[2] It most closely resembles Paracantha culta, which is widespread in the Southeastern United States, but P. gentilis can be distinguished by having smaller spots on the head.
[4] These synonymies were confirmed by researchers constructing a systematic database of Tephritidae, but in addition the species Paracantha sobrina was synonymized with gentilis; no reasoning is given.
[9] Once first instar P. gentilis larvae hatch from eggs, each tunnels to a separate floral tube within the immature capitulum[10] and only feeds there for that life stage.
[9] Unlike many other Tephritidae, even species in Europe that use Cirsium capitula like Urophora solstitialis, P. gentilis does not cause host plants to form galls or host-tissue growth.
[7] First instar larvae always have four spiracular bundles with unbranched hair, which is a character shared with Rachiptera limbata, a member of a genus closely related to Paracantha.
This is possibly to assist in their changed food source, as they leave the floral tube and tunnel through more hardened plant material towards the outer margin of the capitulum.
[7] At this stage, the median oral lobe has fully formed between the mouth hooks, consisting of a heavily sclerotized dorsal rib and two projecting flanges.
Paracantha gentilis is the first species of Tephritidae where the median oral lobe was described; this character has been found to be shared with all other non-frugivorous Tephritinae.
[12] This larval stage avoids intraspecific competition for plant resources based on the density of third-instar P. gentilis larvae within a given capitulum.
The teneral adult rests on the outside of the capitula for about a half an hour once pupation has been completed, while excavating dark metabolic waste 1~2 times.
A portion of the population of these emerged adults are reproductively immature and/or inactive in during midsummer, and instead of immediately ovipositing, they move towards higher elevations and remain with non-host plants.
When not defending, P. gentilis engages in "agnostic" wing behavior, consisting of slow and asynchronous movements that seemingly don't act as communication.
The female then slowly retracts the ovipositor while terminalia are still touching, and the pair moves to a shady leaf and remain in copula for 2.5 to 4 hours.
[10] Paracantha gentilis is only known to use the flowerheads of Cirsium plants as a larval host, where larvae engage in an aggregated attack on the closed capitula.
P. gentilis also gradually surrounds itself with a mixture of dried feces and plant debris, which protects the third instar larval form and puparium.
[9] Multiple introduced species of European Tephritidae have become established in the Western United States, and as a result compete for host plants with native flies.
[18] The larvae of these species don't eat the same parts of the flowerhead, but P. gentilis may make the head unusable for C. undosa during the process of consumption.