The average number of beetle generations produced each year is specific for every individual species (Table 10, Taxonomy of the elm bark beetles). A high number of beetles present from early spring till late autumn will increase the possibility of DED transmission. Species, such as S. scolytus, that require only a short time for development of each generation, produce up to four generations per year, and will therefore be more effective as a vector than beetles with only one generation per year. In addition, the actual number of beetle generations emerging per year is dependent on the environmental conditions, abundance of breeding material and the density of the beetle population. H. rufipes generally produces one generation a year. However, if there is enough brood wood and a high number of beetles present, a second generation may be developed{[452]}.

Like H. rufipes,  S. multistriatus and S. scolytus tend to fly at temperatures around 20 °C (Photo 51, {[230],[358]}). In the southern half of The Netherlands the temperature exceeds 20 °C earlier in the spring (usually in April) than in the northern half. As a result the S. multistriatus and S. scolytus species produce an extra generation in the south {[381]}.

Photo 51:        S. scolytus beetles tend to fly at temperatures around 20 ˚C.