286 MASS. EXPERIMENT STATION BULLETIN 246 



Precautionary Measures 



When either red spider or powdery mildew is discovered on transplante.l 

 plants, the recommended control measures should be applied immediately. 

 Since workmen are common means of carrying these pests, it is unwise for 

 those who have come in contact with infested vines to enter other houses 

 without first making every eifort to rid their clothing and tools of any pests 

 which may be on them. Picking baskets should l)e given careful attention. 

 These precautions may appear impractical in comn\(ercial establishments but 

 they will aid in reducing the spread and abundance of red spider and powdery 

 mildew. 



Mixed Crojys in the Greenhouse 



Host plants of powdery mildew other than cucumber are seldom grown in 

 the greenhouse, but small lots of miscellaneous plants in benches and pots 

 are often infested with red sj^ider. The origin of serious red spider attacks 

 on cucumbers has been traced to violets, carnations, and mint, and in one case 

 infested duckweed was strongly suspected. Such plants should be examined 

 at regular intervals and if infested, destroyed or treated. 



Surrounding Vegetation 



The ground immediately surrounding the greenhouse should be kept clean 

 of vegetation or at least free from plants which harbor cucumber pests. It 

 is imwise to grow plants commonlj^ infested by red spider or powdery mildew 

 close to a greenhouse cropped to cucumbers, as any pests present out-doors 

 may be blown or carried bj' insects into the greenhouse. 



Specific Measures for Red Spider Control 



Partially Effective Treatments 



Spraying and dusting have long been the principal methods of combating 

 the red spider and probably more different materials have been used for this 

 purpose without complete success than for any other greenhouse pest. These 

 materials, if used thoroughly and frequently, when conditions are favorable 

 for their use, will hold a light or moderate infestation in check but the con- 

 trol obtained from them is not permanent and eradication is impossible. 



Some of these temporary remedies are washing with water, spraying with 

 salt solution, flour paste, dissolved soap, nicotine, derris extract, pyrethrum 

 extract, pine oil, lemon oil, linseed oil enmlsion, miscible oil, and various com- 

 binations of these materials. Some check the development of the spiders by 

 washing them from the plants; others stick them to the leaves; and many kill 

 or injure the more susceptible stages of the pest. Some of them are injurious 

 to foliage and none of them give safe and economical control of the red spider 

 on greenhouse cucumbers. Derris extract, pyrethrum extract, lemon oil, and 

 linseed oil emulsion are the best of these partially effective materials, yet 

 each has several disadvantages. 



Dusting or spraying with various compounds of sulfur has been used ex- 

 tensively in the past to combat the red spider both in the greenhouse and 

 outside, but the effectiveness of sulfur is limited by its dependence on high 

 temperatures. Sulfur sprays do not kill red spider eggs and have little 

 effect on the transforming stages. Dusting is not effective because of the 

 difficulty in placing the dust on the under side of the leaves where the spiders 

 congregate. 



