90 



THE AGRICULTURAL NEWS. 



Mai:( h 23, 19iy. 



INSECT NOTES. 



THE RED SPIDER 



(tetranychis telarius, L.) 



The common red spider or spider-mite is of almost world- 

 wide distribution, and sometimes becomes numerous enough 

 to be a pest on certain cultivated crops. It is usually 

 troublesome wherever plants are grown in greenhouses. 

 Within recent years it has come to be recognized as a period- 

 ically serious pest of cotton in ihe South-eastern United 

 States, and during a red-spider year the losses from the 

 ravages of this small mite in the cotton belt are severe, 

 amounting, it is estimated, to 82,000,000. Much of the fol- 

 lowing information on this species is taken from BiiJk/in 416 

 by Messrs. E. A. McGregor and F. L. McDonough, of 

 the United States Department of Agriculture, and from 

 Far/ncis Bulktiii Sji by the first of these two authors. 



The name Tetranychus telarius, L., is that given to the 

 original European red spider, and is the one that is apparently 

 being used at present. The minute size of these creatures, 

 and their great variations due to age, host plant , environment, 

 etc., have led to a confusion of names. Among the commoner 

 scientific names used in literature for the red spider are 

 T, bimaculatus, Harvey, and T. ghnvri, Banks. 



DESCRIPTION OK THE RED .SPIDER. 



This minute creature is not a true spider but a mite, 

 and is more closely related to the ticks than to the 

 spiders. As is usually the case with mites, the full-grown 

 male and female have eight legs but no wings. The 

 colour of the female varies considerably. 'At times it is 

 rusty green, sometimes greenish amber, occasionally yel- 

 lowish, at times almost black, but most often brick-red, 

 and a large spot of much darker colour is usually sein 

 along the back half of each side of the body.' The 

 females are less than one-fiftieth of an inch in length, while 

 the males are considerably smaller, more pointed behind, of 

 a rusty salmon colour, and the spots at the sides are not 

 conspicuous. 



LIFE-HISTORY .SUMMARY. 



ii,?,(,^ The red spider lays its eggs on the under 

 surface of the leaves of a great variety of plants. The 

 eggs are almost complete spheres. When freshly laid 

 they ate quite clear and almost transparent, but gradually 

 become opujue, and are a dark honey -yellow just before 

 hatching. They are deposited singly on the underside of 

 the leaf surface or on the fine webbing sometimes spun by 

 the mites just above the surface of the leaves. 



Femnle. The egg hatches in about four days 

 to a minute, colourless, six-legged creature known as 

 a larva. This feeds, and in about two days under summer 

 conditions ehcds its skin, and becomes an eij;ht-legged form 

 called the primary nymph, which is usually greenish or 

 yellowi.sh. After feeding there is another moult in about 

 two day.s, and the secondary nymph appears. This gradually 

 becomes liiore like the adult, and changes to the female in 

 a little less than t wi days. Thus under favourable condition.^ 

 the female takes between nine and ten days to develop. 



ATalc. The development of the male is very similar to 

 that of the female, except that there is no secondary nymphal 

 stage. It is found, however, that the other stages are 

 slightly lengthened, so that tiie males usually take one day 

 less than the females to develop. 



lEErdNi; AND INJURY. 



The feeding is done by means of sharp, slender, 

 lance-like mouth parts which pieicc the leaf-surface. 



usually on the underside. The injury is caused by the 

 e.\traction of the plant juices, and in a heavy infestation 

 the leaves curl up, wither, and usually drop. In the case of 

 cotton, the presence of the red spider is usually first indicated 

 by the appearance of red blotches on the upper surface of 

 the leaves, and in severe ca.ses the foliage withers and drops, 

 and this dropping is accompanied by boll shedding. Other 

 plants attacked by red spider do not as a rule show the red 

 blotching, but yellowish patches appear on the upper surface, 

 which gradually spread. 



KOOD-l-LANTS. 



The red spider has been found breeding in the United 

 States on over ISO different kinds of plants, including 

 weeds, ornamental plants, garden and field crops. A few 

 of these plants are specially attractive to this pest, and 

 it is on these that the infestation of the red spider in the 

 cotton belt is mainly concentrated. Some of these serve as 

 over-winteiing ho-^ts of this pest, while others support it 

 during the spring and summer. It was found that the culti- 

 vated violet is one of the most important wintering hosts. 

 and serves as a source of dispersion to adjacent weeds and 

 neighbouring cotton fields in the spring. The garden beans 

 {Phaseohn spp.) are important host plants during the 

 summer months, and are therefore a constant menace to 

 surrounding crops. The red spifer is generally found <in 

 cotton in the south eastern Siates when this plant is availab'e, 

 but in the great majority cf cases the infestations on this 

 plant are very slight. Outside the cotton season the red 

 spider is supported on a number of plants, both weeds and 

 cultivated |ilants, from which it spreads again to the cotton. 



DISPERSION. 



It has been shown that when the food supply becomes 

 exliausted on one set (f host plants, the red spider is 

 able to migrate by crawling 10 another set of plants, and 

 so on all through the year. For instance, when cotton and 

 other annual plants die in the autumn months, the red spiders 

 migrate to various weeds and to certain cultivated plants, 

 such as the violet, which remain green throughout the com- 

 paratively mild southern winter- With the coming of spring 

 this pest returns to the weeds which are often left growing 

 on borders of cotton fields, and thence to the cotton as it 

 appears. Red spiders may also be washed off by heavy 

 rains and carried on the surface water to other parts of a 

 field, where the survivors are able to start a new infestation. 

 They may also be blown from place to place by heavy winds, 

 or they may travel directly from one plant to another by 

 means of the interlacing branchec. 



NATURAL ENK.MIKS. 



In the United States the red spider on cotton is known 

 to be attacked by thirty-one predaceous enemies, including 

 <jther mite.s, thrips, predaceous bugs, lacewing tlies, larvae of 

 syrjihid Hies, larvai' of midges, coccinellid beetles, etc. The.se 

 predatory enemies, however, are known to be attacked by 

 seventy-five spvcies of predaceous and parasitic enemies, 

 .so that their usefulness is .somewhat lessened 



IIEMEDIES FOl; rUK IIKI. M'lDKl!. 



The measures against the red spider in the cotton belt 

 nf the United States fall under two main headings, preventive 

 measures and repressive measures 



J'nvcittivc iiUiisiircs. These are far more economical 

 and practical than measures of repression. They include the 

 destruction of weeds round cotton fields iluring winter and 

 early .'•pring, the control of the red spider on cultivated 

 plants in yards, etc., by spraying, the maintenance in cotton 

 fields of a finely fiulverized surface mulch in order to retard 

 the migration of the red spider.^ the judicious u.se of fertil- 

 izers 10 keep the plants in good health and therefore more 

 resistant to injury. The u.se cf a fine surfa e muhli would 



