Why Do They Smell, and What Is the Yellow Stain?

The smell, the yellow fluid, and the stain are not three separate topics — they're one defensive reaction, described in peer-reviewed detail as reflex bleeding: hemolymph released from the tibio-femoral leg joints of a threatened beetle, in a quantity that can amount to a substantial fraction of its body weight (Schmidtberg et al. 2019, Ecology and Evolution). That hemolymph carries the defensive alkaloid harmonine along with volatile alkyl-methoxypyrazines at high concentration, which is where the foul odor and bitter taste come from, and the same fluid is what stains porous, light-coloured materials on contact. UNH Cooperative Extension confirms the practical result: the yellowish fluid "can damage light colored rugs and upholstery." It's worth correcting one specific folk mix-up folded into this question: the yellow substance some homeowners describe finding indoors is this defensive fluid, not fecal waste — the beetles are not "pooping" on walls or furniture, they're reflex-bleeding when disturbed.

Because the strongest release of fluid happens when a beetle is crushed, crushing is exactly the wrong response to a cluster — /fall-invaders-faq grades crushing counterproductive, and this page agrees rather than re-litigating it: crushing maximizes staining, releases the most odor, and adds allergen-carrying dust to indoor air. There's a second reason to avoid leaving dead beetles in place: accumulations of dead insects in wall voids and around windows are a known food source for scavenging dermestid beetles, commonly called carpet beetles, which is one more reason dead clusters shouldn't be left where they fell (Colorado State University Extension). Vacuuming, rather than sweeping or crushing, reduces staining considerably, per UNH Extension's own guidance — but the full step-by-step removal approach belongs to /top-diy-methods/asian-lady-beetle, not to this page, which exists to explain the mechanism, not to walk through a cleanup procedure.