Plant galls: how insects co-opt plant development to build themselves homes

(Image credit: Lahvak via Flicker/CC BY-NC-SA)

Note: This is a science outreach piece belonging to a series I wrote for the newsletter of the Fredericton Botanic Garden. I’d be happy to see it adapted for use elsewhere and so am posting the text here under a CC BY-SA 4.0 license. If you use it, though, I’d appreciate hearing where and how.

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With winter upon us, a walk in the Garden has gotten less colourful: nothing is in flower, and most plants have died back or dropped leaves. But I hope you’ll see this as an opportunity to notice things that aren’t as easy to spot in the full flush of summer vegetation. Plant galls are such a thing, and they’re a piece of natural history that especially fascinates me. A “gall” is an abnormal growth on a plant, caused by an attacking natural enemy such as a bacterium, a virus, or an insect herbivore. I’m a particular fan of insect galls, because they tip us off to a complex web of developmental, ecological, and evolutionary interactions between the insect and its plant host.

The story of an insect gall generally goes like this: a female insect lays an egg on or into the host plant, and when it hatches, the developing larva chews its way into the plant. The feeding insect provokes a developmental response by the plant, which thickens and elaborates growing tissues to produce the characteristic tumour we call the gall. The gall provides the insect larva with food, a hospitable environment, and protection from enemies – and there the larva stays (who wouldn’t?) until it emerges as an adult to continue the cycle. The interaction between larva and plant is an intimate one: the larva spends its life bathed in plant tissue, while also co-opting the developmental machinery of the plant to force it to build the gall. Some gallmakers take charge by secreting analogues of plant hormones, others by chewing on just the right cells in just the right ways; but either way, the result is a gall that’s distinctive and easily recognized as the product of a particular insect attacking a particular plant. The pictured round gall on goldenrod, for instance, instantly betrays its maker as the goldenrod ball-gall fly (Eurosta solidaginis), while a more spindle-shaped gall lower on the stem is made by the goldenrod spindle-gall moth (Gnorimoschema gallaesolidaginis – and there’s a name that will never make my “Wonderful Latin Names” series) . Some of the most elaborate galls are made by gallwasps on oak leaves, twigs, and acorns: different species produce bristly ping-pong balls, wrinkled warts, fuzzy sausages, and a hundred other fantastic and instantly-recognizable forms.

The intimacy of the gallmaker-host interaction has major implications for both ecology and evolution. Because the gallmaker has to tolerate immersion in plant tissue and must direct plant development, gallmakers tend to evolve very narrow diets: often, each insect species attacks a single organ or tissue on a single plant species (but does it very well). With specialized diet comes high diversity: rather than one gallwasp that attacks all oaks, for instance, we have hundreds of gallwasp species, each attacking a different tissue on a different oak. In turn, this means a gallmaker has to be exquisitely attuned to finding and attacking its single host. A goldenrod ball-gall fly, for instance, may probe plant after plant with her ovipositor before settling on just the right host for her offspring. Diet specialization also means that attacker and host plant can co-evolve, each adapting to the other over millennia. Research in my lab, for example, suggests that gallmakers evolve over time to reduce their impact on their hosts, while hosts evolve to better tolerate being galled. Enemies they may remain, but even enemies can get along better tomorrow than today!

So take advantage of winter and the chance to spot a gall – a little growth with a big story.

© Stephen Heard (sheard@unb.ca) Feb 23 2015 but licensed CC BY-SA 4.0.

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