The present work was conducted in order to study the population density of aphids, leafhoppers and mites infesting barley plants and their associated predators and its relation to the prevailing temperature and relative humidity at Salhia District, Sharkia Governorate, Egypt during the two successive seasons of 2013/2014 and 2014/2015. The obtained results showed that the main pests on barely plants were the aphid species, Rhopalosiphum padi (L.), Rhopalosiphum maidis (Fitch), Schizaphis graminum (Rondani) and Sitobion avenae (Fabr.). Leafhoppers included Empoasca decipiens (Paoli) and E. decedens (Paoli) in addition to the mite Petrobia tritici Kandeel, El-Naggar and Mohamed. The associated predators were Coccinella undecimpunctata (L.), Chrysoperla carnea (Stephens), Syrphus sp. and Orius sp. The aforementioned pests and predators were collected by two different methods from barley plants as follows: using plant sample and insect sweep net. The plant sample proved to be better to collect aphids, mite and associated predators of the present work, while sweep net proved to be better for the leafhoppers. The seasonal abundance of R. padi and R. maidis on barley plants showed one peak (354,178 and 210,189 insects/sample) occurred at the 2^nd week of February in 2013/ 2014 and 2014/2015 seasons, respectively. Also, S. graminum appeared one peak (134 and 114 insects/sample) at the 1^st week of February during the two seasons of study, respectively. On the other hand, one peak occurred at the 1^st week of March for both E. decipiens (38 and 27 insects/sample) and E. decedens (31 and 24 insects/sample) during the two seasons of study, respectively. The most abundant species of predators were C.  undecimpunctata showing three peaks at the 4^th week of January, 1^st, 4^th weeks of February, the 2^nd week of March, the 1^st week of April and C. carnea appearing one peak in the 4^th week of February during the two seasons of study, respectively. Obvious relationships between the population density of pests, associated predators and prevailing weather factors including the effect of mean temperature and relative humidity as well as their total combined effect on the numbers of certain investigated pests was recorded.