Insect Galls

Downy Woodpecker snacked hereImagine being able to release a chemical that makes a house grow around you.   There are critters that can do this.  Insects, mostly.  In winter, when the foliage has fallen away from the plants, you can find plenty of evidence.  The “houses” they create are called a galls.

It seems the larvae of gall-producing insects release powerful growth hormones that cause plants to grow in unsual ways… creating perfect shelters for themselves.  Each gall-producing species has a preferred host, so when you find the “house” you can also know the insect that produced it.

The Goldenrod Ball Gall is produced by a fly larva (Eurosta solidaginis).  Read more about it by clicking here.  I also wrote about it last March.  You can read that post here.

Chickadee Snacked HereI think the fun thing to notice about Ball Galls in winter is the shape of the hole – if any…  No hole means the larva is still inside.  A pinhole means this is a 2-year-old gall and the adult escaped last spring to lay more eggs.  Larger holes usually mean bird activity; birds know there’s a tasty snack inside!  The picture above shows a gall that was visited by a downy woodpecker.  With that long pointy beak, a downy can get right to business finding the larva inside.

This gall was visited by a chickadee…  Shorter beak and much less precision in finding the snack!

Willow Pine Cone GallThe Willow Pine Cone Gall is another fairly conspicuous gall that you might find on a winter walk in the Great Lakes region.  This one is caused by a tiny midge (Rhabdophaga strobiloides).  The adult lays eggs at the branch tips.  The larva burrows in and the gall is formed right at the end of the twig.  It so resembles a pine cone – the seed forming part of conifers – that many people mistake it for the seed-producing part of this plant.  Nope!  It’s an insect house.

Willow Pine Cone Gall Cross-section




Slap me if you are getting tired of references to Stokes Guide to Nature in Winter.  I can’t help it if it has a lot of interesting stuff in it, like this:

What has attracted attention to this gall, besides its beauty, is the tremendous number of other insects that use it for overwintering and breeding…  For one study 23 galls were collected and 564 insects were reared from them.  Only 15 contained the original host gnat, but in addition there were 6 wasp parasites, 169 other guest gnats, and 384 eggs of the Meadow Grasshopper…

It’s all too fascinating… Nature never ceases to amaze me.

Learn more about this topic:

11 thoughts on “Insect Galls

  1. I only started to notice galls this year … then I saw them everywhere … many different kinds. We even had a couple of those willow galls in our very own yard!

  2. The Stokes guide is incredible. I didn’t know about galls until I read the guide and have been noticing them ever since. I like the idea of a gall hike with kids, what a great way to get them paying attention to the what is around them unnoticed.

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