Oecologia

DOI 10.1007/s00442-009-1439-1

PLANT-ANIMAL INTERACTIONS - ORIGINAL PAPER

Counterintuitive size patterns in bivoltine moths: late-season larvae grow larger despite lower food quality

Tiit Teder · Toomas Esperk · Triinu Remmel · Anu Sang · Toomas Tammaru

Received: 13 February 2009 / Accepted: 24 July 2009© Springer-Verlag 2009

Abstract Within a season, successive generations ofshort-lived organisms experience diVerent combinations ofenvironmental parameters, such as temperature, food qual-ity and mortality risk. Adult body size of e.g. insects istherefore expected to vary both as a consequence of proxi-mate environmental eVects as well as adaptive responses toseasonal cues. In this study, we examined intraspeciWcdiVerences in body size between successive generations in12 temperate bivoltine moths (Lepidoptera), with the ulti-mate goal to critically compare the role of proximate andadaptive mechanisms in determining seasonal size diVer-ences. In nearly all species, individuals developing late inthe season (diapausing generation) attained a larger adultsize than their conspeciWcs with the larval period early inthe season (directly developing generation) despite the typ-ically lower food quality in late summer. Rearing experi-ments conducted on one of the studied species, Seleniatetralunaria also largely exclude the possibility that theproximate eVects of food quality and temperature are deci-sive in determining size diVerences between successivegenerations. Adaptive explanations appear likely instead:the larger body size in the diapausing generation may beadaptively associated with the lower bird predation pres-sure late in the season, and/or the likely advantage of largepupal size during overwintering.

Keywords Phenology · Phenotypic plasticity · Predation risk · Time stress · Voltinism

Introduction

Environmental factors, such as food availability and qual-ity, temperature and mortality risk, may crucially aVectgrowth schedules in juvenile animals either via proximateeVects on ontogeny or as selective forces leading to varioustypes of adaptive responses. In seasonal environments,the temporal dynamics of environmental factors are to aconsiderable extent predictable, and therefore adaptiveresponses to seasonal cues are expected to evolve.

Seasonal adaptations are particularly important for short-lived ectothermic organisms, such as insects, as their Wtnesscritically depends on their ability to adjust the timing andlength of their development period to the seasonal cycle(Tauber et al. 1986; Nylin 1992; Higgins 2000; Danks2007). In particular, the majority of insect species in tem-perate environments are adapted to pass the unsuitableperiod (winter) in a certain, species-speciWc developmentalstage (Tauber et al. 1986; Gotthard 2001; Danks 2007). Toavoid zero Wtness, most insects have to reach this stageprior to the advent of the unfavourable season. Whereasunivoltine insects just have to Wt their single generation intothe favourable season, bi- and multivoltine species have anadditional adaptive challenge: splitting the season betweentwo or more generations of optimal timing and length.

Successive generations developing in diVerent phasesof the seasonal cycle typically experience diVerent combi-nations of temperature, food resources and mortality risk(Nijhout 2003). Due to both proximate eVects and adap-tive responses, growth rates and Wnal body sizes aretherefore not necessarily identical in diVerent generations.

Communicated by Thomas HoVmeister.

T. Teder (&) · T. Esperk · T. Remmel · A. Sang · T. TammaruDepartment of Zoology, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014 Tartu, Estoniae-mail: tiit.teder@ut.ee

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Oecologia

With respect to proximate eVects, the decline in foodquality for herbivorous insects as the season progressesconstitutes a rather universal predictable season-relatedfactor (e.g. Schroeder 1986; Ayres and MacLean 1987;Van Asch and Visser 2007). As a consequence, early sea-son larvae of bivoltine insects are expected to attain alarger Wnal size than conspeciWc larvae feeding on lower-quality plant resources later in the season (Iwasa et al.1994). Furthermore, as a general rule, ectotherms growingat lower temperatures tend to attain larger sizes than theirconspeciWcs developing at higher temperatures (Atkinsonand Sibly 1997; Angilletta et al. 2004). Even if the rela-tive importance of proximate and ultimate mechanismsbehind this pattern is still a matter of debate (Angillettaand Dunham 2003; Karl and Fischer 2008), we prefer hereto classify any direct eVect of ambient temperature onindividual development as a proximate one.

Considering the possible adaptive explanations, theperhaps most obvious selective pressure causing theamong-generation diVerences arises from the occurrence ofoverwintering per se. Intuitively, size optima for a diapaus-ing and a directly developing generation need not coincide.Another possible driver of adaptive seasonal variability inbody size may be diVerential developmental time stressduring the season. As an example, when mortality risk ofjuveniles is high then a long growing period may not payoV, and it could become advantageous to mature at asmaller size (Peckarsky et al. 2001; Stoks 2001, for empiri-cal evidence).

In summary, there is no shortage of ideas about factorscapable of causing predictable size diVerences betweensuccessive generations. In contrast, studies critically com-paring potential explanations in particular systems are sur-prisingly scarce, and even the patterns themselves appear tobe poorly documented. In this study, we examine intraspe-ciWc seasonal variation in adult body size between directlydeveloping and diapausing generations in natural popula-tions of 12 North European bivoltine moths. We criticallycompare the contribution of two major proximate factors(food quality and temperature) to size diVerences betweengenerations, and the role of various adaptive scenarios. Weshow that, in most moth species, individuals developinglate in the season (diapausing generation) attain a largeradult size than their conspeciWcs with the larval periodearly in the season (directly developing generation). In con-cert with the experimental results of laboratory rearing forone of the species, this pattern challenges the idea of thedecisive role of the proximate eVects (food quality and tem-perature) in determining intraspeciWc diVerences in bodysize between successive generations. Instead, the revealedpatterns are consistent with several adaptive explanations,relying on seasonal dynamics of mortality risk and size-dependent over-winter survival in particular.

Materials and methods

Species

Twelve species of moth (Lepidoptera, Heterocera; Table 1),regularly bivoltine in the study area (Mikkola and Jalas1977; Mikkola et al. 1985, 1989; Marttila et al. 1996;Viidalepp and Remm 1996; T. Teder et al., personal obser-vation) were examined to determine diVerences in adultbody size between successive generations. Although theselection of species was “passive”, i.e. it adhered strictly tothe criterion of suYcient abundance in moth-monitoringcatches (see below), a wide array of major macrolepidopt-eran lineages was represented among the studied species(families Geometridae, Drepanidae, Notodontidae, Arctiidaeand Noctuidae).

Life cycles of the studied species are typical of temper-ate moths (Svensson 1993; Viidalepp and Remm 1996;Table 1). Most of them overwinter as pupae. Three species,however, overwinter as larvae, and in two of them, feedingcontinues in spring prior to pupation (Table 1). Adults ofthis diapausing generation (i.e. the spring- or Wrst-genera-tion adults, as frequently referred to in entomological litera-ture) thus represent individuals which underwent theirlarval development, wholly or for the most part, in latesummer. In Estonia, the adults of the diapausing generationeclose from the beginning of May (Falcaria lacertinaria,Drepana falcataria, Selenia dentaria, Selenia tetralunaria,Clostera curtula) to the middle of June (Xestia c-nigrum).Thus larvae, hatching from the eggs laid by the adults of thediapausing generation, feed predominantly in June. After ashort pupal period, the adults of this directly developinggeneration (i.e. the summer or second-generation adults)eclose in late July or early August of the same year. Most ofthe studied species feed on the foliage of deciduous treeswhile larvae of four species develop on various herbaceousplants (Table 1).

Seasonal diVerences in body size between generations: patterns

Forewing length was used as the index of body size whencomparing size diVerences between generations. For eachspecies £ sex £ generation combination, we aimed to mea-sure at least ten individuals (up to more than 200 dependingon availability; Table 1). However, for a few combinations(especially females) there were not as many individualsavailable (Table 1). Altogether, forewing lengths weretaken from 1,832 individuals.

Most adult moths (93%) measured in this study wereobtained from the catches of automatic moth-monitoringtraps, a network of which is regularly operated as a partof the Estonian Environmental Monitoring Programme.

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Oecologia

Tab

le1

Mea

n fo

rew

ing

leng

ths

(mm

) of

mot

hs in

two

succ

essi

ve g

ener

atio

ns

Not

e th

at a

dult

s of

the

diap

ausi

ng g

ener

atio

n re

pres

ent i

ndiv

idua

ls th

at, w

holly

or f

or th

e m

ost p

art,

unde

rwen

t the

ir la

rval

dev

elop

men

t in

late

sum

mer

or a

utum

n. A

dults

of t

he d

irec

tly d

evel

opin

gge

nera

tion

stem

fro

m th

e la

rvae

, dev

elop

men

t of

whi

ch o

ccur

red

in la

te s

prin

g or

ear

ly s

umm

er. M

eani

ngfu

l sam

ple

size

s co

uld

not b

e re

trie

ved

for

fem

ales

of

a fe

w s

peci

esa

Incl

udes

add

ition

al s

peci

men

s ob

tain

ed f

rom

col

lect

ions

bSi

gniW

cant

onl

y w

ith m

easu

rem

ents

on

coll

ectio

n sp

ecim

ens

incl

uded

Spec

ies

Fam

ily

Ove

rwin

teri

ng

stag

eT

ype

of h

ost p

lant

Mal

esF

emal

es

Dia

paus

ing

gene

rati

onD

irec

tly

deve

lopi

ng

gene

ratio

n

t-te

stD

iapa

usin

g ge

nera

tion

Dir

ectly

de

velo

ping

ge

nera

tion

t-te

st

Mea

n§

SD

nM

ean§

SDn

tP

M§

SD

nM

ean§

SD

nt

P

Fal

cari

a la

cert

inar

iaD

repa

nida

eP

upa

Dec

iduo

us tr

ees

16.6

§0.

624

16.1

§0.

669

3.59

0.00

0517

.6§

1.1

6a17

.2§

0.5

10a

0.89

0.39

Dre

pana

falc

atar

iaD

repa

nida

eP

upa

Dec

iduo

us tr

ees

17.1

§0.

830

16.5

§0.

816

83.

740.

0002

19.1

§0.

914

a18

.4§

1.0

132.

030.

053

Tim

andr

a co

mae

Geo

met

rida

eH

alf-

grow

n la

rva

Her

bace

ous

plan

ts15

.5§

0.5

3014

.5§

0.6

242

7.98

<0.

0001

14.5

§0.

411

a13

.9§

0.9

252.

110.

04b

Epi

rrho

e al

tern

ata

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ts12

.4§

0.5

2011

.7§

0.4

183

7.42

<0.

0001

12.0

§0.

315

a11

.8§

0.3

13a

1.28

0.21

Sele

nia

dent

aria

Geo

met

rida

eP

upa

Dec

iduo

us tr

ees

19.3

§1.

134

15.1

§0.

715

727

.16

<0.

0001

18.7

§1.

614

a16

.1§

0.9

165.

72<

0.00

01

Sele

nia

tetr

alun

aria

Geo

met

rida

eP

upa

Dec

iduo

us tr

ees

19.2

§1.

035

16.0

§0.

825

121

.32

<0.

0001

20.4

§0.

73a

17.7

§0.

99a

4.67

0.00

09b

Not

odon

ta

drom

edar

ius

Not

odon

tida

eP

upa

Dec

iduo

us tr

ees

19.3

§1.

010

a18

.8§

1.0

251.

270.

2121

.2§

1.7

7a21

.8§

1.4

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¡0.

840.

41

Not

odon

ta z

icza

cN

otod

onti

dae

Pup

aD

ecid

uous

tree

s18

.1§

1.0

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0.7

40¡

0.45

0.66

20.1

§0.

513

20.2

§0.

821

¡0.

650.

52

Clo

ster

a pi

gra

Not

odon

tida

eP

upa

Dec

iduo

us tr

ees

11.7

§0.

426

11.3

§0.

526

2.55

0.01

––

––

––

Clo

ster

a cu

rtul

aN

otod

onti

dae

Pup

aD

ecid

uous

tree

s16

.0§

0.7

21a

15.0

§0.

68a

3.85

0.00

0717

.61a

16.6

2a–

–

Phr

agm

atob

ia

fuli

gino

saA

rctii

dae

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n la

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n la

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01

123

Oecologia

Measurements taken from individuals collected by the pas-sively operating light traps were chosen as the basis of thisstudy as such samples are likely to contain less samplingbias than, e.g. samples from entomological collections. Weused insects sampled in 4 years (1995, 2003, 2004, 2007) atfour sites, representing diVerent regions of Estonia. For anumber of species (especially in the case of females), how-ever, additional (haphazardly selected) specimens from col-lections were necessary to obtain reasonably sized samples(Table 1). In some cases, the larger samples allowed us toattain statistical signiWcance in the generation-wise com-parisons of body size, but in no case did the additionalspecimens change the qualitative pattern (Table 1).

Most individuals could uncontroversially be assigned asrepresenting the diapausing or the directly developing gen-eration. In D. falcataria and Notodonta ziczac, however,the two generations may have had some overlap in phenol-ogy. As both of these species showed some level of protan-dry (i.e. males emerging before females; Wiklund andFagerström 1977), the few females from the overlap zonewere considered to be late individuals of the diapausinggeneration, and the few problematic males early individualsof the directly developing generation. However, as thenumber of insects with uncertain generation identity waslow, the eVect of these individuals on the results was negli-gible.

Seasonal diVerences in body size between generations: mechanisms

Some of the alternative explanations of between-generationdiVerences in moth body size could be critically evaluatedby rearing experiments in controlled conditions. Manipula-tively breaking the ties between the natural correlates of thephenological stage allowed us to identify the environmentalfactors causing the diVerences in body size, and also facili-tated judging a proximate, as opposed to adaptive character,of the response.

In particular, with one of the studied species, S. tetralu-naria, laboratory trials were conducted to determine theextent to which seasonal variation in diVerent environmen-tal factors (temperature, food quality and photoperiod) maycontribute to the body size diVerences between generations.Larvae of S. tetralunaria were reared under controlled envi-ronmental conditions from hatching to pupation in two tri-als performed in May and June 2007 (early season trial) andJuly and August 2007 (late-season trial). Each trial usedoVspring of at least two females of Estonian origin. Rearingtemperature was maintained at 21 § 1°C. Care was taken toensure that diVerent treatments did not diVer in mean tem-perature in a systematic way. Field-collected leaves of Bet-ula pendula Roth and Alnus incana L. (the latter only in theearly season trial) were provided as food to the larvae.

Leaves were young (and, presumably, of considerablyhigher quality) in the early season trial while only matureleaves were available for the late-season trial. In the earlyseason trial, the occurrence of individuals with diVerentdevelopmental pathways was incidental. In the late-seasontrial, to simultaneously induce diVerent seasonal genera-tions, two photoperiod regimes were initiated in tworeplicates: a short-day regime (12-h light, 12-h dark) and along-day regime (18-h light, 6-h dark). As typical of Lepi-doptera, a short photoperiod usually induces a diapausinggeneration while a long photoperiod gives rise to a directlydeveloping generation (e.g. Nylin et al. 1989; Windig andLammar 1999; Fischer and Fiedler 2001). However, as thephotoperiodic treatments did not induce a uniform responsein S. tetralunaria, the developmental pathway (diapausingpupae or not) was conWrmed on an individual basis. In rear-ing trials, pupal weight was used as the index of adult size.

The eVect of developmental pathway (though not of foodquality) on Wnal size could additionally be analysed on thebasis of data from analogous rearing experiments in 2006and 2008. Larvae of S. tetralunaria were reared simulta-neously on B. pendula under the two diVerent photoperiodsspeciWed above, and pupal weights were recorded. In thesetrials, we succeeded in eliciting a uniform response withinthe photoperiod treatments.

Results

Seasonal diVerences in body size between generations: patterns

In most species, the adults of the diapausing generation (i.e.the adults that had developed from late-season larvae, seeMaterials and methods) were larger, on average, than theadults of the directly developing generation (i.e. the adultsthat had developed from early season larvae; Table 1). ThediVerence between generations was largest in S. dentaria,in which males of the diapausing generation were 1.28times larger by linear measure than males of the directlydeveloping generation, with the respective ratio in femalesbeing 1.16 (Table 1). SigniWcant diVerences in male bodysize between the two generations were found in ten species(out of 12), and in female body size in four species (out ofnine; Table 1). In all cases but one (males of Phragmatobiafuliginosa; Table 1) in which signiWcance was attained, themoths of the diapausing generation were larger than indi-viduals having developed directly. The relatively smallernumber of signiWcant diVerences in females was, mostprobably, due to small sample sizes; for some species, nofemales were available at all (Table 1). The between-gener-ation size patterns were highly consistent across years: insix species, for which suYcient data were available,

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Oecologia

individuals representing the diapausing generation werenearly always larger than those of the directly developinggeneration (Table 2). The same data indicate relatively lowamong-year size diVerences in either generation (Table 2).

In general, intraspeciWc size diVerences between genera-tions were qualitatively similar in males and females, withthe exception of N. dromedarius, in which directly develop-ing females were somewhat larger than diapausing females,whereas males showed a reversed pattern (Table 1). Thecorresponding sex £ generation interaction term was, how-ever, not signiWcant (F1,52 = 2.2, P = 0.15) in a two-wayANOVA.

Seasonal diVerences in body size between generations: mechanisms

In the 2007 experiment, both food quality and develop-mental pathway (i.e. directly developing vs. diapausingindividuals) had a strong eVect on pupal weight in

S. tetralunaria. As predicted, irrespective of the develop-mental pathway the larvae reared on late-season foliageresulted in smaller pupae than those growing on youngleaves of early summer (Tables 3, 4). However, the eVectof food quality was clearly overridden by the develop-mental pathway chosen by the larva: irrespective of foodquality the pupae entering diapause were considerablylarger than those developing directly. In the replicatedlate-season trial, the pattern was quantitatively consistentamong the subsets (Table 3). Moreover, one of the char-acteristic developmental diVerences between the larvaeof diVerent generations was the diVerent number of lar-val instars prior to pupation. In particular, the majority ofthe directly developing larvae had four instars whilediapausing larvae usually developed through Wve instars,the diVerence in instar number being statistically signiW-cant (�2-test: �2 = 89.9, df = 1, P < 0.0001). The strongeVect of developmental pathway on pupal weight wasconWrmed both in 2006 (diapausing pupae 1.67 times

Table 2 Yearly generation-wise means of male forewing length (mm) in the studied moths. SuYcient data were available for six species and 3 years

The data indicated limited among-year variation in wing length, and consistency of the between-generation diVerences across years: thegeneration £ year(species) term in the respective ANOVA, though signiWcant (F12,1056 = 2.82, P = 0.0008), accounted for just 0.58% of the totalvariance in wing lengths. Sample sizes are given in parentheses

Species Generation 1995 2003 2004

F. lacertinaria Directly developing 16.1 (38) 16.1 (10) 16.0 (22)

Diapausing 16.6 (15) 16.4 (6) 16.9(2)

D. falcataria Directly developing 16.6 (37) 16.5 (101) 16.4 (29)

Diapausing 16.5 (7) 17.2 (20) 17.8 (3)

T. comae Directly developing 14.6 (140) 14.5 (79) 14.4 (23)

Diapausing 15.2 (14) 15.7 (13) 16.0 (3)

S. dentaria Directly developing 15.3 (107) 14.3 (15) 14.9 (35)

Diapausing 19.4 (21) 19.0 (9) 19.4 (4)

S. tetralunaria Directly developing 16.2 (133) 15.3 (45) 15.9 (73)

Diapausing 19.1 (13) 19.4 (14) 19.2 (8)

C. pigra Directly developing 11.5 (11) 11.1 (12) 11.7 (3)

Diapausing 11.8 (8) 11.6 (14) 12.0 (4)

Table 3 The dependence of pupal weight of S. tetralunaria on developmental pathway, sex and timing of rearing trials, type III GLM (SAS PROCMIXED; Littell et al. 2006; SAS Institute 2008) with brood (oVspring of an individual female) as a random factor

a Denominator dfb Calculated as SSerror/SStotal (SAS PROC GLM)c Diapausing or directly developing individualsd Early season (May and June 2007) or late season (July and August 2007); the data on two host plants were pooled as there were no signiWcanteVects; in the late-season trial, the photoperiod regimes to induce diVerent developmental pathways were applied in two replicates; the replicatehad a minor eVect on the results

EVect dfa r2 (%)b F P

Developmental pathwayc 1 8.0 76.4 <0.0001

Sex 1 19.5 185.2 <0.0001

Triald 1 2.6 24.3 <0.0001

Error 285 29.9

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heavier, P < 0.0001, n = 73) and 2008 (1.58 times,P < 0.0001, n = 70).

Discussion

In most of the 12 moth species examined, there were con-siderable intraspeciWc diVerences in body size between thetwo successive generations. The pattern was not random,however. In all statistically conWrmed cases but one, indi-viduals with their juvenile development late in the season(i.e. the diapausing generation) attained a substantiallylarger Wnal body size than individuals with their juveniledevelopment early in the season (i.e. the directly develop-ing generation). The pattern was broadly similar for bothmales and females within each species. The generality ofthe pattern suggests a general explanation: even if we can-not exclude the occasional importance of species-speciWcphenomena, these are unlikely to produce a pattern that islargely invariable across species representing just distantlyrelated phylogenetic lineages. Similar patterns have alsobeen found in some butterXies (Karlsson and Wickman1989; Fischer and Fiedler 2001) but, for that group ofinsects, the opposite pattern may be even more frequent(Tanaka and Tsubaki 1984; Wiklund et al. 1991; Scriber1994; Windig 1999). Ideally, therefore, the explanation forthe pattern characteristic of moths should also be able toaccount for the, at least occasionally, diVerent situation inbutterXies.

IntraspeciWc variation in insect body sizes has been mostfrequently associated with variation in food quality, tem-perature, time constraints, mortality risk and/or overwintersurvival. Seasonal changes in these factors are the mostplausible contenders to explain the diVerences between thetwo generations of moths as well. In what follows, we criti-cally evaluate respective explanations for the detected pat-tern, primarily contrasting the proximate (food quality andtemperature) and adaptive (time stress and overwinteringsuccess) mechanisms.

Food quality

The reported intraspeciWc diVerences in mean adult bodysize between successive generations cannot be assigned toseasonal variation in food quality. Indeed, there is plenty ofempirical evidence that quality of larval food of herbivo-rous insects, especially of those feeding on deciduous trees,declines with the progressing season (Schroeder 1986;Ayres and MacLean 1987; van Asch and Visser 2007).Hence, early season larvae typically feed on a more favour-able substrate than late-season larvae. The observed patternof between-generation diVerences in adult body size is thusinconsistent with seasonal changes in food quality: in thestudied species, late-season larvae attained a larger Wnalsize than the early season larvae.

Moreover, our original empirical data on one of the stud-ied species, the geometrid S. tetralunaria, suggest that foodquality is not the reason behind the detected size diVerencesbetween the generations. Indeed, the diVerences induced bythe season-speciWc food quality followed the typical pattern(smaller pupae later in the season, opposite to the naturalsituation). Even if the absence of among-year replicatescautions against overinterpreting our empirical results withrespect to the eVect of phenology-related food quality, theevidence of a strong eVect of developmental pathway(direct vs. diapause development) on pupal weight is con-vincing. A systematic diVerence in the number of larvalinstars between the developmental pathways furtherstrengthens the conclusion about the adaptive nature of theobserved size diVerences. Even if body size is a trait readilyaVected by various environmental factors, leading to plas-ticity which may well result from non-adaptive eVects (e.g.see reviews by Atkinson and Sibly 1997; Blanckenhorn andDemont 2004; for temperature), plasticity in the number oflarval instars is most likely an adaptive phenomenon(Esperk et al. 2007a, 2007b).

Temperature

In S. tetralunaria, the between-generation size diVerence,quantitatively corresponding to the natural one, wasattained in our rearing trial with standardized environmen-tal conditions. This allows us also to disregard seasonalvariation in temperature as the primary mechanism under-lying the size diVerences between generations in thatspecies. The decisive role of the proximate inXuence oftemperature is unlikely also in other studied species. In var-ious animals, including insects, individuals growing atlower temperatures tend to grow larger than those at highertemperatures (Atkinson and Sibly 1997; Angilletta et al.2004). However, in the study area, average temperatureduring the main developmental period of the late-seasonlarvae (August) is typically even somewhat higher than at

Table 4 Cross-tabulation of mean pupal weights § SE (corrected forsex by SAS PROC GLM, LS means; SAS Institute 2008) of diapausingand directly developing individuals of S. tetralunaria reared in earlyand late-season trial (see Materials and methods for details). Samplesizes are presented in parentheses

a Performed in May and June 2007b Performed in July and August 2007

Diapause developmentMean § SE (mg)

Direct developmentMean § SE (mg)

Early season triala 335.1 § 10.2 (21) 252.0 § 18.9 (6)

Late-season trialb 294.1 § 3.8 (103) 182.6 § 4.4 (112)

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the time of the early season larvae (June) (long-term aver-ages 15.5ºC and 14.5ºC, respectively; EMHI 2008). Thelarval period of at least some of the studied species may,however, extend into September (Svensson 1993), which isa considerably cooler month (average 11.1ºC, for Estonia;EMHI 2008). Nevertheless, if the proximate inXuence oftemperature was the main factor behind the between-generation diVerences, we would expect a highly irregularamong-year pattern as a response to weather conditionsspeciWc to each particular year. The limited data we have(Table 2), together with our Weld experience, do not supportthis scenario, though the possibility that temperature maycontribute to the within-season size diVerences remains.

Bird predation

Our Weld estimates of bird predation intensity (using artiW-cial prey items, modelled upon Lepidopteran larvae;Remmel et al. 2009; Remmel and Tammaru 2009) suggestthat a plausible selective agent behind the reported patternis season-dependent mortality risk during the juveniledevelopment. In the study area (hemiboreal forests ofnorthern Europe), the risk that larvae will be preyed uponby birds peaks in June, and then declines quite rapidly to aplateau value. The predation peak thus coincides with thelarval period of the directly developing generation. In latesummer, when late-season larvae appear, the daily preda-tion risk was estimated to be 3–5 times lower than at thetime of the peak (Remmel et al. 2009), and is likely to fur-ther decline after insectivorous birds have started to migratesouth. High juvenile mortality creates an obvious selectivepressure against longer developmental periods, and thus forsmaller adult sizes (Abrams and Rowe 1996; Benard 2004;Relyea 2007). We have demonstrated (Remmel et al. 2009)that realistic seasonal diVerences in the mortality risk are,indeed, large enough to lead to seasonal diVerences in bodysize of the magnitude reported in this study. In particular,increasing daily mortality from 5 to 10% is suYcient toelicit even a threefold reduction in pupal weight as an adap-tive response (see Appendix 1 in Remmel et al. 2009).

Overwintering survival

Alternatively, the larger body size in the diapausing genera-tion might be adaptively associated with size dependence inoverwintering success. Data on several insect species sug-gest that larger individuals may, indeed, have a greaterprobability of over-winter survival than smaller ones(Gotthard et al. 1999; Liu et al. 2007, but see Zvereva2002). Liu et al. (2007) suggest that this is simply becauselarger individuals can accumulate larger energy reserves,and the latter directly aVects the chances of survival. How-ever, additional empirical data are needed to conWrm the

potential of over-winter survival to create a selection pres-sure that is strong enough to explain size diVerencesbetween directly developing and diapausing generations.Indeed, for the time being, there appears to be no studycomparing the size dependence of pupal survival in diVer-ent development pathways.

Time stress

Another largely open question is can the two generationsexperience diVerent levels of developmental time stressfor reasons other than predation risk? DiVerences in justthe amount of time available for development could poten-tially lead to respective diVerence in (optimal) adult sizebetween the generations. However, if a species is regularlybivoltine in a predictable environment, there appears to beno reason to expect a between-generation diVerence in thelevel of time stress. In particular, the Wrst generation couldbe time stressed (to leave suYcient time for the secondgeneration to develop) only if the second generation isstressed as well (by the risk of not having suYcient time tocomplete the development). However, if the number ofgenerations is variable within a population, individualsrepresenting diVerent developmental pathways areexpected to have diVerent size optima, much in line withthe sawtooth pattern (RoV 1980; Nylin and Svärd 1991)and compound interest (Fischer and Fiedler 2002; Angillettaet al. 2004) hypotheses. Although there is no indication ofdeviations from the regular two-generation developmentin the studied species (Mikkola and Jalas 1977; Mikkolaet al. 1985, 1989; Marttila et al. 1996; Viidalepp andRemm 1996; T. Teder et al., personal observation), thepossibility may deserve attention in a more generalcontext.

Conclusion

We found a nearly universal pattern of between-genera-tion size diVerences in temperate moths, with diapausingindividuals being larger than directly developing ones.Based on our experimental data, we can largely ruleout that the qualitative patterns found are caused byproximate eVects of food quality or developmentaltemperature. Instead, our data suggest that adaptivemechanisms are involved. The estimated seasonal diVer-ences in predation risk are alone suYcient to elicit theobserved diVerence as an adaptive response though thecomplementary role of other factors cannot be excluded.Comparing life histories of the species used in this studywith those showing a reversed pattern of between-genera-tion size diVerence (various butterXies) may provide auseful contrast for further distinguishing between alterna-tive hypotheses.

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Acknowledgements We are thankful for the possibility to use thematerial accumulated through the Estonian Environmental MonitoringProgramme, coordinated by Erki Õunap. We thank Rein Karulaas andTaavet Kukk for technical help. The study was supported by the Esto-nian Science Foundation (grants 6619, 7406 and 7522), the EstonianMinistry of Education and Science (targeted Wnancing projectSF0180122s08) as well as by the European Union through the Euro-pean Regional Development Fund (Center of Excellence FIBIR). Allthe experiments complied with the laws of Estonia.

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