiluted diet to 13% dw on the most diluted diet, and was primarily responsible for the decline in RGR. The increases in fw consumption and AD, while not preventing a decline in RGR on the two most diluted diets, mitigated the impact of dietary dilution (e.g., without these increases, RGR on the most diluted diet would have been only 43% of that attained). These results indicate that the consumption and utilization of food are dynamic processes, and that caterpillars of A. gemmatalis, like many other insects, exhibit compensatory responses to changes in dietary quality.

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Résumé Peu de travaux ont examiné l'aptitude des insectes à modifier leur consommation et l'utilisation des aliments en fonction des variations qualitatives et quantitatives connues de l'aliment. On a rarement examiné si ces modifications sont adaptatives, si elles maintiennent le taux de croissance et le niveau des ressources, ou si elles limitent au moins les effets nuisibles du changement d'aliment. En utilisant une technique gravimétrique, nous avons mesuré le gain des poids, l'aliment consommé et les excréments produits par des chenilles de A. gemmatalis Hübn. (Lep, Noctuidea), important ravageur du soja aux USA.La consommation en poids frais (fw) a augmenté presque 2 fois quand le régime artificiel a été dilué progressivement avec de l'eau (65, 79, 86 ou 89% d'eau); le poids sec (dw) et le taux de consommation relative (RCR) ont diminué néanmoins avec la dilution. L'efficacité de digestion et d'assimilation de l'aliment consommé (digestibilité approchée, AD) a augmenté pour les trois dilutions, l'efficacité de conversion en biomasse de l'aliment digéré (ECD) a augmenté dans les dilutions à 79 et 85%, mais a diminué pour les régimes plus dilués. Par conséquent, le gain dw et le RGR, — produit de RCR x AD x ECD-, étaient identiques avec le régime à 79% fw à ceux de 65% fw, mais ont diminué pour les régimes plus dilués. Le taux relatif d'efficacité de consommation d'azote a diminué aussi avec la dilution, mais il a été compensé par un accroissement de l'efficacité de l'utilisation de l'azole tel, que le produit,-taux relatif d'accumulation de l'azote-, était le même pour les quatre régimes. La teneur en lipides des insectes a diminué, de 32% pour le régime sans dilution, à 13% pour le régime le plus dilué; elle a été le principal responsable de la diminution de RGR. L'accroissement de la consommation fw et de AD, tout en n'empêchant pas la diminution de RGR pour les deux régimes les plus dilués, a limité l'effet de la dilution (sans ces accroissements, le RGR du régime le plus dilué n'aurait été que 43% du RGR obtenu).Ces résultats indiquent que la consommation et l'utilisation d'aliments constituent un processus dynamique, et que les chenilles de A. gemmatalis comme beaucoup d'autres insectes, présentent des réactions compensatrices au changement de qualité de l'aliment. De telles variations dans la consommation d'aliments ont des conséquences écologiques (consommation accrue de substances allélochimiques potentiellement toxiques et exposition accrue aux causes biologiques de mortalité pendant l'alimentation), et intéressent l'organisation de la protection contre les insectes:-en permettant la manipulation de l'alimentation des insectes pour augmenter la consommation à court terme d'agents bioactifs, et, par ce moyen, réduire la consommation totale,-en permettant d'améliorer la modélisation des dégâts alimentaires prévisibles, qui, pour être efficace, devrait inclure les variations de l'alimentation en fonction de la qualité alimentaire de la plante cultiveé.

     

Compensatory responses of the fall armyworm (Spodoptera frugiperda) when fed water- and cellulose-diluted diets

Abstract Caterpillars of the noctuid moth, Spodoptera frugiperda (J. E. Smith), reared on artificial diets diluted with cellulose and water, increased fresh weight (fw) consumption 2.2–2.5-fold over those on the undiluted diet. At moderate levels of water- or cellulose-dilution, the increased consumption, combined with increased digestion and absorption of nutrients (ADNU), sufficiently compensated for the reduced nutrient intake to achieve pupal biomass equivalent to that of pupae from the undiluted diet. At higher levels of water- and cellulose-dilution, fw consumption and ADNU increased further but pupal dry weight declined on the water-diluted diets. At each level of dilution fw consumption and ADNU increased similarly on the water- and cellulose-diluted diets, but biomass gain was reduced on the water- compared with the cellulose-diluted diets. This was due in part to lowered food conversion efficiency on the water-diluted diets, possibly caused by increased costs of metabolizing the wetter diets. Our data support the hypothesis that consumption rates are regulated by a nutrient feedback mechanism. Our data also suggest that digestive enzyme activity is correlated with consumption. Furthermore, the cost of increased consumption rates on diets of reduced energetic value may constitute a substantially greater energy expenditure than previously believed. However, this cost was insufficient to reduce relative growth rates but was apparently manifested in lowered lipid accumulation.     

Effects of potassium levels over the range occurring in host plants on the growth of an insect herbivore (Manduca sexta)

Third instar tobacco hornworms (Manduca sexta L.: Sphingidae) on low dietary potassium had a lower relative growth rate than individuals on diets with potassium concentrations reflecting those in host-plants, due to decreased consumption rate, lower efficiencies of conversion of ingested and digested food (ECI and ECD), and a prolonged growth/feeding phase. Furthermore, these larvae, when placed on a diet with a moderate potassium concentration through the fourth stadium, ended up being smaller due to lower ECI and less biomass gained, and had a prolonged growth phase, which suggest an irreversible cost of the previous low potassium diet. Third instar hornworms on high potassium diets had lower ECI and ECD, and they had a prolonged growth phase. These individuals, when placed on a moderate potassium diet in the fourth stadium, gained less biomass, than those previously offered hostplant-like-potassium diets. Body potassium concentrations (% dw) at the end of the third stadium were similar among treatment groups. With increasing potassium concentrations in the diet, utilization efficiencies of potassium decreased and potassium concentrations in the frass increased. Correspondingly, water content (% fw) of the newly-molted fourth instar larvae declined with increasing potassium, indicating a passive loss of water during potassium excretion. Low and high dietary potassium reduced survivorship of third instar larvae; fourth instar caterpillars previously fed the low potassium diet also had poor survivorship. We conclude that, within the normal range of potassium concentrations in the hostplants, caterpillar performance is largely unaffected by potassium concentration, but that potassium-poor and potassium-rich diets, such as those hornworms may sometimes experience, can reduce growth and survivorship.     

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.     

Discriminating between energetic content and dietary composition as an explanation for dietary restriction effects

A reduction in dietary calories has been shown to prolong life span in a wide variety of taxa, but there has been much debate about confounding factors such as nutritional composition of the diet, or reallocation of nutrients from reduced reproduction. To disentangle the contribution of these different mechanisms to extension of life span, we study the effect of caloric restriction on longevity and fecundity in two species of sugar-feeding parasitoid wasps. They have a simple diet that consists of carbohydrates only, and they do not resorb eggs, which rules out the proposed alternative explanations for beneficial effects of caloric restriction. Two caloric restriction treatments were applied: first, dietary dilution to investigate the effect of carbohydrate concentration in the diet; and second, intermittent feeding to examine the effect of feeding frequency on longevity and fecundity. Only the dietary dilution treatment showed an effect of caloric restriction with the highest longevity recorded at 80% sucrose (w/v). No effect of dietary regime was found on fecundity. We also measured the weight increase of the parasitoids after feeding to obtain an estimate of consumption. A constant quantity of the sugar solution was consumed in all dietary dilution treatments, hence caloric intake was proportional to sucrose concentrations. Although the present study does not disqualify the relevance of nutrient composition in other species, our data unequivocally demonstrate that caloric restriction alone is sufficient to extend life span and invalidate alternative explanations.     

Effects of dextrin and cellulose on feeding and reproduction in female brown-banded cockroaches, Supella longipalpa

Abstract Food consumption and reproduction were compared through two gonotrophic cycles in female brown-banded cockroaches, Supella longipalpa (F.), fed a standard rat food diet which had been serially diluted with methyl cellulose, alpha cellulose or dextrin. Females did not increase daily intake to compensate for dilution, and reproductive rate was highly dependent upon the degree of dilution of the diet. With increasing dextrin concentrations, digestibility increased, but reproductive rate, size and mass of oothecae, and efficiency of food utilization progressively decreased.
On diets containing 75% rat food and 25% methyl cellulose, females consumed minimal amounts of food and 80% of the females did not mate or produce oothecae within 30 days. On identical diets containing 25% alpha cellulose, food digestibility decreased and females ate significantly more than females fed 25% methyl cellulose, but less than females fed rat food. Many (65%) females fed 25% alpha cellulose produced oothecae which were smaller and took longer to form than in control females fed on rat food. At concentrations higher than 50% of either methyl or alpha cellulose daily intake was further reduced and females died rapidly.     

The interplay between nutrient balancing and toxin dilution in foraging by a generalist insect herbivore

Food mixing by herbivores is thought to balance nutrient intake and possibly dilute secondary metabolites characteristic of different host plant species. Most empirical work on insect herbivores has focused on nutrient balancing in laboratory settings. In this study, we characterize food mixing behaviour of the caterpillar Grammia geneura (Strecker) (Lepidoptera: Arctiidae) in nature and use the observed patterns to design ecologically relevant experiments that reveal the relative importance of these processes in food-switching behaviour. Our design involved both choice and no-choice experiments with chemically defined diets in which primary nutrients and secondary metabolites were manipulated in tandem. We analysed two stages in the process of food-switching behaviour: leaving food and accepting new food. In nature, an individual's rate of leaving host plants was positively associated with its probability of rejecting plant species most recently eaten, but not related to its probability of accepting different host plant species. Furthermore, an individual's leaving rate was negatively related to its average feeding bout duration. This relationship resulted partly from variation in the response of individuals to nutrient imbalance and partly from shortened feeding bouts prior to switching, suggesting that a decline in feeding excitation preceded searching for food that differed from that most recently eaten. Laboratory experiments with synthetic diets indicated the importance of secondary metabolites in the decline in feeding excitation prior to switching. Preference for new food depended strongly on secondary metabolites in a manner consistent with toxin dilution. This is the first experimental evidence for the process of toxin dilution in caterpillars, and for the combined influence of nutrients and secondary metabolites on their foraging patterns in nature. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

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.     

Compensatory feeding in response to variable food quality by Melanoplus differentialis

Abstract. The behavioural mechanisms driving compensatory food consumption in response to dietary dilution as well as the relationship between feeding time and food residence time (i.e. digesta retention time in the gut) were studied using the non-diapausing strain of the grasshopper Melanoplus differentialis (Thomas) (Orthoptera: Acrididae). 3-day-old, sixth-instar nymphs and 3-day-old adults were fed artificial diets containing 1%, 3% and 5% total nitrogen (N) at 30C, LD 14:10h; the feeding behaviour was recorded using electronic monitoring devices connected to microcomputers for 24 h. The percentage of time spent feeding increased linearly as diets were diluted using non-digestible cellulose from 5% to 1% N. This response was due to an increase in the number of meals while the meal duration of a feeding bout was unaltered. Sixth-instar nymphs spent about 40% more time feeding than the larger adults. The increased feeding time in nymphs resulted from both more frequent feeding bouts and longer meal duration. Feeding time and food residence time were highly negatively related.     

Effects of temperature on growth and efficiency of food utilization in fifth-instar caterpillars of the tobacco hornworm, Manduca sexta

Fifth-instar larvae of Manduca sexta were reared from hatching on artificial diet at 15, 20, 25, 30 and 35°C. Total development time decreased with increasing temperature. Very few larvae (12%) survived at 15°C, so this temperature was not considered further. There was some mortality at 30°C (11%), and at 35°C (50%).The absolute rate of growth in the fifth instar was faster at 25 than at 20°C, but was similar at 25, 30 and 35°C. This was true both for caterpillars that were chronically exposed to experimental temperatures (i.e. since hatching) and for those acutely exposed (i.e. reared up to fifth instar at 25°C).There was a progressive decrease with higher rearing temperatures in both the initial and final sizes of chronically exposed fifth-instar larvae. Acutely exposed caterpillars matched for initial size showed smaller temperature related differences in final size. Because of these size differences there were differences in relative growth rate which did not reflect true differences in absolute growth rate.Total food consumed by chronically exposed caterpillars was greatest at the lowest temperature (20°C), and decreased progressively with increasing temperature. The absolute rate of food consumption increased from 20 to 25°C, but did not vary significantly between 25 and 35°C. Differences in the sizes of the insects at the different temperatures meant that there were differences among relative measures of consumption that did not reflect absolute food consumption.For chronically exposed caterpillars, none of the three usual indices of food conversion efficiency (AD, ECI and ECD) varied significantly with temperature between 20 and 35°C. This implies that the effects of temperature on metabolic costs are closely matched to food consumption.Oxygen consumption increased with temperature between 20 and 25°C but was temperature compensated between 25 and 35°C.These findings are discussed in terms of their implications for the optimal temperature for growth in Manduca.     

Role of corpus allatum on the modulation of feeding rhythm in the semilooper caterpillar, Achaea janata (L)

A circadian feeding rhythm which may be entrained by photoperiod was found in fifth instar caterpillars of the lepidopteran, Achaea janata (L.). Allatectomy reduced the amount of food consumed; this consumption was significantly lower during the fifth instar in both allatectomized and sham-operated insects. An apparent circadian feeding pattern appeared on day 2 in the sham-operated caterpillars. Topical application of the anti-allatin, Precocene-II (50 micrograms/animal) also reduced leaf consumption significantly compared to the respective controls, although these controls maintained the same apparent circadian feeding rhythm on day 2.     

Nutritional regulation in nymphs of the German cockroach,Blattella germanica

Synthetic foods varying in protein-carbohydrate ratio and total nutrient concentration were used in food selection experiments to investigate the ingestive and post-ingestive regulation of macronutrients by male and female nymphs of the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae). For both nutrient imbalance and food dilution, food ingestion varied between treatments with the effect that nutrient ingestion was regulated. However, this mechanism was insufficient to compensate for some food dilution treatments. In those cases, the regulation of protein intake was prioritised over that of carbohydrate intake, and two additional regulatory responses were seen. Firstly, cellulose digestion supplemented shortfalls in dietary soluble carbohydrates, and secondly the feeding period within the stadium was prolonged. These ingestive, post-ingestive and developmental responses were orchestrated in such a way that, in all treatments, nutrient gain approached similar levels, despite the variation in food properties.     

Instar duration, adult consumption, oviposition and nitrogen utilization efficiencies of leafhoppers feeding on different quality food (Auchenorrhyncha: Homoptera)

Abstract. 1. Plant material of Holcus lanatus L. removed from an artificially fertilized trial was used to rear leafhoppers of three feeding types: Dicrano-tropis hamata Boheman and Ebmana sulphurella Zetterstedt, (phloem feeders), Eucelis incisus Kirschbaum (xylem feeder) and Zyginidia scutellaris Herrich-Schaeffer (mesophyll feeder).
2. In the laboratory, leafhopper instar durations, adult consumption rate, oviposition rate, and nitrogen utilization efficiency were influenced by the quality of the food consumed. Adult longevity was unaffected by the level of food quality normally found in plants.
3. Z.scutellanis consumed mesophyll contents equivalent to 72% of its body weight each day but only digested about 30% of its intake. Eincicus had a low dry weight consumption rate (12% body weight/day) but digested its food more efficiently (60%) than the other feeding types. The two phloem feeders consumed mounts equivalent to 25% of their body weight each day and digested 40–50% of their intake.
4. Each species reached maximum nitrogen utilization efficiencies at different plant nitrogen levels. This suggests that leafhopper species may be associated with a particular nitrogen concentration in the grasses.     

Ontogenetic changes in the rate of ingestion and estimates of food consumption in fourth and fifth instar Helicoverpa armigera caterpillars

We present the second in a series of experiments investigating the behavioural mechanisms used by Helicoverpa armigera caterpillars to fund the increased nutrient requirements associated with growth and development. In the work reported here, we measured ontogenetic changes in the rate of ingestion (amount of an artificial food ingested per unit time when the insect is actually feeding) in fourth and fifth (penultimate and ultimate) instar caterpillars. These data are used together with those obtained in a previous study on ontogenetic changes in the proportion of time spent feeding to estimate the total amount of food ingested over three 33.3% temporal segments of the period from ecdysis to the cessation of feeding in the two stadia. Overall, the rate of ingestion in the fifth stadium was about three times that in the fourth. Rate of ingestion was constant over the fourth stadium but increased over the course of the fifth. Total consumption in the fifth stadium was about 3.5 times greater than in the fourth, mainly due to the greater rate of ingestion. In the fourth stadium, consumption in the third segment was greater than in either of the first two segments because the time spent feeding was greater. In the fifth stadium, consumption in the second segment was greater than in the first because of an increase in time spent feeding. In contrast, the greater intake in the third segment as compared with the second was due to an increase in the rate of ingestion. Our results demonstrated that the larvae, through increasing the rate of ingestion, were able to satisfy their increasing nutritional requirements without there being, necessarily, a commensurate increase in the time spent feeding.     

Seasonal insectivory by Black‐headed Trogons,a tropical dry forest frugivore

ABSTRACT Fruit‐eating birds are important seed dispersers in tropical forests, but little is known about the extent to which they rely on insects or how their diets vary seasonally. We used field observations of focal adults to quantify the diets of adult and nestling Black‐headed Trogons (Trogon melanocephalus) at nine nests in a lowland dry forest in Costa Rica. From May 2004 to August 2004, we documented 540 food deliveries to nests and 1080 food items consumed by adults. Adult and nestling trogons were largely insectivorous, feeding mainly on moth caterpillars (Lepidoptera). Fruit accounted for only 10.5% of items consumed by adults and 2.2% of items delivered to nestlings (6.1% and 0.6% of estimated dry mass, respectively). Adult and nestling diets differed significantly in both composition and prey size, with adults consuming more fruit and fewer large insects (Phasmatodea and Mantodea) than nestlings and eating more types of arthropods and fruit. Although both adults and nestlings relied heavily on moth larvae, adults preferentially consumed small caterpillars and delivered large ones to their nestlings. In addition, the proportion of large caterpillars delivered to nests remained constant throughout the nestling period, whereas the proportion of large caterpillars eaten by adults declined significantly with nestling age. Overall, arthropods delivered to nests averaged 70% heavier than those consumed by adults (estimated dry mass). Our results suggest that Black‐headed Trogons time reproduction to coincide with arthropod rather than fruit abundance, a pattern that may be more common among omnivorous forest birds than previously recognized.     

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.     

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.     

Combined effects of allelochemical-fed and scarce prey on the generalist insect predator Podisusmaculiventris

1. The simultaneous effects of allelochemicals ingested by herbivorous insect prey and prey scarcity on the performance of a generalist insect predator were examined.
2. Fifth-instar predatory stinkbugs ( Podisus maculiventris : Pentatomidae) were fed caterpillars ( Manduca sexta : Sphingidae) in three prey scarcity treatments: every day (unlimited amount), one caterpillar every third day, one caterpillar every fifth day. The caterpillars were fed either a plain diet or a diet containing rutin, chlorogenic acid and tomatine, which are three of the major allelochemicals in tomato leaves ( Lycopersicon esculentum : Solanaceae), the preferred food of these caterpillars.
3. Food consumed, efficiency of conversion of ingested food to biomass (ECI), biomass gained, stadium duration and relative growth rate (RGR) of predators were negatively affected by prey scarcity. The allelochemicals negatively affected food consumed and ECI.
4. There were prey scarcity by allelochemical interactions for ECI, biomass gained and RGR. For ECI, the allelochemicals had a greater negative impact on the predatory stinkbugs when prey were scarce. When prey diet contained allelochemicals, biomass gained and RGR declined more steeply with increased prey scarcity. There was an allelochemical by predator gender interaction for biomass gained. Allelochemicals had no effect on biomass gained by female stinkbugs, whereas biomass gained declined more steeply with increased prey scarcity for male stinkbugs fed caterpillars containing allelochemicals than for males fed control caterpillars.