216 THYSANOPTERA AND ORTHOPTERA. 



Canada thistle (Cirsium arvense) and the common mullein (Verbascum 

 thapsus) were among the most badly infested plants found. Where 

 a hundred or more of Euthrips tritici can be taken from a single flower 

 of the former plant, and 874 larvae and adults of Anthothrips verbasci 

 from one small spike of the latter, it would seem that they could do con- 

 siderable damage. Infested plants, however, were apparently as 

 healthy as others which were nearly or quite free from thrips. I do not 

 know what effect their feeding may have on seed-production in the in- 

 fested flowers. 



It would seem, from the abundance of the various species, that 

 damage to cultivated plants is chiefly to be expected from Euthrips 

 tritici and Thrips tabaci. Owing to their apparent tendency not to 

 migrate, marked attacks may be expected to be more or less local. A 

 general plague is hardly probable. The fact, furthermore, that these 

 two species are very general in food habits, probably aids in preventing 

 serious outbreaks. For they are as likely to attack weeds as culti- 

 vated plants, and in such cases mas' even prove to be beneficial. 



In the case of other species, such as the poephilous group, which are 

 more restricted in their choice of food, outbreaks are more to be feared. 

 Thus, some of the principal crops of this region, as wheat or oats, are 

 open to the attacks of grass-inhal^iting thrips, especially Anaphothrips 

 striatus; and timothy is often attacked by Chirothrips manicafus, though 

 I have not, for reasons stated under the heading of Ecology, in- 

 cluded it among the poephilous species. 



The manner of taking food is still somewhat in doubt. It is generally 

 supposedthat they subsist chiefly by sucking plant juices, though partly 

 by biting or rasping the tissues themselves. Thrips are too small to 

 observe readily in the act of eating. Sections of a few individuals 

 which I have made, however, show the contents of the digestive tract 

 to be rather too homogeneous to contain much of the plant tissue. If 

 they live by sucking juice, the standard method of combating them is 

 by contact poison, for example, kerosene emulsion. This may well be 

 applied in the case of superficial species, but the interstitial ones are 

 usually too well concealed to be reached. It would be well nigh im- 

 possible to attack thrips in this manner, even if it were desirable to do 

 so. on plants like the Canada thi.stle or dandelion. In case contact 

 poison can be applied, the practicability of the method depends on the 

 value of the crop in danger, and the character of the season. Hooker 

 (1906, p. 4) estimates the maximum cost of spraying, in the case of 

 tobacco, at twenty dollars per acre per season. Cultural methods are 

 also to be recommended. For details of these and other methods, the 

 experiment station literature should be consulted. 



