What specific risk does the specialized relationship with *Bombus affinis* impose on the parasite?
Evolutionary vulnerability due to host decline.
The survival of the Yellowish Cuckoo Bumblebee is directly tied to the success and availability of its host species, such as *Bombus affinis* (the Rusty Patched Bumble Bee). When a primary host species faces severe threats, such as habitat loss or disease, the specialized parasite population relying upon it experiences disproportionate pressure. This situation illustrates a critical concept: evolutionary vulnerability. A trait that is highly advantageous in a stable ecological context—in this case, specializing on one successful host—becomes a severe liability when that host population crashes. Consequently, the parasite faces either immediate local extirpation or intense selection pressure to rapidly shift to an alternate, potentially less-preferred host.
