THE CLOVER ROOT BORER 37 
TaBLE 6.—Ezxtent of damage done by clover root borers in selected areas in 
Washington County, Oreg. 
COUNTS ON CLOVER OF FIRST CROP YEAR, ONE TEN-THOUSANDTH OF AN ACRE 
Condition of plants 
collectec bane ive 
plants | Dies | _ in- and | Weak | Dead Place Soil type! 
| fested | fested strong 
| I 
Roa 
1921 ; ; 
Sept. 27___- 36 | 0 Ap peice 17 13 | Verboort....| Amity silt loam, un- 
| | drained. 
Sept. 26____ 62 | 17 45 | 16 16 13 | Hillsboro...| Amity silt loam, tile- 
drained. 
Sept. 29____ 45 11 | 34 | 11 10 13 | Forest Grove| Chehalis silt loam. 
| 
+ 
COUNTS ON CLOVER OF SECOND CROP YEAR, ONE TEN-THOUSANDTH OF AN ACRE 
[ | 
Sept. 24___.| 53 | ) | 53 | Sole: 
Sept. 27_.__| 37 | 0 | 37 | 7 
Melbourne loam. 
Chehalis silt loam. 
25 
23 | Forest Grove 
15 
15; peesee dos 
VSoil Survey of Washington County, Oreg., by E. B. Watson, in charge, and E. C. Eckman, of the 
United States Department of Agriculture, and A. L. Fluharty and C. V. Ruzek, of the Oregon 
Agricultural Experiment Station. Bureau of Soils, Washington, 1923. 
SECOND CROP YEAR 
The count indicates that, under severe infestation, unless the field 
is self-seeded there will be practically no clover left at the end of the 
second crop year. In the second crop year the clover is sometimes 
killed out after the hay crop is harvested, no further cutting for hay 
or seed being possible. 
It has been the general practice in the Pacific Northwest, and is 
still so to some extent, to maintain clover fields for two crop 
years, and often for three (23), occasionally even for four, without 
plowing or reseeding. This probably was practicable in the early 
days, because of the scarcity of destructive clover insects and the 
continued self-seeding which replaced plants killed by animals, insects, 
and disease. However, land costs at present are so high as to lead 
progressive growers to keep their land as nearly as possible at maxi- 
mum production. In the Willamette Valley the clover root borer is 
largely responsible for the fact that as a general rule more than one 
crop year of clover is no longer profitable. 
The number of borers necessary to kill a root is not easy to deter- 
mine, as many other factors are involved, such as the age of the clover, 
soil conditions, rainfall at critical periods, and attacks of other insects 
and fungous diseases. As many as 45 borers in various stages have 
been found on a single root, and 25 to 30 on a root are commonly 
observed in the severely infested field. Sixty-three borers of the new 
generation, 56 of them mature, in a cage where they had developed 
trom eggs on the roots, killed 3 out of 5 roots by August 23; the 
2 other roots were alive, 1 being uninfested. In another case 47 
borers infested 3 out of 5 roots, and all roots were alive on August 
23°; only 1 root showed serious injury. In still another case 57 
borers infested 2 out of 5 roots, and on August 26 1 of the 2 roots 
was still alive and looked fairly healthy with 26 young adults on it; 
but the root was so severely injured that it would probably have died 
before winter. 
