414 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 5 



THE OCCURRENCE OF THE CITRUS RED SPIDER, 



TETRANYCHUS MYTILASPIDIS RILEY, ON STONE 



AND POMACEOUS FRUIT TREES IN OREGON 



By H. E. EwiNG, Agricultural Experiment Station, CorvaUis, Ore. 



In November of last year, while examining some leaves and twigs 

 of apple trees at Corvallis for the common red spider of our eastern 

 states and of Europe, Tetranychus telarius L., I came across another 

 species which could easily be distinguished from T. telarius L. by the 

 use of my hand lens. Upon examining specimens with the compound 

 microscope in the laboratory, I was much surprised to find that this 

 species was no other than Tetranychus rnytilaspidis Riley^ the well- 

 known red spider of citrus trees in southern California. Since this 

 initial observation several records have been made of this species in 

 Oregon. The writer submits the follo^ving notes on its biology and 

 economy under the influences of its new host plants and new climatic 

 conditions. 



In the autumn when the leaves of the deciduous fruit trees begin to 

 brown, die, and fall, these mites change their summer habit of deposit- 

 ing eggs upon the leaves of the trees, and deposit nearly all of them on 

 the twigs. After egg deposition the adults fall to the ground with 

 the leaves and die. These eggs are always deposited singly, and may 

 be placed very close together so that at times scores or even hundreds 

 will be found almost touching one another. They are laid preferably 

 at the bases of the last year's shoots or around the bases of the fruit 

 spurs. Here they frequently become so numerous as to make these 

 portions of the twig look quite red. 



The eggs remain on the twigs over winter, and in Oregon suffer 

 little from the, hardships of moisture or low temperature, and are, at 

 present, remarkably free from the attacks of predaceous enemies. 

 During the month of April the eggs hatch, and the newly emerged, 

 six-legged larvae run about hunting for the opening leaf buds or the 

 first tender leaf shoots where at once they begin to feed. By the 

 seventh of May, at Corvallis, I found all instars present, as I did 

 from this time on until the end of the season. These mites continue 

 to feed and reproduce from the time of hatching in April until the end 

 of the rainy season but with very little vitality. So few individuals 

 are present at this time of the year that they are completely overlooked 

 by fruit growers. 



It is during July that the decrease in the humidity and the great 

 increase in temperature causes these mites to reproduce and feed with 

 great vigor. Now the adults become very active, and eggs are found 



