Last fall, I noticed the squirrels were leaving many acorns from our Quercus agrifolia on the ground, rather than stashing them.
Typically they grab nuts as soon as they fall, scamper to a hiding place, glance around,
and then quickly bury the nut. Sometimes they double-back, dig up the nut, and
rebury it somewhere else. Instead, they were ignoring many nuts, and
burying only a few.
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| Acorns from Quercus agrifolia in the Bay Area. |
Naturally, this behavior change caught my attention. My
folks were visiting from Alaska, so we launched an investigation. The first
thing we noticed is that almost all of the ignored nuts had a bore hole in
them. My dad split open a healthy nut, and several nuts with bore holes. The
unhealthy nuts were filled with frass, but no inhabitants.
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| Acorn with a bore hole. |
Two insects that target acorns in the San Francisco Bay Area,
are filbert worms (Cydialatiferreana)
and several species of filbert weevils (Curculio
spp.). After mating in spring, the female filbert weevil drills a small hole
in an immature acorn, and inserts her eggs; the female filbert worm lays her eggs
on the outside of an acorn, and the larvae bore into the nut. In both cases, the
larvae develops and feeds in the acorn, leaving its frass; and then bores out in
the fall, moving to the soil until spring. The number of infected acorns varies by year,
environmental conditions, and the individual tree. The infestation does not harm
the tree, but the acorns are infertile.
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| Left: healthy acorn. Middle and right: infested acorns. |
Interestingly, birds and squirrels can discriminate between
the healthy and infected acorns. Much like humans, they separate the infested nuts,
and preserve the healthy!
Learn More:
- Filbert Pests in Live Oaks (prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc)
- Pests of Acorns (prepared and edited by Richard B. Standiford and Pamela Tinnin)
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