An Analysis of Threshold Temperatures for the Development of Oncopeltus and Tribolium Eggs

Abstract

Eggs of Oncopeltus fasciatus and Tribolium confusum were incubated at various controlled temperatures either constantly at one temperature or at various alternating temperatures. Experiments were terminated at hatching or failure thereof. The time temperature curves for development are not true hyperbolas and hence hatching thresholds have to be determined experimentally. At constant temperatures and 75% R. H., hatching of O. fasciatus eggs averaged 24% at 15[degree], 65% at 16[degree], 82% at 17[degree], about 90% at 20-30[degree], and 13% at 35[degree], but zero at 14[degree] and 38[degree]. For T. confusum eggs hatching averaged zero at 16[degree], 21% at 17[degree], about 78% at 20-25[degree], 80% at 30[degree], 88% at 35[degree], and 27% at 40[degree]. When eggs are placed at temperatures slightly below threshold for a portion of development, both histological examination and time considerations show that a large fraction of development occurs at subthreshold temperatures even though no eggs hatch. Accordingly, summation of temperatures above a constant temperature threshold cannot be used to predict hatching dates accurately unless allowance is made for fractional development occurring at subthreshold temperatures. Lower and higher humidities are unfavorable and raise the threshold temperature several degrees. There is little or no acceleration of development due to alternating temperatures, in contrast to constant temperatures, provided the calculations include percentages of development at each temperature used. If only one or a few shifts in temperature are made, hatching will occur when about half of the calculated development takes place at subthreshold temperatures. But if shifts are made daily to a favorable temperature, then some hatching will occur when as little as 4% of the developmental time is at an above-threshold temperature. Over 99% of eggs hatching at minimal temperature conditions subsequently die without the insects reaching maturity even though the larvae are kept under favorable conditions. Attempts to analyze the nature of the threshold temperature effect gave negative results but eliminated some possibilities. The subthreshold temperature does not block embryonic development or tissue differentiation of the hatching process per se. There is no "sensitive period" anywhere in development for the deleterious effect of slightly subthreshold temperatures. Low hatching percentages at near minimal conditions seem not to be due to selection within a genetically diverse population. It seems that minimal conditions give debilitated individuals almost all destined to die before, during or after hatching. Clearly hatching, while convenient to measure, is not a good index of conditions under which the spp. can perpetuate itself.