8 Journal of Agricultural Research voi. x, no. r 



Both moths died on August 23, ha\nng lived 7 days. In the cooler 

 weather of spring and fall the life of the moths and the egg-laying 

 period are lengthened and may cover from 10 to 14 days. 



NATURAL ENEMIES ^ 



The diamond-back moth is a striking example 01 a potentially serious 

 pest normally held in repression by parasites. The most effective 

 species engaged in this beneficial work is an ichneumonid, Angitia plutellae 

 Vier. .(PI. 2, A). The writer reared this parasite from larvae of the 

 diamond-back moth collected at various points in the Arkansas Valley, 

 at Colorado Springs, Colo., and at Phoenix, x\riz. In Colorado it is 

 active whenever Plutella larvae are in evidence. In Arizona it appears 

 to work throughout the year. With the later generations of the dia- 

 mond-back moth it is not unusual to find from 50 to 70 per cent of the 

 larvae infested by this parasite. 



Parasitized larvae of P. macidipennis reach maturity and form their 

 cocoons. The larvae of Angitia plutellae, one from each infested Plutella 

 larva, issue and spin compact gray cocoons within the Plutella cocoons. 

 Unfortunately the larvae of A . plutellae are occasionally infested by a 

 chalcidid hyperparasite, Spilochalcis delira Cresson (Pi. 2, B). The 

 writer had A . plutellae under observation at Rocky Ford during the years 

 191 1 to 1916, and at no time during this period was the hyperparasite 

 sufficiently numerous to reduce noticeably the numbers of its host. 



Other parasites reared by the writer at Rocky Ford from the larvae 

 of the diamond-back moth were identified as Mcteorus sp. and Meso- 

 chorus sp., and a new species of Micro plitis. These parasites were, how- 

 ever, too rare to be of noticeable benefit in reducing the numbers of the 

 larvae of P. maculipennis . 



No parasites were reared by the writer from the eggs or pupae of the 



diamond-back moth, and no predacious or fungus enemies have been 



observed. 



EXTENT OF INJURY 



Although the larvae of the diamond-back moth feed on many crucifer- 

 ous plants, they seldom cause appreciable damage to any except cabbage, 

 cauliflower, and rape. In Colorado the larvae are usually sufficiently 

 numerous on these plants during June or early July to riddle the leaves 

 badly. The later generations are so effectively held in check by para- 

 sites that little or no damage results. Whenever damage occurs, it is 

 always most serious with young plants. Sometimes the larvae crawl 

 in among the leaves of newly transplanted cabbage and feed on the 

 tender leaves at the center. In such cases the heads, if any develop, 

 are usually deformed and worthless. 



1 The ichneumonids were identified by Mr. A. B . Gahan, of the Bureau of Entomology, and the chalcidid 

 by Mr. J. C. Crawford, of the United States National Museum. 



