Download Extreme Individuals in Natural Populations

Extreme
Individuals in
Natural
Populations
H. V Danks
Abstract
Extreme individuals (forexample, the peripheral 5% or less of the population) may have specificadaptive values, particularly in unpredictable environments. The extremes, stemming from broad, especially asymmetrical, variation or from genetic polymorphism, may therefore
be actively selected, rather than reflecting simply general variation about some mean value, or environmental and genetic accidents.
Evidence from several fieldsof entomology supports this possibility. Consideration of extreme variants may contribute important information
about the structure of populations. The potential significance of extreme variation should therefore be considered explicitly during ecological
experiments of all kinds, rather than dismissed routinely when central measures such as the mean are emphasized.
M
ostwork in the biologicalsciences
bases generalizations or comparisons on the normal behavior
or state of the populations under study.
Because this is so, "central" measures such
of biological material also encourages neglect of extreme data, for variation adds to
the difficulties of detecting real differences
among populations. Differences between
population means are normally considered
as the mean are emphasized.
significant statistically, therefore, when the
Nature and Meaning of Extreme
likelihood is as great as 5% that the differences resulted by chance. "Extreme" individuals might thus conveniently be defined as
those from the peripheral 5% of the population, although they may be very significant even when their incidence is much
lower than 5%.
Two questions arise in examining variation. First, what are the relative contributions of environmental conditions and genetic differences, or their interactions, to the
observed variation among individuals? Second, and the main theme developed below, are the variations adaptive as varintions in any distinctive sense, rather than
reflecting merely the range of values about
some mean value that is adaptive? This essay illustrates the likely adaptive or ecological significance of extreme values of several different kinds, and points out that in
some circumstances extreme individuals
have an importance out of proportion to
their numbers. Whether or not extreme individuals are ecologically important should
not, therefore, be left only for geneticists to
consider. In many other kinds of study, the'
significance of extreme individuals should
be addressed explicitly, rather than simply
Variation
Variation
among individuals is most often expressed
in terms of the spread on either side of the
mean (e.g., standard deviation). This procedure indicates or assumes that the frequency distribution of the measurements
essentially is continuous, as in a normal
distribution. Variation may also appear as
more than one different form (polymorphism) or as scattered extreme or unusual
forms differing greatly from the norm.
These cases are reflected in frequency distributions as modality, and as discontinuous or unusually prolonged and often
asymmetrical "tails" to the distributions,
which cannot be described satisfactorily by
measures derived from their variance.
Little attention has been paid by most
entomologists who are not geneticists to the
meaning of the "extreme" or "atypical" individuals in normal, polymodal, or peripherally extended forms of variation. Indeed,
the extremes are sometimes deliberately
avoided or omitted, or data on them are
otherwise suppressed during data collection
or analysis on the assumption that they represent merely "noise" irrelevant to the
main pattern. This assumption is not always justified. The variation characteristic
SPRING 1983
dismissed through conventional procedures
that emphasize the properties of central individuals.
Extremes Resulting from
Environmental Van'ation
For natural populations, the belief that
variation stems from chance environmental
circumstances is more frequently assumed
than proven. Certainly, manipulating laboratory conditions can greatly change individual responses. Starvation of larvae of the
midge Chironomus decorus slows their
development and retards the emergence of
adults until more food is available, and this
distorts the emergence curve so that it
becomes asymmetrical (see Fig. 3 in Danks
[1978a)). A somewhat less restricted food
supply partitions the emergence into two
sections, before and after supplementary
feeding.
Such dramatic effectsof distinctly different environments have been demonstrated
in other experiments (for further examples
see, e.g., pp. 139-144 in Mayr [1%3)).
However, the environmental contributions
to variation, including the general deviation
about the mean within a treatment, often
cannot be distinguished from genetic components.
41
Vanation in 'Time
Seasonal emergence patterns vary in the
degree to which the emergence of the members of the population is synchronized (see
Corbet [1964] for an introductory review).
Emergence patterns may also differ in the
degree of symmetry, especiallywith respect
to peripheral individuals which appear well
after the main emergence has taken place.
Thus, Ulfstrand (1969) showed that most
individuals of some stonefliesfrom northern
Scandinavia emerged together, but a few
individuals, especially females, emerged
sporadically much later than the rest. Such
asymmetry can often be eXplained in
general terms. For example, except within
certain limits, development is more readily
retarded than accelerated, since acceleration beyond a certain point is physiologically impossible. This does not explain
whether the extremely retarded individuals
result from substandard environmental circumstances, or from delays that are
genetically programmed, with or without
environmental cues. However, several examples suggest the genetic explanation.
Eggs of some species of stoneflies hatch
irregularly, following a variable period of
dormancy. Single egg masses produced
small numbers of hatchlings at intervals
over periods of more than a year in some
species from streams in Australia when the
eggs were maintained under conditions of
photoperiod and temperature that approximated seasonal progressions in the streams
(see Table 4 in Hynes and Hynes [1975]).
This extended hatching period appears to
be an adaptation to the fact that rainfall is
unpredictable (Hynes and Hynes 1975).
Several experimental studies of dormancy in chironomid larvae (summarized by
Danks [1978a])have shown that a few individuals become dormant under conditions
that normally permit emergence, and small
numbers of individuals emerge under conditions that inhibit emergence in their siblings. These "atypical" responses involve
only 1 to 5% of individuals, and are
thought to insure the main population
against intermittent catastrophes (see Tables 1 and 2 and p. 297 in Danks [1978a]).
In the saturniid moth Hyalophora
cecropia, however, later-emerging individuals comprise a distinct second peak of
emergence, which is usually much larger
than the first peak (Waldbauer and Sternburg 1973, Sternburg and Waldbauer
1978).This bimodal emergence, also due to
dormancy responses, was interpreted as
adaptive to possible differences in weather
42
or other factors from one season to the next;
for example, early emergents might be
more successfulin a year when late summer
was dry, but later emergents might be at an
advantage if the early summer was cold.
Similar temporal polymorphism, apparently adaptive to annual differencesin spring temperatures, occurs in the chaoborid
Chaoborus amen'canus (Bradshaw 1973).
In this species, the morph that responds
rapidly to long days and food is favored in
warm, continuous springs. The more conservative responses of the "slow" morph
cause it to predominate after springs in
which the pond habitat refreezes after an
early thaw.
On a longer time scale, prolonged dormancy (dormancy lasting for more than
one adverse season) partitions the population into several cohorts that are not equally
susceptible to conditions in any single growmg season.
Prolonged dormancy is relatively common in several situations: in the arctic (p.
288 in Danks [1981]), where summers are
very short and temperatures vary close to
the limits of development, so that the
available growing season is uncertain;
among cone-feeding insects (Hedlin et a!.
1981), which depend on cone crops that
may vary greatly from season to season; in
some other phytophagous species, such as
sawflies (e.g., Philogene 1971), which depend on foliage development; in certain
tropical or desert moths subjected to
seasonal drought (e.g., Powell 1974),which
depend on rainfall to produce plant growth
suitable as larval food; and in certain Aedes
mosquitoes inhabiting temporary pools
(e.g., Clements 1%3), in which the supply
ofwater to support larval development may
not be adequate every year. Some larvae of
'Trogoderma spp. (dermestid beetles infesting stored products) enter prolonged
"dormancy" in which starvation can be
resisted by retrogressive moulting with a
decrease in size (e.g., Beck 1971).Development can thereby be interrupted for years if
necessary.
Variation in the duration of dormancy
over the long term thus appears to be an
adaptation to unpredictability, and so
parallels similar variation in emergence
time within a season.
Although individuals can remain dormant for up to 12 years in some species of
Sitodiplosis gall midges (Barnes 1943),dormancy persists for many seasons in relatively few individuals. Rather, most emergence
takes place after one or two adverse seasons
only. For a given generation, the complete
pattern of emergence over a period of
several years is therefore asymmetrical.
The longest-lived individuals are able to
reproduce even if conditions are locally or
temporarily adverse, but their deferred
reproduction reduces the potential rate of
CUlTentpopulation growth. They may also
use nutrient reserves that otherwise would
contribute to egg development. Denlinger
(1981) showed that female flesh flies, Sarcophaga crassipalpis, that had passed the
winter as pupae in diapause were much less
fertile than those which developed directly.
Such costs explain why individuals with extremely delayed development are relatively
rare; resistance is usually obtained only at
the expense of other characteristics, for example, reproductive output, so that the
selectiveadvantage of delayed development
is even smaller than might be expected for a
given chance of catastrophe when development is not delayed.
Van'ation in Space
Population studies normally focus on
areas of maximum density. However, areas
of low population might reflect not only unsuitable habitat, but also recent immigration or reservoirs of recolonization, both of
which may be important to future population development. Spatial differences may
also interact with the quality of local
populations; individuals of the moth
Malacosoma pluviale (Lasiocampidae) differ in larval activity and adult dispersal
ability (Wellington 1964), and these trflits
are represented in different proportions in
different localities.
Areas of low density may thus figure in
the dynamics of populations. The least
suitable sites for growth of the introduced
weed Hypen'cum per/oratum (St. John's
wort or klamath weed) in some regions of
North America-shaded sites-are even
less suitable for the imported phytophagous
chrysomelid
beetle
Chrysolina
quadngemina, so that stands of the plant,
formerly extremely abundant in open
pastures but now controlled there by the
beetle, have become virtually confined to
shady habitats (Huffaker 1957). In other
words, the "extremes" became important
when conditions changed.
Such heterogeneity is important for
understanding population processes, as
evidenced by Huffakers (1958) classic experiments in which phytophagous and
predaceous mites coexisted only if the exBULLETINOFTHEESA
peri mental universe was sufficiently
heterogeneous. The phytophagous species
could then continuously establish subpopulations that were not immediately
found and extinguished by their predators.
In other words, coexistence depended on
variation in space and not simply on the
mean density.
Nevertheless, sampling procedures are
usually designed to measure the mean density of the population most accurately by
reducing variation among samples. Considerable effort has been put into testing
and refining procedures to obtain valid
estimates of mean population size, and
often estimates that are relatively imprecise
require very large numbers of samples, particularly for smaller and more aggregated
populations (e.g., Resh 1979, Roberts et al.
1982). Samples with small numbers of individuals are normally considered significant chiefly because they increase the
variance of the estimate, however, not
because they indicate heterogeneity of
potential significance in population processes.
Variation in space is also reflected by
variation in dispersal ability within species.
In the milkweed bug Oncope/tus fasciatus,
in which dispersal and reproductive diapause are concurrent, a spectrum of diapause-dispersal responses has been identified(Dingle et al. 1980,etc.). Where habitats-which determine foodplant suitability-change
unpredictably, individuals
vary widely in their response to photoperiod, but photoperiod determines dispersaldiapause rather reliably in populations of
bugs from regions where habitats are more
stable. Frequency distributions of the delay
of reproduction (for more than 200 days in
rare individuals) are often asymmetrical (d.
Fig. 2 in Dingle et al. [1980]).
Increased dispersal-or diversification in
space-therefore corresponds to the unpredictability of habitats (d. Southwood 1977);
it is analogous to the diversification in time
already attributed to unpredictable circumstances. Individual insects adapted for
long-distance dispersal may differ greatly
from normal individuals, since the dispersal
faciesincludes modifications of reproductive
status, longevity, size, form (e.g., wings),
and behavior. As in the case of individuals
that defer emergence itself (see above),
deferred reproduction in adults generally
reduces individual reproductive output.
In sum, individuals that are extreme in a
spatial context, as represented by rare longdistance migrants or denizens of sparsely
SPRING 1983
populated habitats, may contribute to the
long-term success of the species.
Van'ation in Physiology
Physiological characteristics such as
longevity and fecundity that interact with
environmental changes in time and space
have already been noted. Individuals resistant to certain extreme conditions are noted
here.
Only a few highly resistant individuals
may survive applications of insecticide.
Mortality responses are usually expressed
by indicators of the median such as LD50
(dosage required to kill half the population),
and often by LD90, but this gives no information on the dose required to kill the last
few individuals. Again, central measures
such as LDSO are used to compare
treatments at low temperatures. Nevertheless, Salt (1970) showed that the frequency distributions of freezing points in
freezing-susceptible
species were not
always normally distributed. Indeed, a few
individuals may survive levels or durations
of cold temperatures that kill all others.
Kuuzik and Kopvillem (1970) indicated
that more than half of the (freezingsusceptible) eggs of the European pine
sawfly, Neodiprion sertlfer, froze at
- 35°C within a few hours, but up to 25
days was required for all eggs to freeze.
Especially resistant individuals are
responsible for the evolution of resistance to
insecticides (e.g., Georghiou 1972), and the
development of cold-hardy strains. Individuals that are very resistant to cold
would also be favored in latitudes or
habitats that make unpredictable demands
for cold-hardiness from one season to the
next. However, these individuals would be
expected to constitute a minority of the
population, since higher levels of coldhardiness appear to be more costly
metabolically (see pp. 1183-1184in Danks
[1978b]).
Variation in Size
Within some species, especially those
such as carrion-feeding insects that develop
on somewhat ephemeral food supplies, individuals emerge successfully over a wide
range of sizes. Although emergence may
take place only beyond a minimum size
(e.g., p. 26 in Klomp [1964]), the lower extreme in such species is much smaller than
the average (e.g., a puparial weight of 15
mg, compared with a weight of 50 mg in
uncrowded individuals of the blow fly
LUCl1iasericata [Ullyett 1950]). Temperature as well as food can change the mean
size and its variation in different populations. In some aquatic species, larvae grow
more slowly and reach a greater size at
lower temperatures (e.g., Mackey 1977).
This variation in size usually indicates
simply that the genetic program allows
wide phenotypic flexibilityin the face of environmental constraints, although perhaps
extremes of size are also programmed
genetically as fixed morphs. In any case,
great flexibility of size appears to correspond (as in blow flies) with uncertain
resources of food or temperature. Downes
(see p. 297 in Downes [1964]) drew attention to similar flexibilityin some arctic butterflies, in which dwarf specimens emerge
successfully; dwarf individuals are very
rarely met with in related species from
temperate areas, where conditions are more
predictable.
Size interacts with rate of development
(in some species, smaller individuals
develop faster and so reproduce earlier),
larval food supply (smaller individuals require less food), and fecundity (smaller individuals are less fecund). It is only the extreme individuals that reproduce (albeit at a
reduced level)when conditions are poor, or
that fully exploit unusual riches, and these
individuals may also escape the temporal
constraints of the majority which is
destroyed in especially adverse years (see
above).
Van'ation in Reproductive Pathway
Some individuals of normal sexual
species can reproduce by parthenogenesis
(e.g., p. 291 in Oliver [1971] for several
species of ticks; Bergerard [1962] for some
stick insects). Rare parthenogenetically
produced individuals are often lessviable in
early instars than sexually produced forms,
and may then comprise only a few percent
of the adJlt population (e.g., pp. 507-508 in
Kaufman [1970],for the chrysomelid beetle
Pyrrhalta nymphaea). Several other sexual
species produce small numbers of parthenogenetic individuals (e.g., Grodhaus
[1971] for some Chironomidae), but large
samples have not been reared to determine
whether any individuals develop into viable
adults. However, sporadic parthenogenesis
as in P. nymphaea would allow at least
some species to reproduce when males are
temporarily absent.
Other species reproduce parthenogene43
tically most of the time, which is one way of
ensuring rapid increase, but occasional individuals reproduce sexually (e.g., Ananthakrishnan [1979] for some Thysanoptera). Rare males of uncertain status occur
in several otherwise parthenogenetic species
(e.g., Corbet [1966b] for the caddisfly
Apatania zonella), and although the rare
males of the beetle Micromalthus debiiis
(Micromalthidae) are sexually functionless
in North America (see Smith 1971), occasional production of males in other species
may ensure genetic recombination. Such a
system occurs in most aphids as an annual
event in which sexual reproduction is confined to a particular morph that appears
late in the year, and its adaptive value has
been commonly accepted. In the aphid
Myzus persicae in southern England, some
parts of the population suppress sexual
reproduction even further, continuing to
reproduce parthenogenetically in fall, but
also producing a few males (d. pp. 291-292
in Blackman [1972]).
The rare appearance of alternative reproductive pathways may well be an essentiallife cycle strategy for some other arthropods, but because it is rare tends to be considered as an "accidental" phenomenon of
limited interest. The value of alternative
pathways has been fully recognized only
when the alternative is relatively frequent
(e.g., "facultative parthenogenesis").
Treatment of Extreme
Variation
The preceding examples suggest that extreme variation is often adaptive rather
than "accidental." However, several conventional means of selecting material and
measurements or treating data discriminate
against extreme individuals.
For example, limiting study to relatively
homogeneous laboratory strains (or even
clones) facilitates comparison between the
effects of experimental conditions, but the
results may not apply to natural populations in which variation is much greater.
Some sampling techniques introduce
bias against extreme individuals and thereby invalidate the assumption that samples
are random. Even ordinary searching for
specimens (or sorting of samples) allows
habituation to a search image, as in birds
and some other predators (d. Tinbergen
1960, Holling 1965), so that "different"
specimens are more easily overlooked than
"normal" ones, especially if both types are
relatively cryptic. Similarly, more in44
dividuals are overlooked at the lowest densities (Morris 1955).
The deliberate or inadvertent selection of
measurements may introduce unwanted
bias. The efficiencyof some techniques can
be improved by eliminating samples when
no individuals are likely to occur, but such
choice of sampling techniques or sites may
miss unusual individuals: sampling only at
night for a nocturnal animal overlooks occasional activity during the day (for example, light traps collect only night-flying
specimens); particular mesh sizes may lose
unusually small (or retarded) individuals
from sieved samples; emphasis on particular host plants or host animals, such as
crops or pests, may undervalue the significance of alternative hosts. despite their
potential value as occasional reservoirs of
pests or parasitoids.
Arbitrary class boundaries set up during
measurement or analysis, especially "larger
than" and "smaller than," can also obscure
extreme variation at the expense of central
classes. Peripheral individuals may thereby
be included in groupings which are heterogeneous, though this cannot always be
determined from limited numbers of measurements.
Even when they are not obscured in this
way, extreme variants have often been dismissed with little explanation during analysis. For example, Brittain (see p. 122 in
Brittain [1982])dismissed the occurrence of
occasional parthenogenesis in mayflies by
claiming that "because of the low level of
hatching success, [non-obligatory] ... parthenogenesis is unlikely to be of importance
in population dynamics." Again, Stinner et
al. (see p. 1170 in Stinner et al. [1975]),
developing a model for insect development,
omitted the few very slow individuals with
the statement "Since one or two individuals
often develop extremely slowly (presumably
due to injury or genetic inferiority), we considered ... the time required for 99%
rather than 100% of the population to develop." This simplification may well be justified to develop a tractable model; it may
be justified also when the model is used in
an ecological context, but then it deserves
explicit rather than parenthetical consideration.
The greatest difficulty in treating extreme variation is simply that enormously
large samples are required to discover
whether certain types of extreme individuals occur consistently and to inventory their true proportions. It is more convenient to regard one or two unusual in-
dividuals out of several hundred as spurious
or accidental, representing merely experimental noise. The many thousand individuals that this might portend in natural
populations of most arthropods may nevertheless be significant. Study of peripheral
variants, therefore, requires specificor protracted sets of data. Detailed information
on several populations of several species of
Oncopeltus bugs (Dingle et al. 1980)shows
how flight capacity is related to several
other life cycle variables including the time
of reproduction. This exemplifies the
broader approach that is required to test alternative explanations for extreme variants.
Discussion
The frequent occurrence of extreme
forms for which adaptive value has been
suggested in this paper confirms that
populations are dynamic; survival may
thus depend on extreme individuals. Such
extremes include late emergers or "stragglers," individuals with greatly prolonged
dormancy, scarce vagrants, survivors after
intense or prolonged physical or chemical
adversity, very small individuals, and unfamiliar sexual forms. Extreme forms are
often associated with unpredictable environments.
In most of these cases extreme individuals, including those outside the expected range of a normal distribution, are
characteristic rather than atypical. Usually
they appear to represent "insurance"
against infrequent but not unexpected
catastrophes that may kill the majority of
central individuals. Their very rarity accords with this insurance function. When
the main population is destroyed, the few
extreme individuals gain disproportionate
significance, for they may produce a major
fraction of the next generation. Consideration of extreme individuals may therefore
contribute useful information about
populations, and these individuals cannot
be regarded simply as a nuisance and
dismissed as "noise."
The adaptive value of extreme individuals has been most readily accepted
when they are relatively frequent, and
when their characters are closelyconnected
to the genotype (e.g., parthenogenesis) or
confer survival under heavy selection (e.g.,
insecticide resistance, industrial melanism).
Other characteristics are controlled by
genes, but the selective value of extreme
variants may be less obvious. Although the
extremes can safelybe ignored or attributed
BULLETIN OF THE ESA
to environmental vanatJon in some circumstances, depending upon the questions
being asked by the investigator, I believe
that the possible significance of peripheral
variants has been insufficientlyappreciated.
Although the genetic mechanisms by
which the variation can be maintained are
beyond the scope of this essay, these ideas
are consistent with the realization that
selective forces interact in complex ways.
For example, the selective value of some
characteristics (size, form, behavior, etc.)
may be inversely related to their frequency
in the population. In such cases-"evolutionarily stable strategies" (d. Blum and
Blum 1979, Maynard-Smith 1981)-the
rarity of extreme individuals would be
maintained by active processes, rather than
resulting simply from chance departures
from some mean value. Again, the dynamic nature of selection for realized reproductive capacity, involving trade-offs in time
and space among rates of development,
size, mating strategies, and so on, more accurately reflects reality than artificially
homogeneous measures of mean fecundity
(see also Labeyrie 1978).
The characteristic genetic variation of
organisms has usually been attributed
chiefly to the fact that many genes in most
generations are not fully expressed phenotypically and so are protected from direct or
drastic selection in various, mainly genetic,
ways; a lesser and rather general role in
maintaining variation has been accorded to
ecological protection and disruptive selection caused by diversity of the environment
(e.g., pp. 237-252 in Mayr [1%31, pp.
116-117 in Dobzhansky et al. [1977]). In
this view, variation contributes in selection
chiefly by providing a source of general
variation about the population mean, by
which that mean can be shifted. The examples cited above suggest that variations
as such, and even extreme individuals of
certain types, are also selected and maintained as part of the requirements for persistence in time and space. In other words,
variation itself not only is raw material for
evolutionary change, but also can be
stabilized in the population for its current
adaptive value.
That general point prompts a specific
recommendation: the basis for the treatment of variation in biological studies
should be explicitly considered, lest the important information that can be provided
by taking account of extreme individuals is
lost through routine emphasis on central
measures.
SPRING
1983
Acknowledgment
Several authors nurtured these ideas
because their otherwise interesting papers
failed to follow my closing recommendation; Anthony Downes encouraged me to
write the ideas down. I also thank him and
John Spence for helpful comments on the
manuscript.
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Georgbiou, G. P. 1972. The evolution of resistance to
pesticides. Annu. Rev. Eco!. Syst. 3: 133-168.
Grodhaus, G.I971. Sporadic parthenogenesis in three
species of Chironomus (Diptera). Can. Entomo!.
103: 338-340.
BioI. 1980. 4R5 pp.
Mayr, E.1963. Animal species and evolution. The Belknap press of Harvard University Press, Cambridge, Mass. 797 pp.
Morris, R. F.1955. The development of sampling techniques for forest insect de/oliators, with particular
re/erence to the spruce budworm. Can.]. ~!. 33:
225-294.
OHver, J. H., Jr. 1971. Parthenogenesis in mites and
ticks (Arachnida: Acan). Am. ~!.
II: 283-299.
Pbilogene, B. J. R. 1971. Revue des travaux sur les
formes de dl'apause chez les 'Tenthredinoides
(Hymenopteres: symphites) les plus communs.
Ann. Soc. Entomo!. Que. 16: 112-119.
Powell, J. A. 1974. Occurrence of prolonged dt'apause
in ethmiid moths (Lepidoptera: Gelechoidea).
Pan.-Pac. Entomo!. 50: 220-225.
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features: basic considerations in the design of
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Roberts, S. J., R. D. P~
R. J. Barney, and E. J.
Armbrust 1982. Effect of spatial distn'bution on
determining the number of samples required to
estimate populations of Hypera postica, Sitona
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Salt. R. W.I970. Analysis of insect freezing temperature distributions. Can. ]. ~!. 48: 205-208.
Smith. S. G. 1971. Parthenogenesis and polyploidy in
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Stemburg. J. G.. and G. P. WaIdbauer. 1978. Phenological adaptations in diapause termination by
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Appl. 23: 48-54.
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30
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