GREENHOUSE RED SPIDER. 169 



linseed oil emulsion derives its efficiency. The spray as made (see "Re- 

 pressive Measures") contains the amount of soap necessary to hold the 

 oil in suspension and give the spray material excellent spreading qualities. 

 Raw linseed oil contains two types of oils, — (1) drying oil and (2) resinous 

 oil. Upon this fact is based its usefulness in paints, as well as its efficiency 

 as a red spider spray. 



A leaf thoroughly covered by the spray soon becomes dry, the water 

 evaporating, while the oil and soap become more and more concentrated 

 as this evaporation continues. Finally there is formed a very thin layer 

 of oil and soap which gradually settles down on to the leaf surface, cover- 

 ing all mites which were feeding on the leaf at the time of application. 

 This film gradually envelops the mite, and the volatile parts of the linseed 

 oil are given off, leaving behind a resinous or waxy oil which securely 

 cements the legs of the mite to itself and to the leaf. Thus the mite is 

 helpless, and the waxy residue of the linseed oil remains, sticking the mite 

 until it dies of starvation. Without doubt some of its effectiveness may 

 be due to its being a contact poison, but its most important quality is its 

 adhesiveness. 



Summary of Materials found to be Efficient Experimentally. 



No fumigant was efficient in killing red spiders without severely damag- 

 ing cucumber plants. 



Sulfur burned to form sulfur dioxid proved to be very effective in 

 killing all stages of mites. Although this gas is deadly to plant life, its 

 appUcation as a fumigant to rid empty houses of all mites is extremely 

 useful. 



Many spray mixtures proved to be efficient in controlling actively 

 feeding mites, but did not affect those in the quiescent stages of develop- 

 ment. For the control of all stages above the egg stage lemon oil, a com- 

 mercial product, and linseed oil emulsion proved to be the most satis- 

 factory. Soapy solutions should also receive some attention as among 

 the most readily prepared spray materials, although their efficiency is 

 only temporary and treatment must be repeated often in order to control 

 these mites. 



EXPERIMENTS CONDUCTED IN COMMERCIAL GREEN- 

 HOUSES. 



The materials found to be most efficient in the laboratory experiments 

 were applied to cucumber plants in commercial establishments in order 

 to determine the practicability of spraying for the control of these mites 

 before any recommendations were made. 



It was found impossible for the writer to be stationed at these green- 

 houses during the whole spraying period. Therefore the efficiency of 

 these sprays under commercial conditions has been determined largely 

 by the statements of the growers, checked by more or less frequent per- 

 sonal observations. 



