1 Dendroctonus micans is an invasive species that has spread throughout Britain, which prompts the question: how is the voltinism of this pest and its biocontrol agent Rhizophagus grandis affected by climate? 2 Dendroctonus micans and R. grandis were reared at a range of constant temperatures.Lower developmental threshold temperature (LDT) and day degree (DD) requirements for the egg, larval and pupal stage of D. micans were estimated to be 7.4 ∘ C 153 DD, 6.6 ∘ C 527 DD and 7.2 ∘ C 126 DD, respectively. At 12.5, 15 and 17.5 ∘ C, prior to prepupal cell formation, fifth-instar larvae underwent a diapause, which has not been observed previously in this species. Prepupal development rate also did not increase linearly with temperature. 3 For R. grandis, the LDT and DD requirements for the egg, larval, prepupal and pupal stages were 5.7 ∘ C 84 DD, 6.2 ∘ C 263 DD, 8.1 ∘ C 90 DD and 6.8 ∘ C 178 DD, respectively. At 12.5 and 15 ∘ C, most prepupae failed to pupate, which is indicative of a facultative diapause. 4 The key developmental parameters reported in the present study may be used to model the phenology of the beetles throughout their range, providing better-timed control strategies or to predict outbreak risk under climate change as a result of reduced biocontrol effectiveness.