1909.] Tpie Insect and Allied Pests of the Hop. 623 



The Hop Red Spider. 



(Tetranychus althcece.) 



This small acarus (PI. II., Figs. 4 and 5) in some years 

 does a lot of harm to the hops. The disease produced by it 

 is sometimes spoken of as "fire blast." 



' It is especially in hot, dry seasons that Red Spider does 

 the most damage. Whitehead states that in the excessively 

 dry, hot summer of 1868, thousands of acres in England and 

 on the Continent were ruined by this pest. Again, in 1872, 

 1884, and 1885, much harm was done. The damage is caused 

 by the small acari sucking and gnawing the leaves below, 

 which produces, first a curious mottled appearance, and then 

 a yellow or even dull reddish-brown and grey discoloration, 

 and later the leaves may fall. In addition, these acari spin 

 a fine silken webbing, beneath which they live and breed, 

 and this acts injuriously by choking up the breathing pores 

 of the foliage, as well as by sheltering and protecting the 

 mites. The fine silken webbing is seen attached to a red 

 spider in Fig. 5. 



The life-history is comparatively simple. The adults 

 (PI. II., Fig. 4) are very variable in colour, some being a 

 translucent grey, others green or greenish-brown, others dull 

 red; all have dark spots on the body and scanty indistinct 

 hairs. It is the females which become red towards the end of 

 August and on to the following spring. The mature male 

 and female have four pairs of legs, as seen in the illustration. 

 The females lay their ova under the webbing ; they are spheri- 

 cal, and pale translucent green to pale gold in colour. They 

 hatch in from seven to eight days. The young or larval Red 

 Spiders have only six legs, and can be recognised by this 

 characteristic. The larvae moult their skin and become eight- 

 legged nymphs, and then moult again and become mature. 

 The cast skins remain under the silken webbing. 



Under favourable circumstances Red Spiders breed right 

 through the summer, and hibernate in the mite condition in 

 the soil beneath fallen leaves, in crevices in the poles, under 

 the rind of the snags, or any convenient shelter. 



In spring they appear on the young leaves, but do not 



