587 



must be heavily covered. The contrasting results obtained in northern 

 Illinois, where weaker applications have been continually successful, 

 ma}' be explained by the different climatic conditions. The tem- 

 perature in the south results in the development of a larger number 

 of broods and consequently greater infestation, whereas in the north 

 the low winter temperature often causes the death of many insects. 



Gibson (A.) & Ross (W. A.). Insects afEecting Greenhouse Plants.^ 



Canada Dept. Agric, Ottawa, Buh. N.S. 7 (Ent. Bull. 21), 1922, 

 63 pp., 3 plates, 35 figs. 



It is estimated that about six million square feet of space is used 

 for growing plants under glass in Canada, the crops raised in this 

 way for commercial purposes in 1920 being valued at about £600,000 

 (at par). This buhetin has been written to enable growers to recognise 

 the various kinds of insect pests hkely to be found on greenhouse 

 plants, and to control the infestations as far as possible. A general 

 account is given of insecticides suitable for greenhouse use and their 

 application, the group of insects for which each is intended being 

 enumerated. Special attention is given to hydrocyanic acid gas 

 fumigation and the method of computing the space to be treated. 



ToTHiLL (J. D.). The Natural Control of the Fall Webworm {Hyphan- 

 tria cunea, Drury) in Canada, together with an account of its 

 several Parasites. — Canada Dept. Agric, Ottaiva, Bull. N.S. 3 

 (Ent. Bull. 19), 1922, 107 pp., 6 plates, 99 figs. 



The detailed account of Hyphantria cunea, Drury (fall webworm) 

 in Canada here given shows that some of the outbreaks of this moth 

 between 1912 and 1919 might have been greatly reduced in intensity 

 or even prevented by the timely transfer of insect parasites from one, 

 part of the continent to another. The usefulness of the parasites is 

 greatest when the host is only moderately abundant and a condition 

 of stabilised control is possible. The maintenance of this equili- 

 brium between host and parasite is most important, as should the 

 host become scarce, the parasite will die out and thus enable the 

 remaining host individuals to give rise to a more than usually severe 

 infestation. Both in eastern Canada and in British Columbia some 

 birds assist in maintaining this stability. The most important parasites 

 may disappear from time to time over considerable geographical 

 areas ; it is therefore not sufficient merely to colonise parasites— 

 their stability must also be maintained. Once a parasite has been 

 eliminated from a locality, its return may be prevented for an 

 indefinite number of years by natural barriers. The narrow strait 

 between the mainland and Vancouver Island kept the webworm 

 situation in the two localities distinct for three successive yeairs. 

 When the first flight of H. cunea reaches Vancouver Island, parasites 

 should at once be introduced so that the required equilibrium may 

 be maintained from the beginning. 



The food supply is a very important factor in the distribution of 

 Hyphantria. A large number of the moths are carried by the wind 

 into foodless parts of the surrounding forest, and as only the alders 

 exposed to the wind are found by the moths, large numbers of the 

 trees are never attacked. The increased food supply (apple trees 

 besides the original alders) would seem to account for the greater 

 severity of the outbreak in the Annapolis valley as compared with 



(S175) 2r2 



