133 
holding it completely in check, however, and remedial measures 
must be resorted to. 
Remedies. — Only remedies to be used in greenhouses need be 
mentioned, as this louse has not been found by us of economic im- 
portance elsewhere. 
Fumigation is the only satisfactory remedy. The lice are 
usually within the heads, making it almost impossible to reach them 
with a spray; and at any rate the use of an effective spray would 
probably spoil the lettuce for the market. Tobacco or one of the 
tobacco extracts, or, better, hydrocyanic acid gas, may be used as 
a fumigant. The latter is the more effective, and will also destroy 
the white-fly and the aphis of the cucumber, which plant is usually 
grown in the same houses with lettuce. (See pages 156 and 159.) 
The Imported Cabbage-worm 
Pontia rapcc Linn. 
(Pieris rapce) 
Occasionally the cabbage-worm becomes abundant and destruc- 
tive to greenhouse lettuce, and I have seen beds which were planted 
the last part of August or the first part of September completely 
ruined by it. It has been already sufificiently discussed as one of 
the cabbage insects (pages 104-107). 
Methods of Control. — As arsenicals can not be used on lettuce, 
it is hardly probable that the cabbage-worm in greenhouses can be 
successfully treated by any of the measures advised for its con- 
trol out-of-doors (pages 106-107). ^^ there are cabbage gardens 
near a greenhouse, it is not advisable to plant lettuce until the mid- 
dle or last of September. If, in spite of precautions, a bed becomes 
badly infested, the plants should be thoroly sprayed with Paris 
green, and pulled up within two or three days, after which the 
bed may be replanted. 
ONION 
With the possible exception of the potato, the onion is the most 
important' garden product of Illinois and its insect enemies are 
the most difficult to control. In 1899, 2,563 acres were used for 
the growing of onions in this state, and the acreage has enormously 
increased since then. There are but two onion pests which are of 
general economic interest, the onion-maggot and the onion thrips. 
The Onion-maggot 
Phorbia ceparuni Meigen 
(Pegoiiiyia cepetorum, P. ceparuni, Anthoinyia ceparuni) 
Any one who has ever had anything to do with onion-growing 
is fully aware of the great damage done by the onion-maggot (Fig. 
