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Boneset and gall midges

Stephany Cook and Emily Vincent
Ecology class, BIO 342, Biology Department, DePauw University
Fall semester 2014

Tall boneset, Eupatorium altissimum, is host to a species of gall-forming midge.  The midge lays its eggs in the plant’s tissue and induces the plant to form a gall.  Individual plants may host one, two, or three galls.  Galls may be placed on the main stem of the plant or on the branches of the plant.  The goal of our study was to measure variation in gall size and larval abundance depending on the specific location of the gall within the plant. 

We hypothesized that the position of the gall on the plant (top, middle, and bottom; proximal and distal) would affect gall size, and that larger galls would be on the bottom of the plant and on the main stem of the plant.  We also hypothesized that larger galls would contain more larvae.  

We collected data in three locations, A, B, and C, in the DePauw Nature Park.  We collected plant stems with two or three galls.  We measured plant height, and height, length, and width of each gall.  We counted the number of larvae inside each gall. 

There was no difference in the volume of galls between the top, middle, and bottom of the plants.  Galls on the main stem of plants were significantly larger than galls on branches.  There was a significant correlation between gall volume and number of larvae; larger galls contained more larvae.

The lack of correlation between gall volume and plant height suggests that gall size is influenced by genetic factors.  The volume of a gall may be based on the pairing between the genotype of the insect and the genotype of the plant.  It makes sense that galls were larger on the main stem than the branches of the plant.  The main stem is larger, stronger, and contains more nutrients than the branches.