Opossums are Not That Awesome

Remember the viral meme that was all over Facebook claiming that opossums ate a huge amount of ticks?  

I called bullshit back in 2018, noting that "as for the notion that possums are capable of reducing the tick load in an area, that is complete hokum. Ticks breed at a rate that is astounding even for possums, and possums make no serious dent in tick numbers. Sorry."

Now a couple of researchers have gone through the stomach contents of 32 wild possums in central Illinois, and found no sign of tick consumption.

Using a dissecting microscope, we searched the contents exhaustively for ticks and tick body parts, without sieving or pre-rinsing the stomach contents. We did not locate any ticks or tick parts in the stomach contents of Virginia opossums. We also performed a vigorous literature search for corroborating evidence of tick ingestion. Our search revealed 23 manuscripts that describe diet analyses of Virginia opossums, 19 of which were conducted on stomach or digestive tract contents and four of which were scat-based analyses. None of the studies identified ticks in their analyses of diet items. (Source: "Are Virginia opossums really ecological traps for ticks? Groundtruthing laboratory observations," July 2021, by Cecilia Hennessy and Kaitlyn Hildad at Eureka College).

So where did the "possums eat a lot of ticks" story come from? 

Apparently researchers took 5 captive possums and put a known quantity of larval blacklegged ticks on them and then counted how many fell off over the course of  FOUR days. These researchers found that, on average, only 3.5 larval ticks fell off each opossum having ingested a blood meal, and the rest could not be located in the cage set-up, prompting the authors to assume that the ticks were eaten by the opossums while self-grooming. 

Note that these earlier researchers DID NOT check the possums for ticks before releasing them, and that the four-day period of captivity was too short for the ticks to actually reach repletion and drop off.  

In short, the original work was full of holes and, as Hennessy and Hildad note in their study of wild possum stomach contents, are not supported by over a dozen stomach content analysis of wild possums done to date.