Nutrition interacts with parasitism to influence growth and physiology of the insect Manduca sexta L.
Department of Entomology, University of California, Riverside, California 92521, USA
* Author for correspondence (e-mail: nelsont{at}ucr.edu)
Accepted 24 November 2004
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Summary |
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Key words: insect, growth, nutrition, diet, parasitism, Manduca sexta, Cotesia congregata
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Introduction |
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Previous studies suggest that parasitism alters the normal feeding
responses of M. sexta larvae
(Thompson et al., 2001).
Normal, unparasitized larvae given a choice of two diets, one containing
casein but lacking carbohydrate and a second containing sucrose but lacking
protein, feed in a ratio of 2 parts of the protein diet to 1 part carbohydrate
diet. Parasitized insects given the same choice consume equal amounts of each
diet. This difference in diet consumption is principally due to lower
consumption of the protein diet by parasitized larvae, which explains in part
the slow growth of parasitized insects. When fed individual diets having
different ratios of casein and sucrose, those parasitized larvae feeding on a
diet with equal amounts of protein and carbohydrate produce the greatest
number and biomass of parasites. This suggests that altered host nutrient
intake may be important for optimizing parasite growth and development.
Lacking, however, is precise information on the influence of dietary nutrient
composition on nutrient consumption, growth and development by normal and
parasitized larvae.
Knowledge of information on host blood metabolite concentrations is
essential for understanding the impact of host response to dietary nutrient
ratio and parasitism because these metabolites provide the nutrients on which
C. congregata feed, and their concentrations play a role in
regulating feeding and food choice by the host
(Simpson and Raubenheimer,
1993b). Nutrients digested and absorbed by the gut and metabolites
synthesized from nutrients and released by tissues are transported directly in
the open circulation. They occur at variable and often high concentrations
depending on nutrient intake. The concentrations of circulating metabolites
provide a continuous reading of the insect's nutritional and metabolic state,
information that serves as a basis for maintaining nutritional homeostasis
through regulation of feeding (Schiff et
al., 1989
; Simpson and
Raubenheimer, 1996
). Studies with several species of lepidopteran
larvae, including M. sexta, demonstrate that the concentration of
trehalose, the blood sugar of insects, correlates positively with carbohydrate
consumption. Carbohydrate intake and blood sugar level, in turn, affect the
subsequent consumption of dietary carbohydrate
(Simpson et al., 1988
;
Friedman et al., 1991
;
Thompson and Redak, 2000
).
Food choice is a dynamic process. Many insects offered nutritionally
unbalanced foods continuously adjust their feeding, selecting a combination of
foods that ultimately results in an intake of required nutrients that is
optimal for growth and development
(Waldbauer and Friedman, 1991
;
Simpson and Raubenheimer,
1993a
; Raubenheimer and
Simpson, 1999
).
The present study establishes the relationship between dietary nutrient ratio, nutrient consumption and growth of normal unparasitized M. sexta larvae and larvae parasitized by C. congregata. We fed larvae a series of chemically defined diets all containing the same total level of casein and sucrose but with variable ratios of these nutrients. We also examined the effects of dietary nutrient ratio and nutrient intake on the blood concentrations of protein, total free amino acids and trehalose. Based on the results of earlier studies described and cited above, we predicted that both dietary nutrient level and parasitism would affect diet and nutrient consumption. Further, we predicted that the levels of blood metabolites would reflect the intake of casein and sucrose by larvae fed the various diets. Last, we consider how the effects and interactions of nutrition and parasitism may influence the feeding behavior and ecophysiology of M. sexta larvae. In a subsequent study we will examine how these effects of parasitism directly influence parasite growth and development.
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Materials and methods |
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Mixed-sex populations of approximately 100-150 Cotesia congregata Say adult parasitoids were housed in 3 l glass jars. M. sexta larvae in the second instar were placed in the bottom of the jars and superparasitized two to three times. The parasitized host larvae were removed, placed on artificial diet, and treated in the same fashion as normal larvae described above. Parasitized host larvae did not pupate and parasitoids emerged from hosts in the terminal stadium housed in 160 ml plastic cups. Parasitoid cocoons were carefully removed from the surface of hosts and placed in 30 ml plastic cups, approximately 150 per cup. Cups were then individually placed in 3 l glass jars for emergence. After eclosion adult parasitoids were fed honey and supplied with water from dental cotton wicks in small test tubes.
Insects were maintained in a Precision Scientific incubator at 28°C with a 16 h:8 h light:dark non-diapausing, long-day photocycle.
Experimental rearing protocol
Normal and parasitized larvae were fed a chemically defined artificial diet
(Ahmad et al., 1989)
immediately upon moulting to the fourth stadium. The diet contained casein and
sucrose as digestible protein and carbohydrate, respectively. The stock diet
formulation contained 90 g l-1 of each nutrient. In addition, the
diet consisted of linseed oil, Wesson's salts, ascorbic acid and B vitamins.
Nutrients were principally obtained from Nutritional Biochemicals (Cleveland,
OH, USA) and Bioserve (Frenchtown, NJ, USA). Experiments were conducted with a
series of six diets, each with an equivalent amount of combined protein and
carbohydrate, but with the following ratios of casein to sucrose:
0.125C:1.875S, 0.25C:1.75S, 0.5C:1.50S, 1.0C:1.0S, 1.5C:0.5S and 2.0C:0S. The
values are relative to the amount of casein and sucrose in the stock
formulation, that is 1C:1S is equivalent to 90 g l-1 casein and 90
g l-1 sucrose. The nutrient content of the diets were equivalent in
mass, and because carbohydrate and protein have similar caloric value the
diets were approximately equal in energy content.
Non-feeding pharate fourth instar larvae were removed from the rearing diet
and superparasitized two to four times, as described by Alleyne
(1995). This produced high
parasite burdens or maximal numbers of parasites developing within individual
host larvae. Larvae were synchronized as described by Baker et al.
(1987
). In our studies,
however, parasitized insects were not starved for 10-12 h after moulting, but
immediately placed on the experimental diets. Groups of ten randomly selected
parasitized larvae were maintained on each of the experimental diets for the
feeding studies described below. Normal fourth instar larvae at the equivalent
stage of development served as controls. All larvae were housed individually
in 30 ml plastic cups during the fourth stadium, and pharate fifth instar
larvae were transferred to 160 ml cups for the duration of the experiment.
Insects were maintained at 28°C with a long-day photocycle as described above.
Feeding studies
Larvae were fed the various experimental diets throughout the fourth
stadium until the end of the fifth stadium. In the case of normal larvae, the
experiments were discontinued once after approximately 25% of the larvae had
stopped feeding and entered the wandering phase in preparation for pupation.
Normal larvae wander within 6-12 h of each other and development time on the
various diets was measured in full day increments. At this point, all larvae
had reached a fresh mass of approximately 8-10 g with a total development time
between 7 and 15 days, depending upon diet. In the case of parasitized larvae,
cessation of feeding occurred over a longer time interval, up to 24 h. The
time also varied with diet, between 12 and 15 days.
Determination of nutrient consumption and larval growth
Diet consumption was measured as the difference between the total amount of
diet offered to larvae and the amount remaining in the diet cups at the end of
the experiment together with undigested diet remaining in the gut. Dry mass of
the diet remaining in the cups was determined by drying the diet in an oven at
100°C for 24 h and weighing on a microbalance. Initial dry mass of the
diet offered to the insects was estimated from the known ratio of wet/dry
mass. Protein and carbohydrate consumption were estimated based on the
composition of each diet. Fecal material, collected at the end of the
experiment, was also dried and weighed. The amount of food assimilated was
calculated as the difference between the food consumed and the amount of frass
produced on the different diets.
The guts of normal and parasitized larvae were dissected at the end of the experiment and the diet remaining in the gut was removed. This diet was added to that remaining in the diet cups, for estimation of diet consumption. The insect carcasses, including the gut, were dried as described above, after which they were removed and weighed. Initial dry mass of larvae was estimated from the wet mass and the wet/dry mass ratio of several larvae that were dried at the beginning of the experiments.
Analysis of host blood metabolites
Analyses were conducted to determine the effects of dietary nutrient ratio
and parasitism on blood concentrations of trehalose, protein and total free
amino acids. Groups of five larvae were maintained on each of the diets until
approximately two days before feeding stopped. The time chosen was based on
the results for the feeding studies described above. We did not wait until
feeding had stopped, in order to avoid difficulties in collecting blood at the
later time, rapid blood coagulation in the case of normal larvae and
interference from parasites in parasitized larvae. Blood was collected from
small incisions made with microscissors in one or more prolegs. To remove
blood cells and tissue debris, whole blood was centrifuged at 3000
g for 3 min in a refrigerated Beckman microfuge.
Trehalose was determined by 13C nuclear magnetic resonance (NMR)
spectroscopy following deproteination of the cell-free supernatant by addition
of perchloric acid to 3.5%. Following centrifugation as above, blood plasma
was neutralized with 2 M K2CO3. Samples were
refrigerated for approximately 12 h and then centrifuged to remove
KClO3. An internal standard, 3-(trimethylsilyl)-1-propane sulfonic
acid, was added to the supernatant to 20 mmol l-1. Deuterium oxide,
added to 15%, served as a field frequency lock. 13C NMR analysis
was conducted under non-saturating conditions at 75.48 MHz in a Varian Inova
300 spectrometer as described previously
(Thompson et al., 2002).
Spectra were generated from 4000 data acquisitions and referenced to the
internal standard at approximately -2.67 ppm. Trehalose concentration was
calculated by comparing the signal intensity of trehalose C1 at 93 ppm with
the intensity of the internal standard.
For analysis of protein and amino acids, blood plasma was deproteinized by addition of sulphosalicylic acid to 2%. After centrifugation, the protein pellet was dried for 2 h at 100°C and the crude protein determined gravimetrically with a Sartorius M2P electronic microbalance (Goettingen, Germany). Amino acid analysis was conducted with a Beckman 6300 automated amino acid analyzer (Fullerton, CA, USA), equipped with a lithium ion exchange column. Samples were diluted 50 fold with Beckman Li-S® high performance lithium citrate buffer containing 50 µmol l-1 aminoethyl cysteine as an internal standard. Sample aliquots of 50 µl were loaded into the analyzer. Amino acid separations were achieved by stepwise pH elution with lithium citrate buffers: pH 2.92, 3.65 and 3.75 (Li292®, Li365® and Li375®, respectively; Pickering Laboratories, Mountain View, CA, USA). A variable temperature program of 32°, 50° and 70°C, respectively, was employed upon addition of each elution buffer. A ninhydrin reaction was used for detection. Total free amino acid concentration was calculated by summing the concentrations of the individual amino acids.
Concentrations of all blood metabolites are reported in mg ml-1 blood plasma.
General statistical analyses
Two-way analysis of variance (ANOVA) principally was used to analyze data
from the feeding experiments to establish the effects of dietary nutrient
ratio and parasitism on host growth (mass gain) and blood metabolite levels.
Owing to potential variation in initial mass between treatments, we first
analyzed the growth data by analysis of covariance (ANCOVA), using host
initial mass as the covariate
(Raubenheimer and Simpson,
1992; Horton and Redak,
1993
). There was no initial mass covariate effect, and data were
subsequently examined by ANOVA, with ANCOVA applied in particular cases where
described. Normality and homogeneity of variance were established with the
Shapiro-Wilk `W' test and normal probability plots. All data met the
assumptions of analysis of variance.
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Results |
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Normal and parasitized M. sexta larvae displayed distinct intake
arrays over the fourth and fifth stadia
(Fig. 1). Differences in
development time between diets, described below, contribute greatly to the
overall nutrient intake patterns for both normal and parasitized insects. The
intake array of parasitized larvae was non-linear with the ends curved sharply
inwards demonstrating reduced nutrient intake at more extreme dietary nutrient
ratios. The nutrient intake array for normal larvae generally formed a linear
configuration. The intake point for the 0.125C:1.875S diet, however, was
radically displaced from the overall array. Clearly, larvae were unable to
adjust their feeding response to accommodate the low protein and nutrient
imbalance of this diet and, therefore, the diet can be considered a
`pathological food' (Raubenheimer and
Simpson, 1999).
Total nutrient intake for the various diets was measured by the distance
from the origin of the intake points along the nutrient rail of each diet
calculated from protein and carbohydrate consumption using Pythagoras's
theorem (Raubenheimer and Simpson,
2003).
Differences in nutrient intake between normal and parasitized larvae were established using t-tests (assuming unequal variances), comparing the distances along the individual nutrient rails. Normal larvae consumed significantly more of the nutrients on the 0.25C:1.75S diet than did parasitized larvae (t=7.86, P<0.0001), while parasitized larvae consumed more nutrients on the 1.0C:1.0S diet (t=-3.97, P=0.0019; Fig. 1).
Dietary nutrient ratio affected development time (Fig. 1). Normal larvae displayed longer development times at the extreme dietary nutrient ratios, especially on diets having lesser amounts of protein. A statistical analysis was not possible because feeding of all larvae on the individual diets terminated at the same time. Exempting the 0.125C:1.875S diet, increased development time was associated with increased nutrient consumption by normal larvae over the last stadia. Parasitized larvae showed a similar but less severe trend for development time. Nutrient consumption for parasitized larvae on the higher protein diets increased sharply when compared with normal larvae. In contrast to the results with normal larvae, however, higher nutrient consumption by parasitized larvae was not apparent with increased development time on lower protein diets.
Effects of dietary nutrient ratio and parasitism on host growth and utilization efficiency
Larval mass gain, with or without parasite biomass included, was affected
by dietary nutrient ratio and parasitism
(Table 1). Mass gain was
greatest on the 0.5C:1.5S, 1.0C:1.0S and 1.5C:0.5S diets, and was less on the
other diets having more extreme nutrient ratios
(Fig. 2). There were
significant interactions between dietary nutrient ratio and parasitism,
demonstrating that dietary nutrient ratio affects mass gain differently in
normal and parasitized larvae (Table
1). When parasite biomass was included as mass gain, there was no
difference in mass gain between normal and parasitized larvae on the 1.0C:1.0S
and the 1.5C:0.5S diets, but on the other diets, mass gain was significantly
less for parasitized insects (Fig.
2). Excluding the parasite contribution, the mass gain by
parasitized insects was less than that of normal larvae on all the diets
(Fig. 2); an effect most
pronounced on the most suitable diets above. Further, parasitism had a much
greater impact on host mass gain than did dietary nutrient ratio when parasite
biomass was excluded (as estimated by mean square values;
Table 1).
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To establish whether dietary nutrient ratio and parasitism affected mass gain independent of nutrient consumption, we conducted an analysis of mass gain by two-way ANCOVA using protein and carbohydrate consumption as joint covariates. The analysis confirmed the importance of protein and carbohydrate for mass gain, as there was a significant effect of each nutrient covariate (Table 2). Protein consumption had the greatest influence on total mass gain that included parasite biomass. Without parasite biomass considered, parasitism had the largest effect on mass gain, but of the two nutrient covariates, protein had a greater effect than carbohydrate. There were no interactions involving either nutrient covariate with dietary nutrient ratio or parasitism. Having accounted for differences in nutrient consumption, the effects of dietary nutrient ratio and parasitism on host mass gain are due to altered utilization efficiency, the efficiency of conversion of food consumed to body mass. There was also an interaction between dietary nutrient ratio and parasitism on mass gain, demonstrating that the effect of dietary nutrient ratio on mass gain differs between normal and parasitized larvae.
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Parasitized larvae displayed lesser total mass gain, and thus lower utilization efficiency, than normal larvae on all diets except the 0.125C:1.875S and 0.25C:1.75S diets where the mass gain of normal and parasitized larvae was similar (Fig. 3). When parasite biomass was excluded, parasitized larvae grew less than normal larvae on all diets except the 0.125C:1.875S diet. With or without parasite biomass considered, the analysis accounted for approximately 97% of the variation within the data.
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Multi-dimensional profiles showing the relationship between mass gain and nutrient intake indicate how nutrient consumption affects growth of normal and parasitized M. sexta larvae (Fig. 4). Three-dimensional plots of mass gain with protein and carbohydrate consumption were modeled using SAS (PROC GRID followed by PROC RSREG and PROC G3D. SAS version 8.02, 2001, SAS Institute Inc. Cary, NC, USA). This methodology uses a least-squares approach to fit a quadratic response surface regression model utilizing protein and carbohydrate consumption as independent variables and larval mass gain as a dependent variable (mass gain = protein consumption2 + carbohydrate consumption2 + [protein consumption xcarbohydrate consumption] + protein consumption + carbohydrate consumption). Additionally, contoured surface maps were generated (PROC GCONTOUR) from a 4000 point matrix created by interpolating a simple linear function for the relationship between dietary nutrient intake and estimated mass gain by normal and parasitized larvae (PROC G3GRID). For the analysis of parasitized insects on the 1.5C:0.5S diet, we deleted the result for one larva that had a biomass twice that of the others in this group. The contribution of this larva resulted in a standard error for mass gain of twice that of any other diet group (Fig. 2).
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Three-dimensional models show the nutrient space supporting growth of normal and parasitized insects (Fig. 4AC). The model for normal unparasitized larvae accounted for approximately 96%, and for parasitized larvae 95%, of the variation within the data. Growth of normal larvae is greater over a broader range of nutrient intake. The effects of nutrient consumption are generally consistent with mass gain (Fig. 2) and nutrient consumption (Fig. 1) by larvae on the various diets. Although growth of both normal and parasitized larvae is very low at low levels of protein consumption (Figs 1, 2), the model incorrectly suggests growth of parasitized larvae in the absence of protein consumption. The models, however, confirm that maximal growth of normal and parasitized larvae occur when protein and carbohydrate are consumed in equal amounts, approximately 2 g of each nutrient. The three-dimensional model for parasitized larvae was similar whether parasite biomass was included or excluded (not shown) from the analysis, except mass gain was lower without parasite contribution. The models are only predictive within the range of actual nutrient consumption approximately 0.1-2.0 g casein and 0-4.6 g sucrose for normal larvae and 0.1-1.5 g casein and 0-2.9 g sucrose for parasitized larvae. The contour maps demonstrate more precisely the growth expected for nutrient intake by larvae on specific diets. It is clear that the maximal growth of normal larvae occurs on the 0.25C:1.75S, 0.5C:1.5S, 1.0:1.5S and 1.5C:0.5S diets, while growth of parasitized larvae is maximal on the 1.0C:1.5S diet and to a lesser degree the 1.5C:0.5S and 0.5C:0.15S diets.
Effects of dietary nutrient ratio and parasitism on blood metabolite concentrations
Dietary nutrient ratio and parasitism each affected the concentrations of
blood protein, free amino acids and trehalose
(Table 3). In the cases of free
amino acids and trehalose there were significant interactions between dietary
nutrient ratio and parasitism, indicating that dietary nutrient ratio affects
the concentration of these metabolites differently in normal and parasitized
larvae. Trehalose concentration decreased as dietary nutrient ratio shifted
from the high carbohydrate to low carbohydrate diets
(Fig.
5). Only on the
1.0C:1.0S diet was there a significant difference between the normal and
parasitized larvae, with parasitized insects having a higher concentration.
Over all diets, parasitized larvae had a significantly higher trehalose
concentration than normal larvae, 25.21±0.94 and 20.38±0.97 mg
ml-1, respectively (P=0.0009). In contrast, free amino
acids and protein concentration generally increased as dietary nutrient ratio
shifted from low protein to high protein diets. Concentration of free amino
acids was higher in normal larvae than in parasitized larvae on the
0.25C:1.75S diet and protein concentration was higher in normal larvae than in
parasitized larvae on the 2.0C:0S diet
(Fig. 5). However, protein
concentration, over all diets, was higher in normal than in parasitized
larvae, 20.49±0.79 and 14.30±0.79 mg ml-1,
respectively (P<0.0001). Free amino acid concentration was also
higher overall in normal larvae, 14.48±0.49 and 12.27±0.50 mg
ml-1, respectively (P=0.0028).
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Contour maps illustrated the effects of protein and carbohydrate consumption on blood concentrations of protein, free amino acids and trehalose (Fig. 6). Profiles for nitrogen metabolites in parasitized and normal larvae were generally similar, but parasitized insects had lower concentrations at equivalent levels of nutrient consumption (Fig. 6A,D and B,E). This was most notable at high levels of protein consumption where parasitized larvae exhibited dramatically lower blood levels of both protein and free amino acids. Profiles for trehalose concentration were also similar in normal and parasitized larvae, but in this case, parasitized larvae generally had higher trehalose concentrations at equivalent levels of nutrient intake (Fig. 6C,F). The profiles for trehalose in both normal and parasitized larvae were not as uniform as were those of protein and free amino acid concentrations. Trehalose maps displayed small concentration foci. These reflect the large variation in concentration among larvae. Consequently, plots for trehalose are probably less reliable for predicting trehalose concentrations at those levels of nutrient consumption.
|
ANCOVA analyses of metabolite concentrations using protein and carbohydrate consumption as joint covariates, as described for growth above, demonstrated significant effects of dietary nutrient ratio and parasitism in each case (Table 4). There were interactions between dietary nutrient ratio and parasitism, for trehalose and free amino acids concentration but not for protein concentration. There was a covariate effect for protein consumption in the cases of free amino acids and protein concentrations, and protein consumption had a greater effect on protein and amino acid concentration than did either dietary nutrient ratio or parasitism. No covariate effect of carbohydrate consumption was evident for any of the metabolites. Because of the absence of significant nutrient covariate effects for trehalose concentration, dietary nutrient ratio and parasitism, rather than differences in nutrient consumption, largely explain the effects of protein and carbohydrate consumption on trehalose. There were no interactions involving the nutrient covariates with parasitism and dietary nutrient ratio.
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Discussion |
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Normal unparasitized larvae grow well on diets having a wide range of
nutrient ratios where some diets have protein and others carbohydrate as the
predominate nutrient. Of the diets tested, normal larvae had the greatest mass
gains and shortest development times on the 1.0C:1.0S and 1.5C:0.5S diets,
suggesting that the intake target for normal larvae lies on a nutrient rail
between these two. This conclusion is consistent with studies of
Spodoptera littoralis demonstrating that last instar larvae defend an
intake target having a nutrient ratio of approximately 1.3 parts protein to 1
part carbohydrate (Lee et al.,
2002). Larvae on diets having lower or higher protein increase
their development time and adjust diet consumption in order to complete larval
development at an appropriate size and with sufficient reserves to pupate.
Parasitism of M. sexta by C. congregata reduced host
growth on most diets, and lower consumption of less suitable diets was a major
contributor to this effect. The nutrient intake array of parasitized larvae
was sharply arced and did not approach linearity at the end of larval
development as typically occurs with increased development times and nutrient
consumption on unbalanced diets
(Raubenheimer and Simpson,
1999). Insects were unable to adjust diet consumption as the
dietary nutrient ratio deviated from 1.0C:1.0S where nutrient consumption was
maximal. Thus, insects suffered a large shortfall of a deficient nutrient to
avoid consuming a small surplus of an excessive nutrient. Parasitized larvae
on unbalanced diets apparently decrease their rate of feeding over time.
Dietary nutrient ratio and protein consumption are the dominant factors
explaining differences in host mass gain between normal and parasitized
larvae. Utilization efficiency, however, also affects mass gain among diets,
and after differences in nutrient consumption are taken into account,
parasitized larvae generally display lower mass gain and, therefore, lower
utilization efficiency, than normal larvae on the same diet. For both normal
and parasitized larvae, utilization efficiency was greatest on the 0.5C:1.5S
diet. Based on Waldbauer's
(1968) ECI (efficiency of
conversion of ingested food to body substance) nutrient ratio, the results of
studies by others imply increased utilization efficiency (ECI) in lepidopteran
larvae parasitized by Cotesia spp.
(Slansky, 1978
;
Benz and Barbosa, 1990
;
Alleyne, 1995
). None of those
studies, however, considered the influence of differences in diet or nutrient
consumption between normal and parasitized insects. We did not calculate the
above ratio, or other commonly employed nutritional ratios because of
difficulties inherent with their application and interpretation, first
outlined by Schmidt and Reese
(1986
) and elaborated by
Packard and Boardman (1988
).
ANCOVA analyses offer distinct experimental as well statistical advantages
based on less restrictive assumptions that apply to ratio based parameters
(Raubenheimer and Simpson,
1992
).
Numerous investigators have reported suppression of host growth and food
consumption by parasitized lepidopteran larvae, including M. sexta
(Vinson and Iwantsch, 1980;
Alleyne and Beckage, 1997
;
Benz and Barbosa, 1990
;
Quickie, 1997
;
Nakamatsu et al., 2001
). The
present study, however, is the first to demonstrate that host nutrition
mediates parasitism effects on host growth and consumption; whether parasitism
reduces growth depends on diet. Parasitized larvae on diets having nutrient
ratios displaced from the 1.0C:1.0S nutrient ratio generally display reduced
growth and nutrient consumption. To achieve growth equivalent to normal
larvae, parasitized larvae must consume a diet having a nutrient ratio close
to that of the intake target, but also consume more nutrients than normal
larvae.
There have been a few reports of hosts with large numbers or burdens of
Cotesia spp. attaining greater total final mass than normal
unparasitized larvae at the same developmental stage
(Slansky, 1978;
Beckage and Riddiford, 1983
;
Tanaka et al., 1992
). We
failed to observe larger parasitized larvae under any of the nutritional
conditions investigated during this study. Parasite biomass contributes
significantly to total host gain but the relative contribution varies with
dietary nutrient ratio. Clearly, parasitism compromises host gain, but
parasite biomass alone does not account for the difference in response of
normal and parasitized insects to dietary nutrient ratio. Increased metabolic
expenditure required to support and sustain parasite development may play a
role. Final host mass excluding the parasite contribution is perhaps the more
relevant measure when considering potential effects of host size on the
parasite, as this is the biomass providing nourishment for parasite growth and
development. This issue, however, is the subject of a separate
investigation.
Analysis of the host blood demonstrated the effects of dietary nutrient
ratio and parasitism on some blood metabolites. Generally, the blood
concentrations of trehalose, free amino acids and proteins conformed to
dietary nutrient ratio. As the dietary nutrient ratio shifted toward higher
protein and lower carbohydrate, the blood protein and amino acid
concentrations increased while trehalose concentration decreased. The effect
of dietary nutrient ratio, however, differed between normal and parasitized
insects. Two explanations are generally offered for the effects of parasitism
on host tissue metabolite levels. These are that parasitism alters the
metabolic capacity of host larvae and changes in metabolite levels reflect
this alteration or that parasite absorption and utilization of host
metabolites brings about these changes
(Vinson and Iwantsch, 1980;
Thompson, 1993
;
Nakamatsu and Tanaka, 2004
).
Clearly, the explanations are not mutually exclusive. The finding that host
fat body, the tissue regulating much of the insect's intermediary metabolism,
is often diminished in parasitized larvae
(Dahlman and Green, 1981
;
Nakamatsu and Tanaka, 2002
),
including those of M. sexta, strongly suggests that the metabolic
capacity of the host is compromised. Protein synthesis, for example, occurs
principally in the fat body and the overall reduction of blood protein levels
observed here in parasitized M. sexta is consistent with decreased
fat body content. However, trehalose synthesis also occurs in the fat body but
overall trehalose concentrations were increased in parasitized M.
sexta larvae. Despite any decrease in fat body, this results from
induction of gluconeogenesis and occurs in parasitized larvae even under
nutritional conditions that fail to induce trehalose formation in normal
larvae (Thompson, 2001
;
Thompson et al., 2002
). It may
be, therefore, that redirection of host metabolic capacity in addition to, or
rather than, reduction of capacity plays an important role in supporting
parasite growth and development. In any case, the concentrations of blood
metabolites in parasitized larvae when related to host size reflect the
nutrients available to the developing parasite. How these factors, host blood
metabolite levels and host size, influence parasite growth and development is
the subject of another study.
The results on blood metabolite concentrations are equivocal regarding
their potential to influence short-term feeding preferences. In the case of
trehalose, only on the carbohydrate-free diet, 2.0C:0.5S, were blood sugar
levels low enough to suggest a feeding preference for carbohydrate if these
larvae were offered a dietary choice of a high carbohydrate and a high protein
diet. Trehalose concentration, however, was not different between normal and
parasitized larvae on that diet. Effects of variations in blood free amino
acids and protein concentrations on feeding behavior are not known for M.
sexta. It is not possible at this time to predict how the variations of
these metabolites may affect short-term feeding preferences. Investigations
with other insects, however, demonstrate a relationship between diet, blood
amino acid level and food choice (Abisgold
and Simpson, 1988; Simpson and
Raubenheimer, 1993b
; Zanotto
et al., 1996
).
The influence of diet and nutrient consumption on growth of M. sexta larvae determined here suggests that normal and parasitized insects exhibit differences in dietary breadth that may be reflected in different long-term feeding patterns. The intake arrays indicate that overall, parasitized larvae exhibit less nutritional flexibility, and to attain maximal host size require greater nutrient levels and a more specific nutrient balance than normal larvae. How this might influence the plant feeding habits of parasitized larvae is unknown, but parasitized larvae may prefer to feed at sites within plants that differ in nutrient content from those preferred by normal larvae. Alternately, the feeding by parasitized larvae may be more restrictive than normal larvae. The results with parasitized larvae clearly suggest narrower nutritional preferences than normal larvae, both, however, within the specialist category.
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References |
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