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killed. The slow-growing varieties of apples suffer particularly from 

 Capsids ; badly marked apples only grow to about one-fourth of their 

 normal size and often fall off. 



The stylets of all the species of Capsids dealt with are very similar, 

 and the cause of the damage would seem to be chemical rather than 

 mechanical, the salivary injection from P. rugicollis being injurious 

 to the tissue of apple trees, whilst that of the other species is not. 

 This supposition is borne out by the fact that it is the mesophyll 

 and not the epidermis that shows the first sign of injury, and dead 

 mesophyll may be present under a healthy epidermis and the damaged 

 area may spread for some time after laceration has taken place. In 

 the case of P. rvgicoUis the mechanical injury is probably of Uttle 

 consequence. Psollus ambiguus and Orthotylus marginalis are some- 

 times as numerous as that species, and yet the injury caused by their 

 sucking the juices is of no importance. An insect that causes mechanical 

 injury would probably do so to all plants that it feeds on. but the 

 injection of an insect that causes physiological injury might affect 

 different plants in different ways, and even be harmless to some 

 plants as well as much less harmful to some varieties of plants of the 

 same species than others. 



While certain pests are specific in their attacks, others infest a 

 large number of plants. Lygus pratensis is known to attack fifty 

 different species of plants. Plesiocoris rugicollis was formerly known 

 to attack Salix and Alnvs, but it now attacks apple, black and red 

 currants, and under experimental conditions has been made to attack 

 plums. This change of diet may be comparable with mutations in 

 the morphological characters and is perhaps due to some physiological 

 change in the organism. P. rugicollis may in the future become a 

 more serious pest by extending its host-plants. The change of diet 

 of this species may simply be due to a fertilised female being blown 

 on to a new host, and being unable to reach the original one, the 

 insect may feed upon and oviposit on the new plant. The larvae 

 that hatch from these eggs will probably be able to live on the tissues 

 of the host on which the mother lived and are in any case incapable 

 of reaching the original host. Further experiments in this direction 

 are required, but it is known that nymphs can be made to change 

 their hosts, for example from apple to plum and black currant to 

 apple, and it would be of interest to know whether P. rugicollis 

 would oviposit on a species distinct from that on which it was reared. 

 It is evident that P. rugicollis normally lays its eggs on the host on 

 which it has fed, and does not readily change any host on which it 

 can live, for apples and ^villows are found interlacing and only the 

 willows attacked, or apples and black currants with only the latter 

 attacked. In Cambridge, where apples are not attacked, but only 

 willows and black or red currants, larvae were transferred from 

 black currants to apple trees and, although they did not at once feed, 

 they became accustomed to their new host and completed their 

 development. 



Experiments have shown that P. rugicollis can be kept in check by 

 spraying with soft soap and nicotine. About 1 per cent, of soft soap 

 is sufficient with soft water, or more with hard water, and 0-05 per 

 cent, of nicotine (98-99 per cent.). This rapidly kills all stages of 

 the insect except the eggs. The wash should be applied just after 



