30 MASS. EXPERIMENT STATION BULLETIN 20L 



C. COMBINED INSECTICIDES AND FUNGICIDES. 



Most farm and orchard crops suffer from both insect pests and fungous 

 diseases. This necessitates the use of both an insecticide and a fungicide 

 on the same plant. Frequently, also, the presence at the same time of more 

 than one species of insect requires the application of both a stomach 

 poison and a contact insecticide. If the crucial time for application of more 

 than one should be approximately the same, it is usually possible and 

 profitable to combine them in a single application. Such a combination 

 results in the saving of one-half to two-thirds of the time required for 

 separate applications, and since labor is usually the big item of expense 

 in spraying, the cost is materially diminished. Unfortunately, however, 

 it is not possible to combine indiscriminately the various substances which 

 are used as fungicides and insecticides. Frequently, in combining two or 

 more of them a reaction takes place which results in — 



1. Complete or partial neutralization of the beneficial qualities of one or more. 



2. Formation of a new compound which will injure the plant. 

 .3. Liberation of some harmful element. 



Such substances are said to be incompatible. It should not be under- 

 stood, however, that chemical combinations between fungicides and in- 

 secticides are always harmful or undesirable. Sometimes the reaction is 

 known to increase rather than decrease the fungicidal value; in other 

 combinations the substances have no effect on each other. The possible 

 combinations are discussed below, and the compatibilities graphically 

 represented in the diagram opposite this page. 



In making combinations, the formulas and methods of preparation 

 should be the same as have been previously described for each material. 



1. Bordeaux Mixture with Lead Arsenate or Calcium 

 Arsenate. 



Bordeaux mixture can be combined safely with lead arsenate or cal- 

 cium arsenate. There is, however, some experimental evidence to show 

 that, in such a combination, the fungicide inhibits the action of the ar- 

 senical to a considerable extent (42) . On the other hand, the excess lime of 

 the Bordeaux combines with any arsenic rendered soluble by atmospheric 

 conditions, thus diminishing the danger of foliage injury. Owing to its 

 superior adhesive qualities, lead arsenate is better than calcium arsenate 

 for combination with Bordeaux. On the other hand, calcium arsenate is 

 much cheaper. The choice between these, therefore, seems to be a matter 

 of personal preference. 



