Hawthorn Mealybugs can be controlled two times during the year.  The first is early May when they move from their overwintering spots in bark, out to twigs.  That timing has passed and you're now seeing lots of these insects on twigs and branches.

The second time for treatment is late summer or early fall.  Additionally, horticultural oil can also be used during the dormant season in winter.

Here is more information on treatment.  You might want to call a certified arborist to apply the pesticide, since timing is so important in controlling these mealybugs.

https://webdoc.agsci.colostate.edu/bspm/Hexapoda%20(Insects)/Hawthorn%20Mealybug%20December%202014%20Revision.pdf