Giant ragweed often forms massive stands, especially in moist soil of low-lying fields, river floodplains, wetland margins, etc. Individual plants can reach Jack-in-the-beanstalk proportions, towering to 15 feet in height. Such ragweed groves are typically shunned by all who encounter them.
Like some other amazing borers, this one spends much of its life cycle burrowing through the stems of plants, and Dectes texanus is especially fond of ragweeds. Perhaps it could be dubbed the "ragweed borer".
Festive fruit flies are utterly dependent upon giant ragweed. They lay their eggs in the flower clusters - some blooms of the ragweed are in the upper lefthand corner of the photo - and the larvae ride out the winter within the seeds. Come late summer, and the astonishing adults are capering about the ragweed foliage, engaging in eccentric courtship displays. If there is one compelling reason to inspect giant ragweed stands, it is because of the possibility of seeing this marvelous fly.
The genus of ragweeds is Ambrosia, and that translates to "food of the gods". That may be overstating the case a bit, but festive fruit flies sure like the stuff.