The Influence of Rainfall, Evaporation and Atmospheric Temperature on Fluctuations in the Size of a Natural Population of Thrips imaginis (Thysanoptera)

Abstract

Daily counts of the numbers of T. imaginis in roses in the garden at the Waite Inst., Univ. of Ade-laide, have been made continuously for 14 yrs. The roses used for sampling were highly attractive to the adult thrips but were not favorable for the development of immature stages. They served as a ''trap'' which gave a satisfactory indication of the density of the population in the area. The method of partial regression was used to measure the degree of association between the numbers of thrips present during the spring and the weather experienced during the preceding months. The analysis showed that 78% of the variance of the population could be related to weather. The max. density attained by the population in the spring is largely detd. by the weather during the preceding autumn, but rainfall and temp. during the early spring may modify the tendencies established in the autumn. ''Competition'' plays little or no part in determining the max. density attained in the spring. Rapid multiplication is possible only for a limited period during spring and early summer. During this period the insects increase rapidly but so does the number of situations available to them. The favorable period normally ends long before the thrips have had time to saturate the environment. The onset of summer drought brings about a ''crash'' and the numbers decline to the low level characteristic of late summer and winter.-The ''balance'' of the population is seen as a race against time with the increase in density being carried further in those years when the favorable period lasts longer, but never reaching the point where competition begins to be important. The annual fluctuations in the max. density attained by the population are controlled almost entirely by ''density-independent'' components of the environment. During the period each yr. when the population is decreasing, weather is still the more important influence at work to determine the density of the population. The population never decreases to zero, partly because during this phase of the ''population rhythm'' weather operates as a ''density-dependent'' component of the environment, and partly because the spring supervenes and the population begins increasing under the influence of the changing environment. The movement of the thrips into the flowers is influenced by the daily weather. The movement was greater during warm, dry days and less during cold, wet ones.