I found this--doesn't look like they would be harmful. There was no mention of consuming plant matter, just sipping nectar.
These insects are actually flies (Diptera) of the family Bibionidae, having the scientific name, Plecia nearctica Hardy. The common name "lovebug" has been given to these black colored, orange-backed flies because they are most often seen flying around in mating pairs.
Lovebug larvae breed in moist habitats high in organic matter such as bar ditches and swampy areas. They are harmless as immatures and actually help nature by decomposing dead plant tissues. Mass adult emergence occurs during specific periods of the year as dictated by environmental conditions (prolonged period soil saturation from rains) conducive to their development. Adults spend their time sipping nectar from flowers and searching for mates and mating while hovering in the air. It has been thought that car fumes contained some properties that were attractive to these flies, but they are naturally attracted to open spaces within their generally wooded habitats.
