62 
PAPERS OX IXSECTS AFFECTING VEGETABLES. 
even while the webworms were destroying the foliage. Judging 
from personal observations and from the statements of many growers, 
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 inject known scientifically as Diosphyrm vulgaris Cress., 
a braconid. 
Fig. 13. — Large sugar-beet plants, showing defoliation 
and weakened roots due to attack by the sugnr-be^t 
webworm in August. (Original.) 
