412 



Melander (A. L.) & YoTHERs (M. A.). Twenty-Fifth and Twenty- 

 Sixth Annual Reports for the Years ended 30th June 1915 and 

 30th June 1916, Division of Zoology and Entomology. — Washing- 

 ton State Coll. Agric. Expt. Sta., Pullman, Bulls, no. 127, December 

 1915, pp. 30-38, and no. 136, January 1917, pp. 35-42. [Received 

 nth July 1917.] 



Investigations for the purpose of determining the progressive 

 immunity of insects to insecticides have shown that differences in 

 viability are due less to the strength of the spray than to the locaHty 

 where the spraying is done. Neither can the immense fluctuating 

 differences be ascribed to climate, condition of the trees, water used in 

 the sprays, thoroughness of application, nor apparently to any com- 

 bination of extrinsic factors. Apparently there are inherent biological 

 differences in the insects at various localities ; in some districts the 

 insects succumb readily to the spray, while in others they exhibit great 

 powers of resistance. During the year 1915-16 scales manifested great 

 susceptibility to lime-sulphur spray in contradiction to their behaviour 

 of previous years in the same district ; on the other hand they proved 

 remarkably resistant to standard emulsified oils, which in the previous 

 year had completed their extermination. It is hoped to publish the 

 curves and tabulations of these extended investigations in a separate 

 bulletin. 



The endoparasitism of Aphis brassicae (cabbage aphis) and A. pseudo- 

 brassicae (turnip louse) has been the subject of a detailed study. The 

 commonest parasites of these species are Aphidius piceus and Xystus 

 brassicae, the effects of which are apparently identical. The incubation 

 of the parasitic eggs lasts but a few hours, and the larva develops 

 rapidly. No apparent effects are produced upon the host by the 

 presence of the egg, nor by the larva until it is about a week old, when 

 the host begins to exhibit signs of degeneration. It is not, however, 

 until the parasite reaches an advanced stage and begins to move about, 

 destroying the remainder of the degenerating tissues and the vital 

 organs, that death takes place. Polyembryony, which is characteristic 

 of certain species of parasites, was found not to exist in any of the species 

 studied. Parthenogenesis was definitely ascertained to be possible 

 under certain conditions, the resultant offspring being invariably 

 males. One method of dissemination of the endoparasites was proved 

 to be through the agency of the winged host-forms, which in migrating 

 to other plants or other fields serve in the distribution of the parasites. 

 It was definitely determined that Pachyneuron mimns and Asaphes 

 rufipes are hyper-parasites of the other parasites. Investigation into 

 the problem of whether parasitised Aphids bear further offspring 

 revealed the fact that reproduction does not cease immediately after 

 the oviposition of the parasitic egg, but may continue for some days, 

 several young being produced. The host Aphids are both attacked 

 by a fungous parasite, which destroys many of them, particularly in 

 the late autumn during cool, damp weather, which seems necessary 

 for its proper development. 



The root maggot [Chortophila brassicae] has proved to be one of the 

 most serious pests and one of the most difficult to deal with. Experi- 

 ments in control were mostly negative in result. Cabbage, kale, 

 marrow cabbage and rape are best protected by starting them in 

 autumn and transplanting in February or March. Tarred felt disks 



