4 MISC. PUB. 52 6, U. S. DEFT. 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 

 tfijure 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 materials 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. 



