The Curious Case of the Little Fat Worm: Acorn Weevil, Case Closed

The sudden sight of two little fat worms crawling across my bedroom floor caused quite a fright; especially when my take-out dinner was lying there just moments before while I was watching a movie with the lights off. Feelings of disgust naturally arose – did I just eat a rotten pizza infested with giant maggots?!

After a process of elimination (a scrupulous inspection of the remaining pizza slices and a lack of organic items in the room), all arrows pointed to the acorns piled in the wooden basketOLYMPUS DIGITAL CAMERA next to the window. A few clicks on Google led to countless photos of Acorn Weevil (Cyllorhynchites ursulus) larvae inside, emerging from and outside of acorns. Indeed, the acorns in the wooden basket had little boreholes on them.

There was still something strange about the incident though. Why did the two little worms choose to crawl out at the same time and why were they moving in the same direction? Could it be that the heater directly above the acorns tricked the larvae into thinking it was summer, signalling that their slumber party was over? Was the fridge emitting a smell that the larvae found enticing? These seemed like plausible explanations so the case was put aside and I was left in awe.


Fast forward one month: Some lab mates and I trudged to the peak of Mount Takao in a final attempt to see the Momiji (Acer palmatum). Unfortunately, it was a rainy day and we were a little late in the Momiji  (Acer palmatum) season. We posed a bit in-front of a Momiji  (Acer palmatum), took some selfies and went in Mount Takao Visitor Centre to take more silly photos. As we were leaving, the cartoon strip to the left caught my eye. No. way. That’s the Acorn Weevil (Cyllorhynchites ursulus), a sad acorn and the little fat worm! In an excited frenzy, I explained the Curious Case of the Little Fat Worm to the staff manning the front desk and shot a bunch of questions at him.

From left to right: Adult Acorn Weevil, female Acorn Weevil (Cyllorhynchites ursulus) laying eggs in an acorn, Acorn Weevil larvae inside a mature acorn. Photo Source 

He had all the answers. The female Acorn Weevil (Cyllorhynchites ursulus) most commonly lays eggs in young Kunugi (Quercus acutissima) and Konara (Quercus serrata) acorns due to wide availability in Japan. The larvae develop inside the acorn and eat the cotyledons as a food source, gradually replacing it with faeces (sorry wild boars, black bears and squirrels!). They stay within the confines of the pericarp for an average of 20 days or so and emerge through a borehole to crawl into soil, where they stay for one to two years as they mature into adults. Everything makes sense in light of these facts – the larvae emerged at the same time probably because the eggs were laid around the same time (they were part of the same batch of acorns) and they were crawling towards the fridge because it was in the darker side of the apartment (the larvae crawl towards soil when they emerge, the darker option).

I was not alone – apparently it’s quite a problem in Japan and many amateur acorn collectors get shocked when the little fat worms emerge and crawl all over apartments in search of darkness.




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