Vergara - 1976 - Physiological and morphological adaptability of ri

350 CLIMAtTE AND RICE
constant.” ivhich tiriginated with the botanist De Candolle (1855). was applied
to insects by Abbe (1878).
The principle of temperature summing and the thermal constant is based on
the premise that the developmental rate of an organism is affected by temperature
in a predictable, linear way at temperature levels above a certain minimum
called the threshold or base temperature. No development is considered to occur
below the threshold. If natural temperatures are summed. daily averages rather
than values actually encountered i11 the diurnal cycle are used. For insects this
assumption was accepted and evaluated both in Europe (Blunek, 1914'. Krogh.
1914) and in America (Sanderson, 1908. 1910', Sanderson and Peairs, 1914).
Ilowever. when constant temperatures ivere used under closely controlled conditions,
several investigators (Loeb and Northrup, 191?; Shelford. 1917) cautioned
that the temperature-development relationship for insects and other
cold-blooded animals seemed to be curved. particularly near the threshold. The
use of temperature summing. and all that it implies. is current today in insect
bioclimatie studies. and the utarnings of curvilinearit)". of real development below
the threshold. are also reported periodically Bioclimatic investigators must
take care to note this controversy (Messenger. 1959; Baker, 1972)‘
That temperature also affected other vital life processes besides development
utas also beginning to be recognized at the onset of the 20th Century (Bachmet-
Jew‘. 1901). The first quantitative evidence for an effect of temperature on insect
reproduction was provided by Hunter and Glenn (1909) in respect to the grain
aphid. Schizaphis gramirmm (Rondani). By rearing the grain aphid in temperaturc
incubators. they established that maximum reproduction occurred at 20°C.
while at lower or higher temperatures reproduction was less. and that at the
extremes. near 0° or 29°C. reproduction ceased.
CORRELATTVE STUDIES OF INSECTS AND CLIMATE
The first bioclimatic application of the knowledge of the quantitative relation
between plant and animal development and temperature was made by Merriam
(1894), who invented the fife zone concept to account for the differences in geographic
distribution of species. He concluded that a species would be limited in
its poleward distribution by the duration of and the amount of heat available in
the growing season. That is to say. the species must be able to develop to maturity
and reproduce during the warmer seasons of the year. and therefore will
only be able to persist in places where temperature summations during the warm
period are equal to or greater than the thermal constant for the species. He
concluded further that species should be limited in their equatorward distribution
not by temperature sums. but rather by the summer intensity of heat and its
lethal effects on insect survival. Ile assumed that heat extremes in the summer
could be represented by the mean temperature of the 6 hottest weeks. For a
species. places with summer temperatures hotter than this limit would not be
suitable.