Welcome back to Ask a Naturalist, your own personal Google for information on all things natural on PEI. I’ve had a few questions about the ‘cones’ appearing on roadside shrubs in some areas. These aren’t cones at all – they are far more interesting: insect galls!
Galls are abnormal lumps or bumps on a plant, usually a reaction to infection by insects, fungi, or bacteria. Depending on the cause, galls can appear on roots, stems, twigs, leaves, or flowers, and can be so subtle as to be barely noticeable or so in-your-face that you can’t miss them.
The photos I’ve been sent and the shrubs I’ve seen are all Willows (Salix spp.) that have been infected by an insect called a Gall Midge (Rhabdophaga strobiloides). Those structures that looks a bit like conifer cones are appropriately called Willow Pinecone Galls.
A midge is a tiny insect, about the size of a mosquito. Some species bite, and you may know those as ‘no-see-ums’. Adult Gall Midges don’t bite humans and if it weren’t for these tell-tale galls, you probably wouldn’t know they were around at all.
In spring, adult female Willow Pinecone Gall Midges lay eggs on buds at the tips of willow branches. As the larvae hatch and begin to feed, they secrete chemicals that stop the growth of the bud, as well as any further elongation of the branch. Leaves continue to grow but become thick and scale-like. This forms the galls, which provide protection for the midge larvae but don’t harm the willow.
The Gall Midge larva spends all summer, fall, and winter in a cozy chamber in the middle of the gall. If you cut open a Willow Pinecone Gall, you’ll see the tiny orange or yellow larva, about the size of a grain of rice. In spring, it pupates and emerges as an adult to find a mate and start the cycle all over again.
That’s if it survives the winter. Gall Midge larvae aren’t totally safe inside their protective galls. They have a natural antifreeze (glycerol) to survive our frigid winters without freezing solid, but it doesn’t always work and if they freeze, they die. Additionally, insect-eating birds will pull the galls apart to get the tasty larvae inside, and parasitic wasps will sometimes take over galls for their own overwintering habitat. One-half to two-thirds of Willow Pinecone Gall Midge larvae don’t make it to spring.
Willow Pinecone Galls become most obvious as leaves drop in fall and winter but if you keep an eye out, you’ll notice them year-round. They’re common across the Island, and one of the many fascinating plant-animal interactions happening on PEI untamed!
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