PHYSIOLOGICAL ECOLOGY
Does Dietary P Affect Feeding and Performance in the Mixed-Feeding
Grasshopper (Acrididae) Melanoplus bivitattus?
VIVIANA LOAIZA, JAYNE L. JONAS,
AND
ANTHONY JOERN1
Division of Biology, Kansas State University, Manhattan, KS 66506
Environ. Entomol. 37(2): 333Ð339 (2008)
ABSTRACT Although consequences of limited dietary protein and carbohydrate to performance are
well studied for terrestrial insect herbivores, the importance of phosphorus (P) remains poorly
understood. We examined the signiÞcance of dietary P to performance in Þfth-instar nymphs of the
grasshopper Melanoplus bivittatus fed artiÞcial diets. Consumption, digestion, developmental rate, and
growth in response to different levels of P nested within standard-Protein and carbohydrate diets were
determined. Developmental rate was slowest on high-P diets; protein:carbohydrate concentration and
P in diets affected frass production and consumption. Approximate digestibility and conversion of
digested food were primarily inßuenced by the protein:carbohydrate quality of the diet but not P. Mass
gain was marginally lower in the low-Protein:high carbohydrate diet used in this study. At the
individual level, other than small effects to developmental rate at high concentrations for M. bivittatus,
dietary P otherwise seems to have little effect on nymphal performance. To the degree that it is
important, effects of dietary P depend on the concentrations of protein and carbohydrate in the diet.
KEY WORDS dietary phosphorus limitation to grasshoppers, ecological stoichiometry, nutritional
ecology, multiple nutrients, nutritional indices
A large mismatch exists between the elemental stoichiometry of herbivore tissues and that of their host
plants (McNeil and Southwood 1978, Mattson 1980,
Bernays 1982, Denno and McClure 1983, Strong et al.
1984, Elser et al. 2000a,b). For example, concentrations of nitrogen (N) and phosphorus (P) in plants
may be as much as 10 Ð20 times lower in plant tissues
than that observed in herbivore tissues. Despite the
imbalance between nutritional and physiological
needs, homeostatic mechanisms maintain concentrations of herbivore tissues within narrow limits (Slansky and Feeny 1977, Slansky and Scriber 1985, Abisgold and Simpson 1987, Yang and Joern 1994),
indicating the need to understand the regulation of
limiting nutrients (Sterner and Elser 2003). For insect
herbivores, protein and carbohydrate have been
shown repeatedly to limit through performance penalties if a key nutrient is in short supply (McNeil and
Southwood 1978, Mattson 1980, White 1993, Joern and
Behmer 1998) or if diets are not balanced (Simpson
and Raubenheimer 1993, Raubenheimer and Simpson
1997). A small number of recent studies now document that dietary P may also play a greater limiting
role in terrestrial herbivores than was previously expected (Elser et al. 2000a,b; Schade et al. 2003; Perkins
et al. 2004), a conclusion recognized for some time for
aquatic herbivores (Elser et al. 1988, Elser and Urabe
1999, Sterner and Elser 2003).
1
Corresponding author, e-mail: [email protected].
The imbalance in stoichiometric relationships of
carbon (C), N, and P in herbivore tissues compared
with their food potentially inßuences individual performance (Sterner and Elser 2003), population and
community dynamics (Fagan et al. 2002, Andersen et
al. 2004, Fagan and Denno 2004, Hall 2004, Raubenheimer and Simpson 2004), and nutrient cycling
(Elser and Urabe 1999, Sterner and Elser 2003, Elser
2006). Intriguing phylogenetic, trophic, and size relationships between the elemental composition of consumers and food have also been identiÞed (Elser et al.
2000a, Fagan et al. 2002, Woods et al. 2004, Kerkhoff
et al. 2006). Although many signiÞcant physiological,
ecological, and evolutionary relationships have been
described for aquatic taxa in response to dietary P
(Elser et al. 2000a, Sterner and Elser 2003), terrestrial
species are not well studied, including the issue of
whether P is limiting to terrestrial insect herbivores
(Schade et al. 2003, Perkins et al. 2004, Woods et al.
2004).
Substantial syntheses based on Þrst principles predict that dietary P limits consumers (Elser et al. 1996).
As one example, the growth rate hypothesis (Elser et
al. 1996) predicts that rapid growth rate requires high
investment in ribosomes needed for protein synthesis.
Ribosomes contain a large fraction of cellular RNA
and are particularly rich in P (⬇10% of organismal
mass and most organismal P is found in ribosomes)
(Elser et al. 2000b, Sterner and Elser 2003, Woods et
al. 2003). Dietary P could easily become limiting in
0046-225X/08/0333Ð0339$04.00/0 䉷 2008 Entomological Society of America
334
ENVIRONMENTAL ENTOMOLOGY
Table 1.
Vol. 37, no. 2
Elemental composition of artificial diets used in experiments to cross P concentration with that of protein and carbohydrate
Percent
protein:percent
carbohydrate
Phosphorus
level
%C
%N
%P
C:N
C:P
N:P
14:28
14:28
14:28
21:21
21:21
21:21
28:14
28:14
28:14
Low
Standard
High
Low
Standard
High
Low
Standard
High
41.93
41.93
39.70
42.96
42.96
40.73
43.99
43.99
41.77
1.91
1.91
1.91
2.86
2.86
2.86
3.81
3.81
3.81
0.34
0.58
1.74
0.50
0.75
1.90
0.67
0.91
2.07
21.95
21.95
20.8
15.0
15.0
14.2
11.5
11.5
10.96
123.3
72.3
22.8
85.9
85.9
21.4
65.7
48.3
20.2
5.62
3.3
1.1
5.72
3.8
1.50
5.67
4.2
1.84
Protein:carbohydrate diets are in standard use (Simpson and Raubenheimer 2001, Behmer et al. 2002), and P was modiÞed according to
Perkins et al. (2004). Percentages of elements in diets were determined from stoichiometric calculations of ingredients in diet recipes.
Phosphorus concentrations diets averaged 0.5% for low, 0.75% for standard, and 1.9% for high levels.
taxa with rapid growth rates. Moreover, compared
with N (Fagan et al. 2002), which varies considerably
among insect taxa, the potential for P limitation in
insects seems to be universal. P levels do not differ
systematically between herbivores and predators, and
only a weak phylogenetic signal at the ordinal level is
observed (Woods et al. 2004). A general inverse relationship between P content and body size exists in
many insect groups, but it is only marginally signiÞcant
for Orthoptera (Woods et al. 2004).
In this study, we examined possible P limitation to
nymphs of the grasshopper Melanoplus bivittatus
(Say) during the Þfth instar. Mineral limitation from
N in grasshoppers is well known (Hinks et al. 1993,
White 1993, Joern and Behmer 1997, 1998), and
extensive state-space analyses of the combined effects of protein and carbohydrate on grasshopper
performance indicate a dynamic interaction among
primary nutrients (Raubenheimer 1992, Simpson
and Raubenheimer 1993, 2000, Behmer et al. 2001,
2002). Knowledge of such relationships provides an
ideal background against which to assess the impact
of P because it allows us to assess interactions among
nutrients.
Rapidly growing individuals such as prereproductive grasshopper nymphs must accumulate a signiÞcant amount of new tissues over a short period of time.
Such growth requires a large investment in N-rich
proteins and P-rich rRNA, which constitute large portions of individual N and P budgets, respectively
(Elser et al. 1996, 2000a). Using artiÞcial standard diets
for grasshoppers (Abisgold and Simpson 1987, Simpson and Raubenheimer 1993) modiÞed according to
Perkins et al. (2004), we test two hypotheses: (1) that
dietary P limits nymphal grasshopper performance in
the form of growth and development and (2) that
consumption and postingestive regulation of food is
inßuenced by dietary P levels. Feeding and performance by the mixed-feeding grasshopper M. bivittatus
is assessed on different diets in which dietary P concentrations are varied against a background of diets
with protein:carbohydrate (p:c) ratios known to be
near optimal (21%p:21%c; Behmer and Joern 2008).
Materials and Methods
Study Organism. Melanoplus bivittatus (Orthoptera:
Acrididae) is a large polyphagous grasshopper species
common in grasslands throughout much of North
America, which feeds on both forbs and grasses
(Mulkern et al. 1969, Capinera et al. 2004). Nymphs
develop in midsummer during the period that foliar
nutritional quality is undergoing its greatest change,
and grasshoppers generally encounter highly variable
food in the Þeld (Joern and Mole 2005).
Feeding Trials. Diet consumption and performance
by Þfth-instar nymphs of M. bivittatus was evaluated
using controlled diets that varied in the percentage of
protein (p), carbohydrate (c), and phosphorus (P).
After completing egg diapause, nymphs hatched from
eggs of Þeld-caught adults were reared in an insectary
maintained at 30⬚C on a 14:10 light:dark photophase.
Incandescent lamps (40 W) placed near cages provided light and heat. Before use in experiments,
nymphs were fed unlimited seedling wheat, romaine
lettuce, and wheat bran mixed with brewerÕs yeast.
Within 12 h of molting to the Þfth instar, nymphs were
weighed, transferred into individual cages, and provided with a randomly assigned diet.
Feeding Experiments. Feeding trials were performed as a 3 ⫻ 3 factorial experiment (p:c diet type ⫻
dietary-P level) arranged in a completely randomized
design (Steel and Torrie 1980). Nine dry, granular,
chemically deÞned diets were prepared in a manner
similar to those described previously (Simpson and
Raubenheimer 1993, Behmer et al. 2001). We varied
protein and digestible carbohydrates to provide three
p:c combinations (all values are expressed on a percentage dry mass basis): (1) p14:c28, (2) p21:c21, and
(3) p28:c14. All foods had equal amounts of digestible
macronutrients (protein plus carbohydrate equaled
42% as a proportion of total dry mass, an amount
typical for plant material) and were identical in the
amounts of the other ingredients, including indigestible cellulose powder. The P level of artiÞcial diets was
modiÞed from this standard grasshopper diet (Simpson and Raubenheimer 1993) following Perkins et al.
(2004) and nested as three levels within each p:c diet
(Table 1). Low-P diets were prepared by withholding
April 2008
LOAIZA ET AL.: PHOSPHORUS LIMITATION
the major sources of P (phosphoric acid, iron phosphate, and calcium phosphate) contained in the WessonÕs salts mixtures and raised in the high-P diets by
adding dibasic sodium phosphate, following the approach of Perkins et al. (2004). The standard-P diet is
the P concentration in the standard grasshopper diet.
P levels in the diets averaged 0.5, 0.75, and 1.9% percent for low, standard, and high levels, respectively.
Other major sources of P in the diet were casein and
albumin and were not varied.
A standard feeding protocol was followed (Behmer
et al. 2001). Diets were assigned to arenas in a completely randomized design with 10 replicates of each
diet. Food was placed into food dishes and stored in a
drying cabinet at 32⬚C for 24 h before weighing. After
weighing to the nearest 0.1 mg, foods were placed
inside the arenas, and grasshoppers were allowed to
feed for 72 h, after which the food dish was removed
and replaced with a fresh, preweighed dish of food.
The food dish that was removed was placed in the
drying cabinet for 24 h before being reweighed. This
procedure was repeated until the grasshopper either
molted or died.
Data Analyses. The initial and Þnal insect wet
weights, Þnal insect dry weight, days until Þnal molt,
frass dry weight, and amount of diet consumed were
measured for each grasshopper. Initial dry weight was
estimated by regressing dry and fresh weights of
nymphs at the beginning of the study (n ⫽ 14, dry
mass ⫽ 0.033 ⫹ 0.125 ⫻ wet mass, R2 ⫽ 0.474). Nymphs
that died before the Þnal molt were not included in
subsequent analyses.
We examined the effect of diet over the entire Þfth
instar for the following dependent variables: consumption, frass production, mass gained, developmental rate (time to complete Þfth instar), relative growth
rate, and estimates of food use efÞciency (commonly
referred to as approximate digestibility, efÞciency of
conversion of ingested food, and efÞciency of conversion of digested food). Relative growth rate (RGR)
was calculated as [(ln Þnal weight ⫺ ln initial
weight)/days] (Hunt 1978).
Two-way analysis of variance (ANOVA) tests and
Fisher least signiÞcant difference (lsmeans) were
used for post hoc analysis of pairwise comparisons for
signiÞcant effects in SAS (v.9.1; SAS Institute, Cary,
NC). Only statistically signiÞcant relationships are
presented in the results section except that we indicate when the effect of P is marginally signiÞcant to be
conservative about evaluating its importance. Variables were transformed to satisfy normality assumptions, and the variances of all transformed variables
were homogenous. Time elapsed (days) to complete
the Þfth instar was transformed into developmental
rate using the inverse of the square root. Approximate
digestibility (AD), efÞciency of conversion of digested food (ECD), and efÞciency of conversion of
ingested food (ECI) were calculated according to
Waldbauer (1968). To account for ratios, analysis of
covariance (ANCOVA) was used (Raubenheimer and
Simpson 1992, Horton and Redak 1993) to analyze
consumption and frass production (each with initial
335
dry body mass as the covariate), AD (amount of food
assimilated with food consumed as the covariate), ECI
(mass gained with consumption as the covariate), and
ECD (mass gained with food digested as the covariate).
Results
Feeding Trials. Over the Þfth-instar stadium in M.
bivittatus, consumption and conversion of digested
food (ECD) were primarily inßuenced by the p:c ratio
of the diet. Dietary P inßuenced developmental rates,
whereas weight gain and RGR were not affected by
the diets used in this study. An interaction between p:c
ratio and P was observed for frass production and AD.
Survival. Average survival of Þfth-instar nymphs was
83% and was not inßuenced by either p:c ratio (␹2 ⫽ 0.00,
P ⫽ 1.00) or P concentration (␹2 ⫽ 0.8, P ⫽ 0.96) of the
diets.
Consumption. The p:c ratio in the diet inßuenced
the amount consumed over the Þfth instar (Fig. 1a;
Table 2). Initial dry weight was a signiÞcant covariate.
SigniÞcantly more of the p21:c21 diet was consumed
than either the p14:c28 or p28:c14 diets. P level had a
marginally signiÞcant negative effect on consumption,
but there was no interaction between the p:c diet
and P.
Food Processing and Digestibility. Both dietary p:c
ratio and P affected frass production over the Þfth
instar (Fig. 1b; Table 2). Initial mass was a signiÞcant
covariate. Individuals fed p14:c28 diets produced signiÞcantly less frass than those fed the p21:c21 or p28:
c14 diets (Fig. 1b; t ⫽ 2.62, P ⬍ 0.01 and t ⫽ 2.04, P ⬍
0.05, respectively). The high dietary P diet produced
less frass than either the low and standard levels (t ⫽
2.12, P ⬍ 0.04 and t ⫽ 2.12, P ⬍ 0.04, respectively),
which were not different from one another.
AD was signiÞcantly affected by p:c, whereas there
was a marginally signiÞcant interaction between p:c
and P (Fig. 1c; Table 2). The amount consumed was
highly signiÞcant as a covariate. As a pattern, AD
generally decreases as protein levels in the diet increase. The p28:c14 diet is signiÞcantly different than
the p21:c21 diet and the p14:c28 diet.
ECD varied signiÞcantly with dietary p:c ratio
(Fig. 1d; Table 2). The covariate was highly significant. ECD generally increased for diets with higher
protein. Individuals were more efÞcient at digesting
p28:c14 than p14:c28 diets and marginally more efÞcient than the p21:c21 ratio. No signiÞcant effect
was observed in ECI.
Developmental Rate. Dietary P signiÞcantly affected developmental rate (Fig. 2a; Table 2), measured as the number of days required to complete the
Þfth-instar stadium. More time was required to complete development on the high-P treatment compared
with the standard and low-P levels.
Weight Gain. No signiÞcant effects of diet on total
weight gain or RGR were observed (Table 2). If there
was any effect of diet, p:c ratio had perhaps the greatest inßuence on total weight gain (P ⫽ 0.088), with
336
ENVIRONMENTAL ENTOMOLOGY
Vol. 37, no. 2
Fig. 1. Consumption and whole individual digestion by Þfth-instar M. bivitattus in response to crossed combinations of
standard diets that vary in ratios of percent protein:percent carbohydrate as indicated with three levels of dietary P labeled
as L (low), S (standard), and H (high). Effect of dietary P is shown only if results are signiÞcant or near signiÞcant. The
following responses to diet combinations are presented: (a) dry mass of food consumed (mean, SE), (b) frass production
(mean, SE), (c) approximate digestibility (AD; mean, SE), and (d) efÞciency of conversion of digested food (mean, SE).
nymphs fed the p21:c21 diets gaining more weight
than those fed the p14:c28 diet (Fig. 2b).
Discussion
Determining which combinations of minerals or
nutrients limit insect herbivores is a major challenge
in nutritional ecology, especially given the large number of possible combinations (Clancy and King 1993)
and the structural barriers to extracting nutrients
(Clissold et al. 2006) that may be involved. Here we
studied the problem of whether dietary P limits performance in a developing insect herbivore when combined with diets that differ in combinations of protein
and carbohydrate. This scenario of variable diets is
representative of daily challenges faced by free-ranging individuals in natural environments. In the Þeld,
grasshoppers routinely encounter food that is highly
variable in nutrient content, making it unlikely that an
optimal diet is readily attained by eating a single host
species. Mixed-feeders such as M. bivittatus meet this
challenge in part by feeding on a variety of different
food plants (Bernays and Raubenheimer 1991), which
may permit them to balance nutrient intake. Because
the nutritional background of potential food under
natural conditions is so variable, it is appropriate to
assess the contribution of a single portion of the diet
against multiple diets. We do this by comparing performance by nymphs on a range of diets in which
concentrations and relative abundances of nutrients
are within the range of those observed in naturally
occurring plants (Agren 2004, Kerkhoff et al. 2006).
Moreover, these diets bracket levels of p:c ratios
known to best support performance of this species
(Behmer and Joern 2008).
Our results indicate that dietary P can be marginally
important to M. bivittatus, but our overall impression
of its signiÞcance suggests it is best understood with
respect to other components of the diet (p:c ratios in
this study) and not as the contribution of a single
mineral nutrient acting independently (Clissold et al.
2006). Also, p:c ratio treatments were generally more
signiÞcant than P. Because no-choice diets were used
in these experiments, grasshoppers were not able to
mix foods to balance nutrient demands, although other
studies indicate that M. bivittatus does balance the p:c
ratio in the diet when given a choice (Jonas 2007).
This study examines three components of the problem: consumption, digestion (processing and conversion of diet), and the eventual performance of the
individuals (growth, developmental rate, or survival).
A major conclusion from this study is that the grasshoppers regulated nutritional needs through combined consumption and digestion, with the result that
performance is not greatly impacted by variable diets.
However, we cannot determine directly whether elemental regulation occurred because N or P use efÞciencies were not measured in this study.
As described above, an ecological stoichiometric
framework predicts that both N and P can potentially
limit insect herbivores, but P is less well studied in
terrestrial systems. In rapidly growing individuals, Prich rRNA may be at a premium, leading to the Plimited growth rate hypothesis (Elser et al. 1996),
which predicts that increased levels of P in the diet (to
a point) should support increased growth rate. For
example, elevated dietary P levels increased growth
and shortened developmental time in larval Manduca
sexta (Perkins et al. 2004). For M. bivittatus in this
study, P levels did not signiÞcantly inßuence either
April 2008
LOAIZA ET AL.: PHOSPHORUS LIMITATION
337
Table 2. ANOVA and ANCOVA summary of significant responses in feeding, digestion, and performance by M. bivittatus to
protein:carbohydrate and P diet combinations
Dependent
variable
Consumption
Model
p:c
P
p:c ⫻ P
Initial dry
weight (g)
Error
Frass production p:c
P
p:c ⫻ P
Initial dry
weight (g)
Error
AD
p:c
P
p:c ⫻ P
Amount
consumed (g)
Error
ECI
p:c
P
p:c ⫻ P
Amount
consumed (g)
Error
ECD
p:c
P
p:c ⫻ P
Food assimilated
(g)
Error
Developmental
p:c
rate
P
p:c ⫻ P
Error
Total weight
p:c
gain (g)
P
p:c ⫻ P
Error
RGR
p:c
P
p:c ⫻ P
Error
df
Mean
square
F
2
2
4
1
0.11878
0.08582
0.03223
0.29972
3.32
2.40
0.90
8.38
62
2
2
4
1
0.03578
0.03803
0.03008
0.01404
0.18164
65
2
2
4
1
0.01012
0.03699 12.86 ⬍0.0001
0.00320
1.11
0.335
0.00671
2.33
0.065
0.61906 215.27 ⬍0.0001
62
2
2
4
1
0.00288
0.00088
0.00078
0.00166
0.02717
1.01
0.370
0.89
0.414
1.90
0.122
31.18 ⬍0.0001
62
2
2
4
1
0.00087
0.00331
0.00145
0.00193
0.01560
3.12
1.37
1.83
14.74
0.051
0.262
0.135
0.0003
62
2
2
4
66
2
2
4
66
2
2
4
65
0.00106
0.00043
0.00607
0.00171
0.00088
0.00630
0.00325
0.00513
0.08239
0.00345
0.00197
0.00113
0.00142
0.49
6.88
0.11
0.617
0.002
0.114
2.52
1.30
1.03
0.088
0.279
0.400
2.43
1.39
0.80
0.096
0.258
0.533
P
0.043
0.099
0.469
0.005
3.76
0.029
2.97
0.058
1.39
0.248
17.94 ⬍0.0001
Only responses that are signiÞcant (P ⬍ 0.05) or nearly signiÞcant
(0.05 ⱕ P ⬍ 0.1) are shown. Covariates are omitted if they are not
signiÞcant.
total weight gain or relative growth rate. In the p14:c28
and p21:c21 diets, however, there was a tendency for
the individuals fed the standard-P diet (1% P) to gain
more than those fed the low-P or high-P diets (Fig.
2b). Counter to the Þndings of Perkins et al. (2004),
we found that individuals fed the low-P and standard-P diets developed signiÞcantly faster than those
fed the high-P diets.
In our experiments, P levels in diets ranged from
0.34% at the lowest level to 1.90% at the highest. The
P concentration in the high-P diets was greater than
that observed in most plant tissues (Kerkhoff et al.
2006). If herbivores are homeostatic for speciÞc nutrient levels or ratios, it may be costly if one mineral
nutrient is considerably out of balance from others
because consumers incur a “metabolic handling cost”
resulting in lowered performance; this response was
also predicted for p:c comparisons in the geometric
Fig. 2. Performance by Þfth-instar M. bivittatus in response to diet combinations. Responses include (a) developmental rate measured as the number of days until the
Þfth-instar individual molt to adults (mean, SE) and (b)
growth expressed as dry mass gained (mean, SE).
model of Simpson and Raubenheimer (2000). For example, such a cost is implicated for a variety of consumers where growth rates peaked at P- levels of ⬇1%
of dry mass and decreased at higher P concentrations
in diet (Boersma and Elser 2006). Ultimately, a multivariate framework for understanding nutritional
needs of herbivores is needed where performance
reßects ratios of nutrients, not just the action of individual elements (Simpson and Raubenheimer 2000,
2001). Evidence for the importance of this approach
was shown in our study, but a more reÞned analysis of
digestion is needed to quantify such costs to an unbalanced diet.
Discussion continues about how to best understand
nutrient limitation, either as complex macronutrients
and dietary ratios, individual elements or their ratios,
or some combination of the two. Our results add to the
small but increasing number of examples indicating
the potential importance of P within the context of p:c
ratios for performance in immature terrestrial insect
herbivores. We show that the mineral nutrient P affects developmental rate in growing nymphs of a common grasshopper, M. bivittatus but that nutrient acquisition is regulated to minimize affects of dietary P
on performance on diets with near optimal protein:
carbohydrate ratios. Ecological stoichiometry has focused primarily on elemental responses, in part
because of research interest in nutrient cycling motivating the development of the Þeld (Elser and Urabe
1999, Sterner and Elser 2003), capability of speciÞc
elements to act as a useful proxy for complex nutritional limitation, and the development of compelling
explanations across organizational scales based on elements (Elser et al. 1996). The element-biased view
contrasts with equally compelling physiological argu-
338
ENVIRONMENTAL ENTOMOLOGY
ments that require an understanding of how elements
interact as nutrients (e.g., protein, carbohydrate, lipids) to understand the context in which elements
become limiting (Simpson and Raubenheimer 2001).
Our study with P indicates that, at a minimum, a
multiple resource-ratio approach is needed to understand individual elements (Clissold et al. 2006). Only
small effects of dietary P were detected compared
with other components of the diet, indicating that
more effort to understand the importance of P for
terrestrial insect herbivores is needed.
How do stoichiometric approaches to explain performance Þt with other ecological factors also affecting foraging and the resulting nutritional consequences by insect herbivores? Because inßuences of
feeding and nutrition in natural systems are multifactorial (Pitt 1999, Danner and Joern 2004), nutrient
limitation often emerges even when food is seemingly
abundant and of sufÞcient nutritional quality. Reduced time available for searching and consuming
optimal diets is common for many reasons. Abiotic
conditions are often unsuitable, nonoptimal spatial
arrangement of complementary nutritional resources
limit diet mixing within the time frame available for
foraging, or the effects of predators on feeding behavior may prevent insect herbivores from optimally
exploiting food resources. Each of these factors makes
Þnding quality, balanced diets important. It may be
difÞcult to balance a diet if less time is available for
Þnding it (e.g., predation) or abiotic conditions are
suboptimal for processing it (e.g., digestion).
Acknowledgments
Logistical support from Konza Prairie Biological Station
and the KSU Grassland Ecology REU program is gratefully
acknowledged. We thank J. Higgins (KSU Math and Statistics
Department) for advice on the experimental design and S.
Parsons for assistance with the laboratory feeding trials. Research was supported by NSF Grant DEB0456522 (to A.J.)
and the NSF Konza Prairie LTER and KSU Prairie REU
program (NSF Grant DBI0552930). J. Apple, S. Behmer, and
A. Laws provided helpful comments on the manuscript.
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Received 12 March 2007; accepted 20 December 2007.