Bindweed Stem Gall Midge

Finding a plant gall I haven’t seen before is always a pleasure. Today, while looking at the remains of last summer’s wildflowers along the edge of a small catchment pond at St Olaf, I noticed the nifty spiral of a Field Bindweed stem wrapped around the thin trunk of a Sumac. And toward the bottom of the bindweed stem were several elongated nodules. Plant galls.

Clearly a must have volume for every curious naturalist is the one-of-a-kind guidebook by Charley Eiseman and Noah Charney, Tracks & Sign of Insects and Other Invertebrates (2010). Included among the diverse chapters on droppings, leaf mines, sign on twigs and rocks and shells is a chapter on plant galls. Eiseman and Charney give quick mention of the Bindweed Stem Gall, but also give the general sage encouragement to collect and rear the galls to discover the identity of the gall makers: “If a gall is inhabited, it is often possible to collect it and see what emerges. Bear in mind, however, that what emerges may not be the insect responsible for the gall. Numerous parasitoids and inquilines are often 'guests' in the galls of other insects.” For me, this potential for the parasitoid wasps is all the more reason to gather galls and shelve them in vials.

The other indispensable reference is Margaret Redfern’s New Naturalist volume, Plant Galls (2011). Redfern has studied plant galls for much of her life and this book presents a dazzling breadth and depth of knowledge on the subject, including discussion of the complex and not always well understood biochemistry involved in the formation of galls as well as including information on many of the interesting and bizarre life cycles of the gall makers.

“Understanding how galls develop is a major problem that has exercised cecidologists for years. The gall causer redirects normal growth and development of the plant, the details probably varying in different gall-causing groups. In bacterial galls the processes are fairly well understood but not so in animal galls; in these, several ‘morphogens’ have been suggested but none have been shown conclusively to cause galls. The gall causer manipulates the plant to ensure an adequate, reliable food supply and also to protect itself from adverse environmental conditions including unfavorable climate and predators. How this happens is the subject of active research today.”

Plant galls are fascinating. For overwintering larva they function as space capsules traversing inhospitable seasons. For growing larva the forming galls provide protection and food. It’s as if instead of building a house we could inject some instructive chemical into the trunk of a living tree and it would proceed to grow us a room complete with a well-stocked pantry.

Posted on January 27, 2017 07:46 PM by