What is this? He has bored into the the flower bearing fresh new shoots of a...
We cannot see a pest in your photo. Here are some possibilities. Elderberries have hollow stems and it is possible that rough weather, wind, heavy rains, etc. may have knocked the stem over. Elderberries can be susceptible to possible borers such as the elderberry borer, corn borers, etc. At this point, slit open the stem and look for larvae and/or frass. All you can do is prune back to healthy tissue. Dispose of affected plant material in the trash. Do not leave it on the ground or put in the compost bin.
Thank you for the additional photo. The insect is most likely the European Corn Borer, Ostrinia nubilalis, This is the caterpillar of a moth that prefers corn, but will occasionally stumble onto other hollow, pithy stalks that are similar to corn stalks.
Control is as explained in the earlier response. Simply cut out the affected cane and destroy it. Since it is a caterpillar, it is susceptible to Bt 'Kurstaki', Bacillus thurengiensis is a disease and 'kurstaki'' is a strain of the disease that only kills caterpillars. It is a safe alternative to not-selective insecticides when dealing with caterpillars. In the case of elderberries, however, there is no effective way to deliver the pesticide.