Sunn Pest, Eurygaster integriceps Puton (Heteroptera: Scutelleridae), is a major pest of wheat and barley in West and Central Asia and East Europe. The insect causes a significant reduction in yield and quality of grains. Entomopathogenic fungi, especially Beauveria bassiana are among the most dominant microbial agents that have been used against Sunn Pest. However, there are concerns related to their efficacy and thermotolerance particularly in countries with high temperature regimes. Nine B. bassiana isolates were tested against Sunn Pest to estimate their virulence and speed of infection under laboratory conditions. The isolates were also exposed to 25, 36, 38 and 40 °C, for 24, 48, 72 and 120 hours. The isolate with highest pathogenicity against Sunn Pest and the one with the greatest thermotolerance were selected and their conidia were paired using a cell electrofusion technique to obtain a strain that was both highly virulent and thermotolerant. The infused conidia were first exposed to the foregoing temperature regimes and the most thermotolerant ones were selected to test against Sunn Pest. Three Sunn Pest populations at early, mid and late overwintering period were treated with electrofusion products and the two parental isolates. The efficacy of hybrid isolates was also tested against western flower thrips, Frankliniella occidentalis Pergande (Thysanoptera: Thripidae) as another test organism. The results showed that among the electrofused conidia, some of them tolerated the highest temperature regime (120 h at 40 °C) and their pathogenicity was similar and in some cases even higher than the most efficacious parental isolate. The susceptibility of the Sunn Pest to hybrids was dose dependent as considerable mortality occurred with the highest concentration (1 x 10⁸ spore/ml). In addition, Sunn Pest populations were more susceptible to fungal infection at the end of the overwintering period. That is mainly because Sunn Pest adults do not feed during this period and their survival is entirely depended on the amount of energy and nutrition stored in their body. Depletion of nutrition at the end of the overwintering period and the harsh environmental conditions in overwintering site make the insect more susceptible to the fungal pathogen. Assessing the genetic differentiation of the hybrids with the parental isolates is an important task that requires more investigation in the future. In a separate experiment, the thermotolerance of two selected B. bassiana isolates for the electrofusion were tested under 40 and 45 °C wet and dry-heat conditions for 4, 8, 16, 24 and 48 h. Additionally, the thermotolerance of conidia and mycelia of the two isolates were compared at 38 and 40 °C for 24, 48, 72 and 120 h. In general, fungal isolates tolerated the dry-heat better than the wet-heat condition and the thermotolerance of fungal mycelia was significantly greater than the conidia.