Population growth studies were carried out with Tetranychus urticae(Koch), on clover under laboratory conditions. The population growth potential was studied in relation to various constant temperatures and relative humidities.
Complete development of summerform individuals was possible over the temperature range from 16.5 to 34.5°C at both high (85 to 90 per cent.) and low (25 to 30 per cent.) relative humidity. Rate of development was greatest at 34.5°C but fecundity was greatest at 25.5 or 29.5°C depending on relative humidity. The intrinsic rate of natural increase (rj^) reached a maximum of 0.282 per day at 34.5°C and low relative humidity. Its value depended greatly on the rate of immature development. Values for r were greater at low relative humidity rather than high relative humidity at any one temperature, but the difference was only of the same magnitude as that due to 2°C difference in temperature. A simple method of obtaining provisional values of rm required for the calculation of accurate values is suggested.
The total fecundity and longevity of post-winterform females after they had overwintered under field conditions tended to be slightly less than that for summerform females. High relative humidity was more detrimental to post-winterform females than to summerform females.
The population growth potential was also determined using detached leaves from trees of commercial apple varieties in the Stanthorpe district of Queensland. The varieties with high rm values were the varieties most seriously infested in the field. The technique would provide a means of assessing resistance of new apple varieties to T. urticae infestations.
Field populations were sampled on three apple fertilizer trials. Populations increased with applied nitrogen and applied phosphorus but decreased with applied potassium. Laboratory studies with detached leaves suggested that increased numbers of progeny were produced by females feeding on leaves from trees receiving applied nitrogen. Variability was high and the laboratory technique appeared relatively insensitive. Reducing sugar content of the leaves was not significantly affected by the mineral nutrition and was not correlated with population growth potential.
T. urticae populations in a Delicious apple orchard were studied over a year. Approximately 220 winterform individuals per tree successfully overwintered, chiefly in litter near the base of the tree. Alteration of the concentration of litter during spring altered leaf populations on the apple trees during summer. From the laboratory data on population growth potential a population per tree of 206,000 was possible by January. Actual sampling indicated that the population was 124,000 mites per tree or 60 per cent of the theoretically possible total.
Approximately 23,000 winterform individuals per tree were trapped during autumn. During the winter the mortality of these winterform individuals in cages simulating field conditions was approximately 99 per cent. This suggests a spring population of 230 winterform individuals per tree which was quite close to the initial population.
Females in which the winterform stage was induced early in the autumn failed to overwinter. High populations cause leaf damage which in turn results in premature induction of the winterform. This is suggested as a mechanism by which populations are reduced under conditions of high infestation.