FUMIGATION OF CITRUS TREES. 51 



of many writers to advise a practice based on theory alone. The 

 writer has seen vegetation destroyed by the action of this residue, 

 especially where the amount of residue was large. Its injurious 

 effect on cover crops has been called to his attention by one orchard- 

 ist. One of the most striking examples of its injurious effect that 

 the writer has ever seen was in an orchard of large trees on light 

 sandy soil. The residue was emptied at the trunks of the trees, with 

 the result that portions of the root systems of some trees were 

 destroyed. These examples go to show that the residue produces an 

 immediate injurious effect on vegetation wherever it may come into 

 contact with the same. 



The residue should never be emptied near the base of the tree, but 

 out in the middle of the row. It should be so placed that the tents 

 do not come into contact with it while being moved from one tree to 

 another. The common practice in California is to empty it midway 

 between the two rows of trees in the opposite direction from which 

 the tents are being moved. This prevents their being dragged over 

 the residue. 



DOSAGES FOR VARIOUS SCALE PESTS. 



It has been stated previously in this bulletin that the purple, 

 black, red, and yellow scales were the insect pests of citrus trees 

 against which fumigation was generally practiced. In some instances 

 this treatment has been tried against the mealy-bug. The distri- 

 bution of these insects is such that in one locality the purple scale 

 might be the principal problem of control, whereas in another it 

 might be the black scale, red scale, or yellow scale. Usually a single 

 species will predominate in any one orchard, yet sometimes two or 

 even all require attention at the same time. 



The tendency of citrus-fruit growers is to overlook the fact that 

 this problem of control is one wherein different species of insects are 

 concerned and to believe that whatever treatment the fumigator 

 applies should accomplish the same results in all cases. Only the 

 treatment itself is considered, not the strength of dosage, and this 

 has led many orchardists to complain because fumigation by the out- 

 fits owned by associations or counties sometimes costs as much or 

 even a little more than that by contractors. If one party performs 

 the work much cheaper than another, the real basis of this cheapness 

 is that less cyanid is used. The desired results can not be accom- 

 plished unless the correct dosage requirements are met. 



Many dosage tables for the different scale pests of citrus trees 

 have been published in California and elsewhere, based on a consid- 

 eration of the height and width of the trees. These dosage tables for 

 the most part are very erratic, being calculated largely from hearsay 



