62 



PAPERS ON INSECTS AFFECTIXG VEGETABLES. 



even while the webworms were destroying the foliage. Judging 

 from personal observations and from the statements of many grower^, 

 the writer may state that when sugar beets have been defoliated 

 by webworms during the growing season a loss of from 1 to 5 

 tons of roots to the acre may be apparent at harvest time. The 

 decrease in tonnage is not the only damage, as analyses of such 

 beets have indicated losses of both sugar content and purity, which 

 in some cases have reduced the price $1 a ton. Another injurious 

 feature which follows defoliation is that the soil about the beets is 

 exposed to the direct rays of the sun, allowing the moisture to 



evaporate rapidly, and 

 if the supply of irriga- 

 tion water is limited 

 this may become a seri- 

 ous matter. 



It will be seen that 

 the sugar-beet webworm 

 is a pest capable of 

 causing extensive dam- 

 age and that measures 

 tending toward its con- 

 trol are worthy of care- 

 ful consideration. 



NATURAL ENEMIES. 



Fortunately this spe- 

 cies has natural ene- 

 mies, among the most 

 efficient of which are 

 blackbirds. These birds 

 often gather in enor- 

 mous flocks in the in- 

 fested beet fields and 

 feed on the webworms. 

 Unfortunately the webworms are not thus attacked until they have 

 become nearly full grown and attain a size that renders them more 

 conspicuous. As a result, it generally happens that the infested beets 

 are partially or completely defoliated before the birds have com- 

 pleted their good work. The destruction of the " worms," however, 

 lessens the possible number of the succeeding generation. The web- 

 worms are also reduced in number by true parasites, and in some 

 cases the writer has found fully 50 per cent of the overwintered larva? 

 killed in this way. One of the most common parasites is a little 

 wasplike insect known scientifically as Diosphyrus vulgaris Cress., 

 a braconid. 



Fig. 13. — Large sugar-beet plants, showing defoliation 

 and weakened roots due to attack by the sugar-beet 

 webworm in August. (Original.) 



