Ecology, 88(4), 2007, pp. 1012–1020� 2007 by the Ecological Society of America

FITNESS RELATED DIET-MIXING BY INTRASPECIFICHOST-PLANT-SWITCHING OF SPECIALIST INSECT HERBIVORES

KARSTEN MODY,1 SYBILLE B. UNSICKER,2 AND K. EDUARD LINSENMAIR

Department of Animal Ecology and Tropical Biology, Biozentrum, Am Hubland, D-97074 Wurzburg, Germany

Abstract. Generalist insect herbivores may profit by feeding on a mixture of plant speciesthat differ in nutritional quality. Herbivore performance can also be affected by intraspecifichost plant variation. However, it is unknown whether conspecific plant individuals differsufficiently to promote diet-mixing behavior in specialist herbivores. We experimentally testedthis ‘‘specialist diet-mixing hypothesis’’ for specialist caterpillars (Chrysopsyche imparilis,Lasiocampidae) in a West African savanna. The caterpillars switched regularly between hosttree individuals (Combretum fragrans, Combretaceae). To examine whether switching benefitedcaterpillar performance via diet-mixing, the caterpillars were reared either on leaves fromseveral plant individuals (mixed diet) or on leaves from a single plant. The strongest effect ofdiet-mixing was found for fecundity, with females reared on a mixed diet laying significantlymore eggs than sisters receiving a single-plant diet. In addition, a mixed diet decreasedvariability in egg size and increased the growth of second-instar caterpillars. Supplementaryfood choice experiments were conducted to assess a potential influence of lowered host quality(induced by herbivory) on caterpillar behavior; no such effect was found. By linkingintraspecific host-switching behavior and herbivore performance, this study provides newinformation on the relevance of intraspecific plant variation for herbivorous insects.

Key words: Chrysopsyche imparilis; Combretum fragrans; Comoe National Park; egg size variation;enemy avoidance; foraging behavior; host quality; induced defense; insect movement; intraspecific variability;specialist and generalist herbivores; West African savanna.

INTRODUCTION

Organisms require resources for growth and repro-

duction. Besides the quantity of nutritional intake, the

quality of the food consumed may be crucial for

development time, fecundity, and fitness (Awmack and

Leather 2002). Herbivorous insects can obtain suitable

food by specialist and generalist feeding habits (Schoon-

hoven et al. 1998). Using a wide range of hosts increases

food availability and allows mixtures of different types

of food, which may improve nutrient balance (Pulliam

1975, Westoby 1977, Rapport 1980, Bernays et al. 1994,

Simpson and Raubenheimer 2001, Berner et al. 2005).

Dietary mixing may also help to dilute potentially

poisonous allelochemicals that are unevenly distributed

over different plants (Freeland and Janzen 1974,

Bernays and Minkenberg 1997), or are induced by the

feeding herbivore itself (Karban and Baldwin 1997, van

Dam et al. 2000). To achieve potential advantages via

food-mixing, host-switching is necessary, requiring

mobility and some orientation capabilities to secure

the timely finding of a new food source. Abandoning a

proven food plant and searching for a new one bears

potential risks such as starvation, desiccation, increased

exposure to enemies, and metabolic costs (Schultz 1983,

Bergelson and Lawton 1988, Dethier 1988). Whether

food-mixing is adaptive depends on the ratio of costs

and benefits involved.

A mixed diet incorporating different plant species has

been shown to improve the performance of some

generalist insect herbivores (Waldbauer and Friedman

1991, Hagele and Rowell-Rahier 1999, Miura and

Ohsaki 2004). The extent of beneficial effects, through

nutrient balancing and toxin dilution (Simpson and

Raubenheimer 2001, Behmer et al. 2002, Singer et al.

2002), depends on the range of variability of the relevant

food components. A wide range is clearly given for

heterospecific plants (Hegnauer 1962–1992). A marked

variation in plant chemistry, however, is not restricted to

different plant species, but also exists among conspecific

plants and even within plant populations (Zangerl and

Berenbaum 1993, Hemming and Lindroth 1995, Lawler

et al. 2000). Thus the benefits of dietary mixing are not

restricted to interspecific food plant use but can also

apply for mixing food from conspecific plant individu-

als. As the importance of intraspecific plant variation for

insect herbivores is increasingly acknowledged, the

question arises whether conspecific plant individuals

differ sufficiently to promote intraspecific food-mixing

and thereby to affect herbivore distribution patterns on

conspecific host plants in the field.

Manuscript received 7 August 2006; accepted 9 October2006. Corresponding Editor: S. J. Simpson.

1 Present address: ETH Zurich, Institute of Plant Science-s/Applied Entomology, Schmelzbergstr. 9/LFO, 8092 Zurich,Switzerland. E-mail: karsten.mody@ipw.agrl.ethz.ch

2 Present address: Institute of Ecology, University of Jena,Dornburger Str. 159, D-07743 Jena, Germany.

1012

The caterpillars of the lasiocampid moth Chrysop-

syche imparilis Aurivillius seem very suitable for an

investigation into these issues. They are specialized on

plants in the family Combretaceae and appeared in a

pilot study to switch regularly among Combretum

fragrans F. Hoffm. host plants (see Plate 1). C. fragrans

plants are intraspecifically variable in herbivore-relevant

characteristics, which was demonstrated by the distri-

bution and feeding preferences of herbivorous scara-

baeid and curculionid beetles (Mody 2003). The plants’

foliage was never considerably reduced by caterpillar

feeding. Thus, other reasons than resource depletion

must have set off the switching behavior, despite the risk

for such a specialist with restricted orientation capabil-

ities (Dethier 1993).

In this study, we tested whether (1) diet-mixing

achieved by intraspecific host-switching can improve

the performance of the specialist herbivore C. imparilis

(specialist diet-mixing hypothesis). This hypothesis was

tested in an experiment in which caterpillars were reared

either with a diet originating from a single host plant or

with a diet comprising several conspecific host plant

individuals. Host-switching behavior might also be

caused by (2) decreasing host quality through caterpillar

feeding (induced-defense hypothesis; e.g., Karban and

Baldwin 1997, van Dam et al. 2000) or (3) by

predator/parasitoid avoidance (enemy avoidance hy-

pothesis; e.g., Montllor and Bernays 1993, Karban and

English-Loeb 1997). To address the induced-defense

hypothesis, we performed feeding-choice tests, present-

ing caterpillars with leaves of conspecific plants differing

in certain leaf traits as well as differing in pre-

experimental damage levels caused by other herbivores.

Additionally, we determined the distribution of cater-

pillars and leaf damage in C. fragrans in the field. The

possible role of natural enemies was inferred from direct

observations of caterpillar behavior in the field and from

parasitoid infestation rates of caterpillars.

METHODS

Study area and species

The study was conducted in Comoe National Park,

Cote d’Ivoire, between 1997 and 2001 during 12 months

of field surveys. The vegetation at the study site

consisted of shrubs and trees belonging to more than

20 species, growing several meters apart from each other

(no crown overlap) in a matrix of grass. More

information on Comoe National Park is provided by

Mody (2003) and Hovestadt et al. (1999), and literature

therein. The studied host plant species Combretum

fragrans (Combretaceae) is a medium-sized (maximum

height 12 m), deciduous savanna tree. Caterpillars of the

studied moth species Chrysopsyche imparilis (Lasiocam-

pidae) could be found in several generations throughout

the whole rainy season. They are large (female larvae up

to 4 g) specialists restricted to food plants in the family

Combretaceae and to Monotes kerstingii Gilg, a tree

species not present at the study site (K. Mody and S.

Unsicker, personal observations).

Test of the specialist diet-mixing hypothesis

The larvae that we used for the feeding experiments

hatched from egg clutches that were laid by two females

of C. imparilis in June 2000. In the remaining

paragraphs, we will refer to clutch 1 as the progeny of

female 1 and clutch 2 as the offspring of female 2. The

mothers were reared from caterpillars collected on C.

fragrans trees three weeks before the experiment started.

They were mated with wild-flying males attracted by the

pheromones of the receptive females. The two clutches

were laid with a time lag of two days and the first-instar

caterpillars hatched about 12 days after the mothers’

oviposition. We picked 96 freshly hatched caterpillars

from each of the two clutches and assigned them

randomly to 12 groups with eight individuals per clutch

(resulting in 192 caterpillars altogether). We thus ran a

full sib-design with offspring of two different mothers in

all experimental treatments. Six individual C. fragrans

trees to be used as experimental host trees were

randomly chosen from the C. fragrans population in

the study area. Randomly selecting six trees from the

host population should provide sufficient variation in

food quality of individual plants, as was inferred from

previous studies on herbivore community composition

and on feeding preferences of herbivorous beetles (K.

Mody, unpublished data). The leaves of each individual

tree were then fed to the groups of caterpillars (a) as a

single-plant diet or (b) as a mixed-plant diet (for an

overview see Appendix A: Table A1). Thus six groups of

caterpillars (N ¼ 8 individuals per group) from each

clutch got a single-plant diet comprising leaves from one

of the six C. fragrans individuals (single-plant food), and

the other six groups got a mixture of leaves from these

six trees (mixed-plant food, imitating a regular host

switch) according to the following feeding regime: in the

first four days of the experiment, larvae were reared on

leaves of one of the six plant individuals and they were

additionally provided with leaves of one of the other

five. The additional host was changed daily. Subse-

quently, larvae were reared on leaves of one plant that

was changed at six-day intervals (resulting in 8–9 six-day

plant changes until pupation). The host plants were

changed in a fixed order, with each plant individual

being the first, second, and so on for one group of

caterpillars of each clutch, ensuring that all plants were

used evenly, and that caterpillars received new plant

individuals until all six plants were offered to the

caterpillars. After the sixth plant, the feeding scheme

started anew in the same order as before.

First- and second-instar larvae were kept in groups of

eight in closed Petri dishes (9 cm in diameter) together

with moistened filter paper. Thereafter, they were

transferred to plastic terraria (23 3 17 3 15 cm) with

ventilation screens in the lids and moistened filter paper,

still at a group size of eight; after the fourth instar their

April 2007 1013SPECIALIST HERBIVORE DIET-MIXING

number was reduced to four. Petri dishes and terraria

were held in open laboratory huts, protected from direct

sunlight and exposed to ambient air temperature and

humidity. Rearing continued until all larvae either died

or pupated. Pupae within their protecting silk/hair

cocoon were kept separately until eclosion.

To test if diet composition affected performance traits

of C. imparilis, larval mass at different stages, develop-

ment time from hatching of larvae to eclosion, survival

(number of successfully eclosing moths), sex ratio of

moths, and realized fecundity of female moths were

recorded. For later instars, developmental status (feed-

ing, premolt or postmolt stage) varied strongly among

individual caterpillars, preventing a comparable quanti-

fication of larval mass at a given time. Later-instar larval

mass was therefore not further considered. Eclosing

female moths did not keep their eggs longer than about

three days. Then they laid their eggs independently of

mating status and died a few days later. Realized

fecundity was measured as the number of eggs laid by

the unmated female. In addition, egg size was deter-

mined as egg dry mass for 16 females that were chosen

to represent eight individuals reared on mixed- and eight

individuals reared on single-plant food. Within these

groups, females were randomly chosen under the

specification that they represented all possible host plant

individuals.

Test of the induced defense and the enemy

avoidance hypotheses

Four leaf parameters with the potential to influence

feeding preference of herbivores were determined for

every leaf used in subsequent feeding-choice tests: pre-

experimental leaf damage (PLD), total leaf area (TLA),

leaf water content (LWC), and specific leaf area (SLA;

according to Hanley and Lamont [2002]). To quantify

PLD, the freshly collected leaves were photographed

with a digital camera (Nikon, COOLPIX 950). The area

of the digitized leaves and the missing area (PLD) were

then determined using the graphics package Adobe

Photoshop 6.0. More information on the quantification

of leaf damage is given by Mody and Linsenmair (2004).

LWC and SLA were determined using the remaining

leaf parts not used in the preference tests. The area of

these parts was determined as previously described.

They were stored in airtight plastic bags. Their fresh

mass was assessed within two hours after collection. The

leaves were dried at 408C to mass constancy and their

dry mass (in mg) was determined. LWC was computed

as the difference between fresh and dry mass. SLA was

calculated by dividing leaf area by dry mass. Feeding-

choice experiments were performed to assess whether

caterpillars prefer certain plant individuals and to test

whether feeding is influenced by leaf traits, particularly

the degree of previous herbivory. For further informa-

tion on preference tests, see Appendix B: Table B1.

To additionally test aspects of induced plant respons-

es, we determined the distribution of C. imparilis

caterpillars in the field as follows. In 1997, 1999, and

2000, the arthropod communities of 14 C. fragransplants were repeatedly completely sampled (32 3 14

samples overall) and C. imparilis caterpillars werecounted. In 2001, all Combretaceae plants were mapped

in a 3-ha patch surrounded by strips of vegetationwithout Combretaceae (strips were several hundredmeters wide, with no potential host plants for C.

imparilis). The exact position of each plant was recordedwith a Differential GPS (Leica SR530, Leica Geo-

systems AG, Switzerland). We searched 381 C. fragransplants growing in a part of the study area for C. imparilis

caterpillars and for typical signs of feeding by lepidop-teran larvae (almost exclusively C. imparilis; K. Mody,

personal observation). At this time, the foliage of C.fragrans individuals was about one month of age. In

2001, the host-switching frequency of 29 individuallyrecognizable (by coloration and size) caterpillars was

determined by observations of their feeding site for 11days.

To infer whether host-switching helped to reducenegative effects of enemies, we observed feeding and

resting behavior of caterpillars in the field. In addition,the parasitoid infestation rate of caterpillars was

quantified by rearing caterpillars sampled in the fieldand counting emerging parasitoids.

Data analysis

Prior to data analysis, we confirmed that the data met

the statistical assumptions of normality and heterosce-dasticity for parametric tests (ANOVA, t test). Other-

wise, nonparametric tests (Kruskal-Wallis test,Wilcoxon’s signed-ranks test, paired-sample randomiza-

tion test, chi-square test) were conducted using SPSS11.0 (2005; SPSS, Chicago, Illinois, USA) and SsS1.1a

(1998; Rubisoft Software GmbH, Eichenau, Germany)statistical software. For analysis of growth, development

time, and realized fecundity, average values werecomputed for caterpillars kept in the same Petri dish

or terrarium and were treated as subsamples; hence,average values of these variables were used for statistical

analyses to conservatively ensure independence of thedata. Results however, were identical when all caterpil-lars were treated as independent data. Growth param-

eters (mass) of second-instar larvae and developmenttime data were analyzed with SAS 6.12 (1999; SAS

Institute, Cary, North Carolina, USA) statistical soft-ware by a three-way mixed-model ANOVA, with the

two clutches as blocking factor. The fixed explanatoryvariable was food composition (mixed-plant vs. single-

plant diet), and food-plant individual (six tree individ-uals) was considered a random effect.

RESULTS

The specialist diet-mixing hypothesis

The independent variables ‘‘feeding treatment’’ and

‘‘host individual’’ and the blocking factor ‘‘clutch’’contributed significantly to variation in mass of sec-

KARSTEN MODY ET AL.1014 Ecology, Vol. 88, No. 4

ond-instar Chrysopsyche imparilis larvae (Table 1a). The

mean body mass of second-instar caterpillars reared in

the mixed-plant treatment was significantly higher than

the body mass of larvae reared with a single-plant diet

(paired t test, t¼3.66, df¼11, P¼0.004; see also Fig. 1).

ANOVA post hoc comparisons revealed that caterpillar

growth differed significantly between certain host

individuals, indicating that Combretum fragrans individ-

uals varied in suitability (quality) as host plant (Fig. 1).

The effect of mixing food plants for growth of second-

instar larvae seemed to be the more pronounced, the

lower the suitability of the host plant in the single-plant

treatment (Fig. 1, single- vs. mixed-plant diet, t test, t¼3.99, df ¼ 14, P ¼ 0.001 for C. fragrans plant 1 (Cf1),

clutch 2; t¼ 1.98, df¼ 14, P¼ 0.067 for Cf1, clutch 1; t¼1.94, df ¼ 14, P ¼ 0.073 for Cf2, clutch 1; for all other

comparisons P . 0.1).

There was no effect of feeding treatment or host

quality (based on single-plant diet) on survival and sex

ratio (Table 2). For both sexes, development time also

was not influenced by feeding treatment and host quality

(Tables 1b and 2). The feeding treatment 3 host quality

interaction, however, was significant (Table 1b), indi-

cating that mixing food reduces development time for

low-quality plants (cf. Table 2). There was also a

significant difference in fecundity of C. imparilis females

reared on a single-plant diet vs. a mixed-plant diet.

Moths reared on a mixed-plant diet laid 65% more eggs,

on average, than moths reared on a single-plant diet

(Fig. 2). Individual egg mass and the mass of the

analyzed egg clutches did not differ between the feeding

treatments (t test; for individual egg mass, t¼�1.08, df¼14, P¼ 0.30; for egg clutch mass, t¼ 1.42, df¼ 14, P¼0.18). However, the relationship between the number of

eggs and egg mass differed between the two treatments.

Although there was no correlation between egg number

and egg mass in the mixed-plant treatment (n ¼ 8, rS ¼0.36, P ¼ 0.39), there was a significant negative

correlation in the single-plant treatment (n ¼ 8, rS ¼�0.83, P ¼ 0.011).

Induced defense and enemy avoidance

We found significant differences in all tested leaf

characteristics among the studied C. fragrans individuals

(Kruskal-Wallis tests for PLD, TLA, LWC, and SLA).

Leaves of paired plants differed strongly in PLD and

TLA. LWC and SLA differed in only one plant pair. No

TABLE 1. Influence of treatment (mixed-plant vs. single-plantfood) and food plant individual on (a) mass of second-instarlarvae and (b) development time of the moth Chrysopsycheimparilis.

Source df F P

a) Mass of second-instar larvae

Plant 5, 5 10.83 0.010Treatment 1, 5 9.77 0.026Plant 3 treatment 5, 11 0.87 0.531Clutch 1, 11 140.10 0.0001

b) Development time

Plant 5, 5 0.46 0.792Treatment 1, 6 0.38 0.379Plant 3 treatment 5, 40 2.95 0.023Clutch 1, 40 0.46 0.499

Note: Analysis was by ANOVA, with clutch (differentmother, different age) as the blocking factor.

FIG. 1. Mass (mean þ SE) of second-instar Chrysopsyche imparilis caterpillars of two sibling groups (clutch I and II) with adifference in age of two days at the time of weighing. Caterpillars were reared on leaves of six Combretum fragrans plants (Cf1–Cf6); leaves were derived either from a single plant individual (s, single-plant diet) or from different plant individuals (m, mixed-plant diet). Thus ‘‘s I’’ in the key represents first-clutch caterpillars fed a single-plant diet. Assignment of ‘‘mixed-plant diet’’caterpillars to a host individual refers to the starting plant in the feeding scheme. The mass of caterpillars reared on Cf1 and Cf2was significantly lower (low-quality hosts) than that of larvae reared on Cf5 and Cf6 (high-quality hosts). The mass of caterpillarsreared on Cf4 and Cf5 (medium-quality host) was not distinguishable from that of any other groups. See Methods: Data analysisfor ANOVA details. Results of Tukey’s post hoc tests for C. fragrans plant 1 (Cf1): Cfl vs. Cf5, P¼ 0.003; Cf1 vs. Cf6, P¼ 0.002;Cf2 vs. Cf5, P¼ 0.014; Cf2 vs. Cf6, P¼ 0.010.

April 2007 1015SPECIALIST HERBIVORE DIET-MIXING

significant difference in consumed leaf fresh mass,

consumed leaf area, or consumed leaf dry mass could

be detected for leaves of different plant individuals.

Leaves with higher pre-experimental herbivory seemed

to be more attractive than leaves with lower pre-

experimental herbivory, although this was not statisti-

cally significant. For more details, see Appendix B:

Table B1.

The determination of caterpillar distribution in the

field revealed regular switching among conspecific host

plants. Although C. imparilis females lay eggs in clusters,

caterpillars did not aggregate on individual host plants,

but occurred mostly solitarily (81% of surveyed C.

fragrans plants hosted one caterpillar; Appendix C: Fig.

C1). The monitoring of 29 individually recognizable

caterpillars demonstrated that host plants were regularly

changed within the 11-day observation period (83%

changed host plants, 45% within 1–2 days after first

detection; Appendix D: Fig. D1). Although only a small

percentage of the studied C. fragrans plants hosted C.

imparilis caterpillars at the time of leaf damage

assessment (12.3% of 381 plants; Appendix E: Fig.

E1), almost all trees showed signs of feeding by C.

imparilis caterpillars (96.3% of 381; Appendix E: Fig.

E2).

Concerning enemy avoidance, no indication was

found that C. imparilis caterpillars are subject to high

predation pressure while being on their host plant.

Although conspicuously colored, they were regularly

recorded feeding or sun-basking at highly exposed

positions on the plant. Parasitoid infestation rate also

appeared to be low: less than 4% of C. imparilis

caterpillars observed in the field and taken to the

laboratory were parasitized (five of 220 individuals were

parasitized by Braconidae and three by Tachinidae).

DISCUSSION

Caterpillar field distribution and the specialist

diet-mixing hypothesis

Herbivores usually feed on foods that are nutrition-

ally and toxically heterogeneous and unbalanced,

forcing them to ingest excesses of some nutrients/toxins

in order to avoid a critical shortfall of other nutrients

(Hagele and Rowell-Rahier 1999, Raubenheimer and

Simpson 1999). The study of model herbivore species in

the laboratory has yielded considerable insights on

nutritional requirements and constraints governing

nutritional ecology and feeding strategies of arthropod

herbivores (Raubenheimer and Simpson 1999, Cruz-

Rivera and Hay 2001, Simpson and Raubenheimer

2001, Behmer et al. 2002). However, little information is

available on how the need to deal with unbalanced food

TABLE 2. Number of eclosing Chrysopsyche imparilis individuals, sex ratio, and development times(in days) for mixed-plant vs. single-plant diet and different host qualities. Host qualities werederived from mass achieved by second-instar caterpillars reared on the respective plants.

Host qualitiesand diet type

Sex ratio(female :male)

No. eclosed moths Development time (d)

Females Males Females Males

Overall

Mixed-plant 0.9 23 25 87.8 6 0.8 82.6 6 0.7Single-plant 1.0 26 25 88.6 6 0.9 83.8 6 0.9

High-quality food

Mixed-plant 0.8 5 6 89.8 6 2.4 83.3 6 1.7Single-plant 0.7 8 12 88.5 6 0.7 83.5 6 1.2

Medium-quality food

Mixed-plant 1.7 10 6 87.8 6 1.1 84.7 6 1.3Single-plant 0.9 7 8 86.3 6 1.5 82.8 6 1.8

Low-quality food

Mixed-plant 0.6 8 13 86.5 6 1.0 81.3 6 0.8Single-plant 2.2 11 5 90.2 6 1.6 86.2 6 1.5

Notes: For survival and feeding treatment (mixed-plant vs. single-plant), v2¼ 0.04, df¼ 1, P¼0.84. For survival and host quality, v2¼ 0.82, df¼ 2, P¼ 0.66. For sex ratio and feeding treatment,v2 ¼ 0.08, df ¼ 1, P ¼ 0.78. For sex ratio and host quality, v2 ¼ 1.0, df ¼ 2, P ¼ 0.61. Fordevelopment time, data are given as mean 6 SE.

FIG. 2. Number of eggs (mean þ SE) laid by female C.imparilis moths reared on single-plant diet (75.3 6 14.9 eggs) oron mixed-plant diet (124.0 6 10.1 eggs; t ¼ 2.71, df ¼ 24, P ¼0.012).

KARSTEN MODY ET AL.1016 Ecology, Vol. 88, No. 4

may affect arthropod herbivore distribution and feeding

patterns in the field (Raubenheimer and Bernays 1993,

Singer et al. 2002, Wright et al. 2003). Our study of

specialist Chrysopsyche imparilis caterpillars may reduce

this gap in knowledge and simultaneously contribute to

understanding the influence of intrapopulational varia-

tion in plant nutritive characters on insect herbivore

distribution and performance.

The studied caterpillars generally remained on a given

Combretum fragrans host plant only for a few days. They

then switched to another conspecific host plant individ-

ual, causing a quite homogeneous incidence of caterpillar

damage within the plant population. Large host plant

size (usually several hundred or more leaves) and a short

utilization period resulted in consumption of a very small

fraction of the available suitable foliage by individual

caterpillars (usually much less than 1%; K. Mody,

unpublished data). Mere food limitation thus should

not be a decisive determinant for these caterpillars to

leave a host plant, and balancing of diet composition

should be regarded a major alternative explanation.

To test whether diet balancing may help to explain the

remarkable intraspecific host change of C. imparilis

caterpillars, we conducted no-choice and choice feeding

experiments and assessed distribution patterns of cater-

pillars and feeding damage in the field.Herbivores feeding

onnutritionally heterogeneous host plantsmay adopt two

feeding strategies: they may improve food quality by

either host selection or host-mixing (Westoby 1977, Cruz-

Rivera andHay 2001, Raubenheimer and Simpson 2003).

Host selection applies when herbivores can choose among

plants on the basis of an ordinal ranking and select those

plants that are simply better than others. Host-mixing

enables animals to compose nutritionally balanced diets

by mixing their intake from two or more hosts that are

individually suboptimal but nutritionally complementa-

ry. For the studied C. imparilis caterpillars, host-mixing

appears to be the more plausible feeding strategy. As the

mobility and orientation capabilities of the caterpillars

are restricted, the caterpillars can hardly compare and

then directly and reliably head for a chosen host plant that

usually grows several meters apart from others in a highly

diverse and structurally very complex plant community

matrix. Caterpillars can, however, switch among inciden-

tally chosen host plants and thereby more or less balance

nutrient intake when the individual plants are comple-

mentary with regard to nutrient/toxin content. To fully

understand the quantifiable effects of feeding on different

host plant individuals for the performance of herbivores,

knowledge of the plants’ food quality (specific nutrient

and/or allelochemical levels) would be necessary.Without

this information it is not possible to quantify the relative

influence of nutrient ratios and allelochemical levels on

foraging behavior in the field, and on performance

parameters determined in the laboratory. However, our

results demonstrate that unselective mixing of conspecific

host plants can improve the performance of insect

herbivores. When C. imparilis caterpillars were reared in

a no-choice setup, either with leaves of a single plant

individual, or with leaves of different plant individuals

(simulating a regular and incidental host switch in

nature), they grew better, on average, and had a higher

fecundity on themixed diet. In the choice feeding tests, no

preference for anyplant individualwas detected, although

the plants differed in several characteristics relevant for

host choice in insect herbivores. The same experimental

PLATE 1. Host-plant-switching of a Chrysopsyche imparilis caterpillar (from left to right): increased non-feeding activity, leavingof host, searching for new host (lower arrow points to caterpillar, upper arrow to new Combretum fragrans host plant), feeding onnewly approached host (total distance covered by caterpillar: 15.3 m). Photo credits: K. Mody.

April 2007 1017SPECIALIST HERBIVORE DIET-MIXING

setup with other caterpillars (Spodoptera littoralis)

revealed very strong preferences and a clear ability to

select between leaves of conspecific host plants (K. Mody

and S. Dorn, unpublished data). Hence, we regard the lack

of preference inC. imparilis as an indication that selective

feeding does not commonly occur in host-switching C.

imparilis caterpillars in the field.

Feeding on leaves of different C. fragrans plant

individuals turned out to influence some performance

traits of C. imparilis caterpillars, but not others. Most

striking was an increase in fecundity and a stabilizing

effect on egg size. This shows that a mixed-plant diet can

positively affect important fitness parameters of the

moths (e.g., Fischer et al. 2003). The negative correla-

tion between egg number and egg size in moths reared

on a single-plant diet suggests that a single-plant diet

may often be a lower quality diet, which results in a

trade-off between egg number and egg size and reduces

fitness via either lowered fecundity or reduced offspring

quality. In combination, the positive effect on fitness

parameters of an intraspecifically mixed diet may be a

key for explaining the respective host-changing behavior

of C. imparilis caterpillars. Besides fecundity parame-

ters, growth parameters were determined for the studied

caterpillars. Although some growth parameters were

also affected by diet-mixing, no general effect on growth

was detected for second-instar caterpillars. Development

time was only affected when host quality was low.

This indicates that food-mixing might be especially

adaptive in an environment where low-quality food

resources predominate or where it is difficult to find

high-quality food. The variability of intraspecific vari-

ation in the plants’ content of nutritional or toxic

substances (Osier and Lindroth 2001) may help to

explain why some studies investigating food-mixing in

caterpillars failed to detect any measurable effect. Diet-

mixing is only effective when relevant differences exist in

nutrient composition or toxin content among plants.

For cultivated plants, which are often selected for

homogenous quality and low concentrations in deterrent

or toxic compounds (Barbosa 1993), the prerequisites

supporting diet-mixing are probably not met. Wild plant

populations, however, are usually characterized by high

intraspecific variation in most traits (Karban 1992),

which in turn can be crucial for the development of

herbivorous insects.

Induced defense and enemy avoidance hypotheses

Although intraspecific diet-mixing positively affected

fitness parameters, other factors may also promote

intraspecific host-plant-switching. C. imparilis caterpil-

lars showed no significant preferences in any feeding

experiment, although the tested leaves differed signifi-

cantly in many leaf characters that are potentially

important to herbivores (Schoonhoven et al. 1998).

Contrary to expectations derived from other studies, the

caterpillars neither preferred leaves from plants bearing

bigger leaves (Senn et al. 1992), nor leaves with higher

water content (Scriber 1977) or lower toughness

(Choong 1996). They also showed no preference for

leaves from plants with lower levels of pre-experimental

herbivory, excluding a decisive role of feeding-induced

plant defense. To the contrary, if there was any trend in

preference, then leaves with higher pre-experimental

herbivory were more attractive. This suggests either an a

priori higher attractiveness of these leaves (Neuvonen

and Haukioja 1985), or even an increased attractiveness

induced by previous herbivory (Haukioja 1990, Agrawal

2000). Another argument against the induced-defense

hypothesis is provided by the mapping of feeding

damage. Because almost all C. fragrans plants showed

recent C. imparilis feeding marks, almost every new host

also would have been induced.

The influence of enemies on the distribution of the

caterpillars was assessed only indirectly. The conspicu-

ously colored caterpillars were often observed feeding or

resting at highly exposed plant parts. Therefore,

avoidance of visually hunting (vertebrate) predators

does not seem to be a major factor impacting caterpillar

distribution (Heinrich 1993). The low parasitoid infes-

tation rate of the C. imparilis caterpillars indicates that

they are well protected against parasitoids, perhaps via

host-plant-switching. If host-plant-switching serves as a

defense strategy, two explanatory scenarios seem plau-

sible: Either a regular change of host plants impedes

caterpillar location by parasitoids, or it increases

caterpillar survival via conditional host choice that

improves the performance of parasitized caterpillars

(Karban and English-Loeb 1997, Lee et al. 2006); i.e.,

intraspecific diet-mixing as conditional host choice in the

case of C. imparilis.

Conclusion

The importance of intraspecific plant variation for the

distribution of herbivores and for animal–plant interac-

tions is increasingly acknowledged (Denno and McClure

1983, Fritz and Simms 1992, Mody et al. 2003). Here, we

show that a regular intraspecific host plant change may

provide specialist herbivores with an optimized diet that

positively affects herbivore performance in a way

hitherto only attributed to interspecific diet-mixing.

When herbivores are mobile and can easily select the

best plants within a limited range, intraspecific host

variation should lead to an aggregated herbivore

distribution and marked preferences for certain plant

individuals. However, for less mobile herbivores such as

caterpillars, a regular host change may allow them to

balance nutrient intake and to obtain the best diet

available, on average (as high-quality plants can only be

reached by chance and comparisons are difficult for

caterpillars and probably impossible from some dis-

tance). Therefore the intraspecific diet-mixing hypothe-

sis seems to be a good explanation for host changes in C.

imparilis. It might also apply to a multitude of

herbivores with limited selection ability, feeding on

plants of variable and predominantly low quality.

KARSTEN MODY ET AL.1018 Ecology, Vol. 88, No. 4

ACKNOWLEDGMENTS

We thank E. Strohm, A. Davis, W. Weisser, T. Nuttle, andtwo anonymous reviewers for helpful comments on themanuscript, B. Pfeiffer for statistical advice and performingthe SAS analyses, C. Hauser for identification of Chrysopsycheimparilis, and A. Thiombiano for confirmation of Combretumfragrans. Permission to conduct the research was courtesy of theMinistere de l’Enseignement Superieur et de la RechercheScientifique, Cote d’Ivoire. Access permit to Comoe NationalPark was issued by the Ministere de la Construction et del’Environnement. The study was partly supported by scholar-ships from the DAAD (German Academic Exchange Service,No. 332 4 04 101) and the German Research Council Graduier-tenkolleg 200 to K. Mody, and by BIOTA (BiodiversityMonitoring Transect Analysis in Africa), German FederalMinistry of Education and Research (BMBF), subproject W06,01LC0017.

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Choong, M. F. 1996. What makes a leaf tough and how thisaffects the pattern of Castanopsis fissa leaf consumption bycaterpillars. Functional Ecology 10:668–674.

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Denno, R. F., and M. S. McClure. 1983. Variable plants andherbivores in natural and managed systems. Academic Press,New York, New York, USA.

Dethier, V. G. 1988. The feeding behavior of a polyphagouscaterpillar Diacrisia virginica in its natural habitat. CanadianJournal of Zoology 66:1280–1288.

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Fritz, R. S., and E. L. Simms, editors. 1992. Plant resistance toherbivores and pathogens: ecology, evolution, and genetics.University of Chicago Press, Chicago, Illinois, USA.

Hagele, B. F., and M. Rowell-Rahier. 1999. Dietary mixing inthree generalist herbivores: nutrient complementation ortoxin dilution? Oecologia 119:521–533.

Hanley, M. E., and B. B. Lamont. 2002. Relationships betweenphysical and chemical attributes of congeneric seedlings: howimportant is seedling defence? Functional Ecology 16:216–222.

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APPENDIX A

A table with information on Chrysopsyche imparilis caterpillar assignment to feeding treatments and feeding order (EcologicalArchives E088-062-A1).

APPENDIX B

A table showing leaf and consumption attributes in paired feeding choice tests with Chrysopsyche imparilis caterpillars(Ecological Archives E088-062-A2).

APPENDIX C

A figure showing abundance patterns of Chrysopsyche imparilis caterpillars on individual Combretum fragrans host plants(Ecological Archives E088-062-A3).

APPENDIX D

A figure showing the timing of host-plant-switching of Chrysopsyche imparilis caterpillars (Ecological Archives E088-062-A4).

APPENDIX E

Figures showing the distribution of Chrysopsyche imparilis caterpillars and feeding marks on Combretum fragrans host plants inthe field (Ecological Archives E088-062-A5).

KARSTEN MODY ET AL.1020 Ecology, Vol. 88, No. 4

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Ecological Archives E088-062-A1

Karsten Mody, Sybille B. Unsicker, and K. Eduard Linsenmair. 2007. Fitness relateddiet-mixing by intraspecific host-plant-switching of specialist insect herbivores. Ecology88:1012–1020.

Appendix A. A table with information on Chrysopsyche imparilis caterpillar assignment to feedingtreatments and feeding order.

TABLE A1. First instar caterpillars of Chrysopsyche imparilis out of two clutches assigned to thedifferent experimental groups. N = number of individuals per group. Cf 1–Cf 6 = Combretum fragransindividuals 1 to 6. Starting plants for “mixed-plant diet” feeding treatments in bold.

Feeding treatment No. groups per clutch Clutch 1 (N) Clutch 2 (N)Single-plant diet

Cf 1 1 8 8Cf 2 1 8 8Cf 3 1 8 8Cf 4 1 8 8Cf 5 1 8 8Cf 6 1 8 8

Mixed-plant dietFeeding order:

Cf 1 , Cf 2, Cf 3, Cf 4, Cf 5, Cf 6 1 8 8Cf 2 , Cf 3, Cf 4, Cf 5, Cf 6, Cf 1 1 8 8Cf 3 , Cf 4, Cf 5, Cf 6, Cf 1, Cf 2 1 8 8Cf 4 , Cf 5, Cf 6, Cf 1, Cf 2, Cf 3 1 8 8Cf 5 , Cf 6, Cf 1, Cf 2, Cf 3, Cf 4 1 8 8Cf 6 , Cf 1, Cf 2, Cf 3, Cf 4, Cf 5 1 8 8

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Karsten Mody, Sybille B. Unsicker, and K. Eduard Linsenmair. 2007. Fitness relateddiet-mixing by intraspecific host-plant-switching of specialist insect herbivores. Ecology88:1012–1020.

Appendix B. A table showing results for leaf and consumption attributes in paired feeding choice testswith Chrysopsyche imparilis caterpillars.

TABLE B1. Mean values ( ± 1 SE) of leaf and consumption attributes determined for 10 C. fragransplants used in paired (P1a,b–P5a,b) feeding-choice tests with C. imparilis caterpillars. For inter-plantvariation of leaf attributes, results of Kruskal-Wallis (H) test are shown. For variation within plant-pairs,results of paired t tests or Wilcoxon’s signed-ranks test (indicated by wsr), or of paired-samplerandomization tests (for PLD and LWC) are shown. Number of caterpillars tested: P1 = 15, P2 = 15, P3= 20, P4 = 29, P5 = 18. ns, nonsignificant; * P < 0.05; ** P < 0.01; *** P < 0.001.

Sample

Pre-experimental leafdamage

(%)PLD

Areacomplete leaf

(cm2)ACL

Leaf watercontent

(%)LWC

Specific leafarea

( mm2 mg-

1)SLA

Consumed leaffresh mass

(mg)CLM

P1a 0.6 (0.2) 30.7 (1.5) 69.7 (0.9) 11.7 (0.5) 28 (13)P1b 7.2 (1.5) 40.1 (1.3) 68.1 (0.6) 10.3 (0.3) 72 (19)P (1a vs. 1b) *** *** ns ns nsP2a - - 66.1 (0.7) 9.7 (0.2) 85 (16)P2b - - 67.3 (0.9) 9.6 (0.4) 97 (22)P (2a vs. 2b) - - ns ns nsP3a 2.8 (0.7) 39.6 (1.3) 60.3 (0.5) 8.9 (0.3) 291 (57)P3b 8.8 (1.6) 57.2 (2.1) 61.1 (0.7) 8.6 (0.2) 317 (48)P (3a vs. 3b) ** *** ns ns nsP4a 5.2 (0.7) 60.3 (2.4) 54.8 (0.6) 8.5 (0.1) 269 (50)P4b 11.4 (1.6) 59.4 (2.8) 54 (0.5) 8.1 (0.1) 349 (69)P (4a vs. 4b) ** ns * * nsP5a 2.7 (0.7) 51.1 (2.4) 57.1 (0.2) 6.4 (0.2) 469 (71)P5b 14.2 (1.8) 62.5 (4.1) 57.2 (0.2) 7.1 (0.4) 324 (86)P (5a vs. 5b) *** * ns ns ( wsr) nsInter-plantcomparison:

H ( df):

P

72.5 (7)

***

87.3 (7)

***

156.1 (9)

***

117.1 (9)

***

-

-

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Explanatory information Table B1: Comparisons of leaf characteristics of experimental plant pairs wereconducted using paired t tests when parametric assumptions were met and Wilcoxon’s signed ranks testswhen these assumptions were not fulfilled. For variables expressed as percentage, paired-samplerandomization tests were performed with 10,000 iterations (Windows version of SPSS 11.0). Between-plant comparisons of single leaf characters were conducted using the Kruskal-Wallis test, sinceparametric assumptions were not met by these data even after application of transformation procedures (Sokal and Rohlf 1995, Zar 1999).

Leaves were randomly (by blindly pointing to the plant with a stick and picking the leaf firstencountered) collected from the five pairs of experimental plants. The paired plants were chosen torepresent, judging by appearances, one pair member with high and one with low pre-experimentalherbivory. One leaf of each pair member was offered to single C. imparilis caterpillars for 16 hours(from 1600 till 0800, to ensure day and night-time feeding and ad libitum fresh leaf material to feed).The caterpillars were kept in plastic terraria (18 × 11 × 14 cm; provided with moistened filter paper) andrepresented all larval stadia except first instars. Size of paired leaves was either standardized to squareleaf pieces (9 cm²) or was aligned by cutting the leaves and providing comparably sized apical parts tothe caterpillars. If leaf area was not standardized, it was determined before the experiment following theprocedure described above for leaf area determination. Leaf a rea was also determined for leaf-remainings after the feeding experiment. From differences in leaf area before and after the experiment,consumed leaf area was computed. Leaf specific fresh and dry mass (mg fresh or dry mass per area) wasdetermined to derive consumed leaf mass (fresh and dry) from consumed leaf area.

LITERATURE CITED

Sokal, R. R., and F. Rohlf. 1995. Biometry. Freeman and Company, San Francisco, USA.

Zar, J. H. 1999. Biostatistical analysis. Prentice-Hall, Upper Saddle River, New Jersey, USA.

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Ecological Archives E088-062-A3

Karsten Mody, Sybille B. Unsicker, and K. Eduard Linsenmair. 2007. Fitness relateddiet-mixing by intraspecific host-plant-switching of specialist insect herbivores. Ecology88:1012–1020.

Appendix C. A figure showing abundance patterns of Chrysopsyche imparilis caterpillars on individualCombretum fragrans host plants.

FIG. C1. Abundance pattern of Chrysopsyche imparilis caterpillars on individualCombretum fragrans host plants. The number of caterpillars that simultaneously occur on aplant is shown.

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Ecological Archives E088-062-A4

Karsten Mody, Sybille B. Unsicker, and K. Eduard Linsenmair. 2007. Fitness relateddiet-mixing by intraspecific host-plant-switching of specialist insect herbivores. Ecology88:1012–1020.

Appendix D. A figure showing the timing of host plant switching of Chrysopsyche imparilis caterpillars.

FIG. D1 . Timing of host switching of 29 C. imparilis caterpillars during 11 days ofobservation. Every host switching after the first detection of caterpillars is given.

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Ecological Archives E088-062-A5

Karsten Mody, Sybille B. Unsicker, and K. Eduard Linsenmair. 2007. Fitness related diet-mixing byintraspecific host-plant-switching of specialist insect herbivores. Ecology 88:1012–1020.

Appendix E. Figures showing the distribution of Chrysopsyche imparilis caterpillars and feeding marks on Combretum fragranshost plants in the field.

FIG. E1. Contemporaneous distribution of C. imparilis caterpillars on a subsample (within box) of 381 C. fragrans host plants.

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FIG. E2. Distribution of feeding marks of C. imparilis caterpillars on young foliage (3–4 weeks after leaf emergence) of asubsample (within box) of 381 C. fragrans host plants (plants without feeding marks are indicated).

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