FIELD EVALUATION OF SOME SOYBEAN CULTIVARS FOR THEIR RELATIVE SUSCEPTIBILITY TO THE TWO-SPOTTED SPIDER MITE, Tetranychus urticae KOCH INFESTATION WITH RELATION OF SOME BIOTIC AND ABIOTIC FACTORS AT ASSIUT GOVERNORATE (Acari: Tetranychidae)

Field experiments were carried out in Abnoub district, Assiut Governorate during two successive seasons 2010 and 2011 to study the population fluctuation of the two-spotted spider mite,  Tetranychus urticaeKoch and some associated common predators (Scymnus spp., Orius spp., Geocoris pallidipennis (Costa), Charysoperla carnea Stephens and true spiders) on seven soybean varieties. Also, the effect of different plant growth stages, common predators and some prevailing weather factors on the infestation of soybean varieties and the relative susceptibility of soybean varieties to infestation were studied. The obtained results indicated that Giza 111 variety attain the highest population of T. urticae while the lowest one was recorded with the two varieties Giza 22 and H 32. Moreover, coincidental appearance for both T. urticae and associated common predators. So, firstly appeared in low numbers during July then increased gradually to reach the peak during the end of August. Therefore, the population decreased sharply from September to the end of season, for all soybean varieties during the two seasons 2010 and 2011.
The effects of certain biotic and abiotic factors on the population density of T. urticae were investigated. The obtained data showed that the plant growth stages played the most important roles in regulating the population density of the two-spotted spider mite. Also, it could be substituted equations for predicting the infestation of T. urticae on soybean.
The soybean varieties differed in their relative susceptibility to the infestation with T. urticae. Whereas, Giza111, H30 and Clark were the highly susceptible varieties (HS). While, H117 and H127 varieties were exhibited relatively resistant (RR), but Giza22 and H32 varieties manifested a resistant degree (R).