14 



DECIDUOUS FRUIT INSECTS AND INSECTICIDES. 



Thrips are in the ground all of tMs time, and for the most part within 

 reach of the cultivator, but they mature and arrive on the trees in 

 March and April, before spring cultivating is begun. 



NATURAL ENEMIES. 



The pear thrips is largely protected from ordinary preaaceous and 

 parasitic insects, because it spends so long a time hidden away in the 

 ground. A successfid parasite must in a way parallel the life of its 

 host, and we have found no insect which thus follows the pear thrips. 

 Raphidians, or snake flies, their commonest enemies in the vSanta 

 Clara Valley, feed rather on the younger forms than on the fully 



developed insects, and they do not 

 appear early enough in the spring 

 to constitute an effective check to 

 the pest. To be competent thrips 

 killers they would have to feed on 

 other insects for perhaps ten months 

 in the year and then, when thrips 

 appear, suddenly change their diet 

 and later, after tlu"ips have gone into 

 the ground, as suddenly change back 

 again to aphides or to something 

 else. Such feeding habits are not 

 to be expected in a nredaceous 

 species. 



Ants were at one time thought to 

 be doing much good as an enemy of 

 the thrips. A certain orchardist 

 brought in an ant with a thrips 

 impaled in its jaws — the evidence 

 complete. After a careful investi- 

 gation, however, it was found that 

 only a very small percentage of ants 

 were actually killing thrips. Four 

 hundred ants were examined as 

 they descended a thrips-infested tree. Twelve of these carried 

 something in their jaws and only 4 of these objects were thrips. Thus 

 only 1 per cent of the ants on the tree were actually killing thrips 

 aiul carrying them down. It has been a common observation among 

 orchardists, however, that tlmps are not common where ants are 

 unusually abundant. 



Spiders and mites are active enemies of thrips. In some of our 

 breeding cages almost all of the thrips woidd at times be killed by 

 some small s])ider or mite which had gained an entrance. The writer 

 has observed a red mite {RhyncJiolo'phus sp., determined by Mr. Nathan 

 Banks) actively engaged in feeding on the onion thrips ( Thrips tabaci 



Fig. 7. — Cladosporium sp., ii fungus which at- 

 tacks the pear thrips: a, active fruiting stage 

 on adult thrips; 6, branching mycelia; c, 

 forming spores, a, much enlarged; h, c, 

 highly magnified. (Original.) 



