156 MASS. EXPERIMENT STATION BULLETIN 179. 



as sources of inside infestation, or may in turn become infested from 

 plants or parts of plants thrown out of the greenhouse during or after an 

 infestation. The most important garden crops attacked are the bean, 

 egg plant and celery. Tomatoes grown out of doors are more susceptible 

 to red spider injury than when grown in greenhouses. Strawberry plants 

 are also subject to attack, but usually this does not assume great im- 

 portance undgr New England climatic conditions. The most important 

 plants, as far as the greenhouse man is concerned, are those found around 

 most greenhouses, consisting of clover, grasses and weeds, as these are 

 undoubtedly important factors in causing inside infestation. 



NATURE OF INJURY TO CUCUMBERS. 



The first signs of injury appear soon after the plants have been trans- 

 planted in the greenhouse, and in the majority of cases on the oldest, 

 basal leaves. The pests usually attack the leaves of a cucumber plant 

 progressively; that is, the older, basal leaves first show injury, then those 

 just above are attacked, and thus the ravages of the pest progress upward 

 as the plant grows. As a general rule very young, hairy leaves around 

 the terminal shoot are exempt from attack until the plant becomes very 

 heavily infested. 



The injury is caused by the puncturing of the under surface of the leaf 

 and the extraction of the liquid contents of the leaf cells immediately sur- 

 rounding the puncture, which results in a very characteristic and notice- 

 able injury. In the process of feeding, the green chlorophyll is withdrawn, 

 leaving a small dead area which soon appears on the upper surface of the 

 leaf as a small whitish speck. As the mites continue feeding, the removal 

 of chlorophyll and specking increases until ultimately the leaf becomes 

 yellowish, lifeless and useless for food assimilation. 



The characteristic red spider injury is quite easily recognized, even-in 

 its early stages of development. The normal leaf is opaque, allowing 

 no light to pass through it, while around injured areas considerable light 

 passes through the leaf tissue, due to the lack of chlorophyll in this vicinity. 

 The contrast between the opaque normal leaf tissue and the lightness 

 seen around affected areas is especially noticeable when the cucumber 

 plants have become full-grown and have leaves and terminal shoots 

 running over the top wires, for at this time the leaves are between the 

 source of light and the observer walking beneath them. The appearance 

 on the upper surface of the minute, pitted dead specks or spots, usually 

 arranged in clusters, will also point to infested areas. 



ECONOMIC IMPORTANCE OF THE PEST ON CUCUMBERS. 



The damage caused by red spiders in cucumber houses varies in severity. 

 The factors influencing this have not been determined, but at least they 

 are very complicated. The severest injury seems to occur in houses 

 containing a light sandy soil, while houses having heavy soils are better 



