4 MISC. PUB. 526, U. S. DEPT. OF AGRICULTURE 
not be sprayed with these materials after the head has begun to form. 
If the marketed product is to bear a greater number of loose outer 
leaves than those allowed in U. S. Grade No. 1, these materials should 
not be used after the plants have been thinned or transplanted. If 
all the outer leaves of headed cabbage have been removed, the re- 
mainder should be free of harmful residues. 
Celery.—Arsenicals and fluorine compounds are not recommended 
for use on celery because of harmful residue. 
Tomatoes.—Residues may occur on tomatoes treated with calcium 
arsenate or cryolite, but the washing process in the canneries and wip- 
ing the fruit for market with a cloth will practically eliminate these 
residues. 
Beans.—For green and snap beans, sprays or dusts containing cryo- 
lite or arsenicals should not be applied after the pods have begun to 
form. If treatment is made after this period of growth, two or three 
washings of the harvested beans in water should remove any harmful 
residues. 
Peppers——All peppers that have been treated with an arsenical 
(calcium arsenate) or fluorine compound (cryolite) must be washed 
before they are marketed or processed. 
INJURY TO THE PLANTS BY INSECTICIDES 
Insecticides, especially when improperly prepared or applied, may 
injure the plant foliage and flowers. Promiscuous spraying is there- 
fore very inadvisable. Insecticides are seldom if ever beneficial to 
the plant itself, and they are good and useful only insofar as they 
remove the dangerous and destructive pests with a minimum of 
damage to the plant. © | 
The choice and purity of the materials and the care exercised in 
their preparation and application influence the results obtained. The 
margin of safety between the effective dosage required to kill the 
insect and the tolerance of the plant to the insecticide is often very 
narrow. Some plants will tolerate more insecticides than others; 
for example, beans are likely to be damaged by treatment with 
arsenicals, whereas potato and cabbage will withstand comparatively 
large dosages of such materia!s without injury. The factor of plant 
tolerance, therefore, as well as the effect of the insecticide on the 
insect, must be considered in pest control. Oftentimes the plant 
tolerance limits the general use of an insecticide. For example, lead 
arsenate and calcium arsenate will control the Mexican bean beetle, 
but under certain climatic conditions both these materials may cause 
plant injury. Magnesium arsenate, while satisfactory on beans under 
most conditions, will cause plant injury to peach and apple trees. 
Hydrated lime is used with the arsenicals as a means of offsetting 
foliage injury; but it should not be used with the fluorine compounds, 
with which it is not compatible. 
Sulfur, applied either as a dust or a spray, may cause injury to 
the foliage of squashes, melons, and cucumbers, and to the fruits of 
raspberry, as well as to the tender growth and blossoms of roses and 
various other flowering plants. 
Many plants will not tolerate oil sprays, especially when repeated 
applications are necessary. 
