264 INSECTS AFFECTING VEGETATION 



times drawn to them by reason of the thin white web which is spun 

 on the surface of the leaves, but it is usually only in cases of bad in- 

 festation that the web becomes apparent. The injury to the leaves is 

 quite characteristic. The leaves of an infested plant first become 

 dotted with brownish patches, usually on the under side, and then 

 gradually wither and dry up. 



The eggs of the mite are deposited on the underside of the 

 leaves, from five to ten being laid each day, and one female will lay 

 from 80 to 100 eggs. These hatch in three or four days, under favor- 

 able conditions. Observations indicate that in a greenhouse three to 

 four weeks are sufficient for the completion of a life cycle. It will 

 thus be seen that the rate of increase will be something enormous, if 

 the mites are left undisturbed. Fortunately, the ordinary methods 

 of caring for plants under glass destroy large numbers of them, and 

 thus partially hold them in check. 



The list of food plants of the red spider is a long one, including 

 many of the common greenhouse plants. The carnation, rose, gera- 

 nium and violet are especially liable to attack, however, and it not 

 infrequently causes great damage to these plants. Probably the rose 

 and carnation growers are the greatest sufferers, it sometimes occur- 

 ring that even well-cared-for houses become so badly infested as to 

 render the crops of these flowers almost valueless, owing to the stunted 

 condition of buds which have been robbed of their natural nourish- 

 ment. (Bui. 119, Md. Exp. Sta.) 



Thrips. Carnations are not infrequently badly damaged by 

 tiny insects called thrips, which are closely related to the true bugs, 

 or Hemiptera. The thrips work in the flower buds, causing small, 

 irregular white blotches to appear on the petals, and thus rendering 

 them imperfect. In case of bad infestation, the flowers are rendered 

 unsalable, and the florist sustains a heavy loss. The blotches are 

 caused by the sucking of the sap from the petals before the flower is 

 open, causing the tissue to turn white. 



The insects are less than a tenth of an inch long, very slender, 

 and in the adult form dark colored, almost black, with four small 

 membranous wings carried flat on the back. The head is quite small, 

 and the abdomen long and slender, giving the insect the appearance 

 of being pointed at both ends. The larvae are slightly lighter in color 

 than the adults, and have no wings, but otherwise they look much 

 alike. When disturbed the thrips will frequently elevate the tip of 

 the abdomen, as if intending to sting. They are very active, leaping 

 or flying readily, or wriggling out of sight so quickly among the 

 bases of the petals when the flower is torn open that it is very difficult 

 to capture one of them. The life history of the greenhouse species 

 has never been satisfactorily worked out, but it is probable that the 

 eggs are laid in the tissue of the plant in the leaf or stem, as with 

 other species of the genus, and when the eggs hatch the young larvae 

 crawl out and feed by sucking the sap, as do the adults. The carna- 

 tion is the only greenhouse plant liable to be severely injured, (Bui. 

 X19, Md. Exp. Sta.) 



