Extension News

The Dark Side of Collecting Acorns

acorn weevil larva

Note to media editors: This is the Garden Column for the week of Sept. 28, 2007.

9/24/2007

Laura Jesse
Entomologist
Plant and Insect Diagnostic Clinic
Iowa State University

It was a dark and stormy night, the wind howled, the lightning flashed. A good night to just head off to bed early and snuggle under the blankets. Innocently, I walked into my upstairs bathroom to brush my teeth, only to come across a gruesome scene. There was a maggot-like insect wiggling across my floor. Quickly, I did the gross-thing-on-the-floor tap dance. Luckily my experience in entomology proved useful for once. Before I could really panic I remembered the acorns my toddler had been playing with. He collected several acorns a week ago and for some reason he enjoys washing them repeatedly in the bathroom sink. Sure enough, there was one of his acorns on the bathroom counter with a pencil-lead sized hole in it. My maggot-like bug was the larvae of an acorn weevil.

The adult acorn weevil is a brown colored beetle about 3/8 inch long, and has a very long thin snout. The female uses her long snout to make a small hole in a developing acorn on the tree. She lays several eggs within the hole. Her eggs hatch and the creamy white, grub-like larva feeds on the developing acorn inside the nut until fall. The larva grows to 1/4 to 3/8 inch in length and is off-white in color with a brown head. The legless grub is curved and fat in the middle, tapering toward both ends. The larvae within the acorn on the tree fall to the ground in the nut in the late summer or fall. 

In the fall the fully grown acorn weevil larva chews a perfectly round 1/8 inch hole in the side of the nut and emerges. I would like to see them actually do this since the hole does not look large enough for the larvae to fit through. If the acorn were still outdoors, these larvae would tunnel into the soil to complete development. They remain in the soil for one to two years before emerging as a new adult weevil to repeat the process. 

However, if the acorns have been carried indoors by inquisitive toddlers, the emerging nut weevil larvae are forced to wander across the floor, often "burrowing" into carpeting or under furniture in an attempt to finish the normal process. These misplaced grubs are harmless to the house, its occupants and contents. They cannot damage the furniture, carpets, people or pets. Wandering weevil larvae need only be swept or picked up and discarded.

Part of the reason you find so many "wormy" or "holey" nuts under the trees and so many infested nuts and acorns end up in the house is because the squirrels leave them behind. It appears the squirrels are able to select the good acorns and hickory nuts during their fall frenzy of nut gathering and burial. This leaves only "wormy" nuts for you. If you want to collect the uninfested nuts for yourself, you will have to get up early and do your collecting ahead of the squirrels.

Now my only concern is where my son has put all the other acorns he picked up.  Although the acorn weevil larvae are harmless I don’t particularly want them crawling around in my bed!

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Contacts :
Laura Jesse, Plant and Insect Diagnostic Clinic, (515) 294-5374lrahnsen@iastate.edu

Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu

There is one photo for this week's column.

AcornWeevil9-28-07

Caption: In the fall the fully grown acorn weevil larva chews a perfectly round 1/8 inch hole in the side of the nut and emerges.