THE Influence of the manurial treatment of orchards on the population density of metatetranychus ulmi (koch) (acari, tetranychidae)
1959
BREUKEL, LUUS M. | POST, ANN
The influence of the nitrogen-content of apple leaves on the development of the fruit tree red spider mite (Metatetranychus ulmi (Koch)) was studied under laboratory and semi-field conditions. The experiments in the laboratory showed that a higher egg production per female in 24 hours, a lower mortality rate of the eggs, larvae and nymphs, and a shorter period of development of the male as well as the female mites was obtained on the leaves with a high nitrogen-content. However, these differences were statistically significant only in the case of egg production and mortality rate. From the experiments under semi-field conditions it became evident that the egg-female ratio on leaves with high nitrogen content originating from manured rootstocks was also significantly higher than this ratio on leaves with a low nitrogen content. In both series of experiments the differences gave a factor of 1.6. The experiments confirm the results, we obtained earlier from perennial field trials where an effect of manuring of the apple trees on the development of M. ulmi was found. It is concluded that the mortality of predators of M. ulmi, caused by chemical treatment, is not the only reason why the red spider mite populations are stimulated in well kept orchards. The higher level of nitrogen content of the leaves, brought about by cultural practice, in particular manuring, may be of considerable importance also.
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