The Fates of Ball and Elliptical Galls on Solidago canadensis and Their Importance as Secondary Microhabitats

K. J. Clinton High School Student


A total of 609 galls were collected between the months of May and June in 1990 and May and June in 1991 after the gallmakers, Gnorimoshema gallesolidaginus (elliptical gall moth) and Eurosta solidaginis (goldenrod gall fly), should have emerged.  The summer of 1990 reveled 220 ball galls and 34 elliptical galls; 1991 revealed 339 ball and 16 elliptical galls.  Ball gall exhibited the following fates:  30.4% successful, 22.2% unsuccessful (no exit hole), 19.5% parasitized, 19.5% eaten out by birds, 8.4% unknown.  Elliptical galls exhibited the following fates:  72% successful, 10% unsuccessful, 14% parasitized, 4% unknown (elliptical galls are not eaten out by birds).  Ball gall diameters averaged 20.9mm for successful galls (n=152), 17.5 mm for unsuccessful (n=110), 19.9 mm for parasitized (n=98) and 22.9 mm for eaten out galls (n=107).  Averaged elliptical diameters showed successful galls to average 13.8 mm (n=29), unsuccessful average 13.3 mm (n=5), and parasitized averaged 13.1mm (n=5).  The one unknown gall was 13.0 mm.  Hole diameters were taken and averaged for successful galls, parasitized galls, and galls eaten out by birds.  Inhabitants within the galls were collected after the fly should have hatched.  This collection included thirty-nine ant colonies, Crematogaster lineolata, an individual ant from another species (unidentified), twelve wasps (number of different species unknown at present), three solitary bees, and seven spring tails.  In 1991, fourteen colonies were collected and the number of adults were counted and averaged 144 per colony (SD=106.58).  There were thirteen colonies in ball galls and one colony from an elliptical gall.  The ball galls which they inhabited averaged 23.5 mm (SD=2.89). The elliptical gall was 16.6 mm.  Actively growing plants were measured in 1990 when the ball gall formation was first noticed (26 May) and average 667 mm compared to 691 mm for those without galls.  The average distance up the stem where the gall was located was 595 mm and diameter of these galls averaged 14.4 mm.


The life cycles of the gall fly, Eurosta solidaginis, and the gall moth, Gnorimoshema gallesolidaginus, are very similar but occur at different times of the year.  The fly lays her eggs in the terminal buds of Canada goldenrod, Solidago canadensis, from late May to early June and they will hatch within seven to fourteen days.  The newly hatched larva bores down the meristematic tissue (Uhler, 1951).  The moth lays her eggs in autumn and they will not hatch until the following spring.  Upon hatching, the moth larva crawls up the stem of a Canadensis shoot.  It then bores into the side of the apical bud and tunnels down  4 cm through the meristem (Leiby, 1922).  Secretions from the moth larva cause a long, thin swelling called an elliptical gall to form on the stem while the fly larva’s secretions cause a ball-like growth on the stem, known as a ball gall.  (See Carango et al, 1988 for information on the specifics of the secretions.)  When a moth or fly is developing, it must burrow its tunnel before it begins to pupate.  If it does not, it will not be able to escape when it becomes an adult since adults have no chewing mouth parts. 

The moth begins to pupate in late July, and the adult emerges, mates, and the cycle continues.

Life cycles are never perfect and are often interrupted.  In some cases the fly or moth will not develop into an adult and exit its establishment.  The fate of a gall is determined by a number of factors.  The gall fly and moth are both considered to be parasitic on its host plants (Stinner and Abrahamson, 1979).  However, they too can be parsitized.  These attackers include two species of parasitoid wasps, Eurytoma gigantea and Eurytoma obtusiverntris (Hymenoptera:Eurytomidae), and the inquiline beetle, Mordellistena unicolor (Coleoptera: Mordellidae), (Wies and Abrahamson, 1986).  Parasite escape holes will be smaller than a regular fly or moth escape hole.  In addition, during the winter months, Downy woodpeckers (Picoides pubescens) and black-capped chickadees (Parus atricapillus) peck open galls to eat the larvae within (Weis and Abrahamson, 1986 and Walton, 1998), whether it be the fly, the parasite, or other larvae (Confer and Paicos, 1985).

After the gall has been used and the fly has either escaped, been parasitized, or been eaten out, the gall is  a year old and has become tough textured habitate for a variety of insects.  Spring tails, wasps, solitary bees, beetles, and, most commonly in this projects, ant colonies have been discovered using the gall as a microhabitat.  The gallmakers are not involved in one interaction with their host, but are a small part of a whole community of interactions focused around the gall host, Canada goldenrod.

In a study carried out by a ninth grade ecology class, (unpublished, 1989), 5,702 goldenrod plants were surveyed.  One hundred and thrity-one plants (2.3%) were found to be infected (Table 1) with the presence of either a ball or elliptical gall.  This infection rate has led to a more indepth study of this system.  In this study, I have attempted to quantitatively evaluate gall diameters as related to fates.  In addition, I have expanded my study to look at the relationship of galls to woodpeckers and gall to ants.



A total of 609 galls were collected from the CRESO research site (ACWS) in early summer of 1900 and 1991.  Galls were collected randomly from areas of the research site containing goldenrod.  In the process of collection, an attempt was made to gather both large and small galls with equal effort.  Each gall was classified under a specific fate, which was determined by the size and shape of the escape hole.  Parasites are smaller than the fly or moth and therefore make smaller holes (Uhler, 1951).  Unsuccessful galls were defined by the lack of an exit hole.  In galls eaten out by birds, there was a notably larger hole, exposing ragged edges that had been pecked open.  The diameter was measured with a Spi 5 in.  poly Vern caliper to the nearest 0.05 mm.  Results were calculated under two separate types, either ball or elliptical.  This was determined by the shape and surface texture of each gall.  Ball galls are greater in diameter but shorter in length and have a smoother surface than elliptical galls.  Any inhabitants founds were collected and preserved.  Results from each year were compared and evaluated. 


A growing area of interest is the abundance of ant colonies found in the galls after the departure of the gall fly or moth.  The galls are used as secondary microhabitats, and sometimes the stems are hollowed out above and below the gall to provide more room or possibly an escape route.  When galls were collected, thumping them with the forefinger would cause any ants within to come out of the exit hole.  However, if the ants were not very active, blowing into the exit hole would bring the ants out.  Ants were identified as Crematogaster lineolata by Dr. Will Cloyd, an entomologist at Carson Newman College.  In an effort to determine plant height differences, twenty-seven actively growing plants with galls and thirty plants without galls were measured in height from the ground to the tallest growing point of the plants.  To determine gall location on the stem, measurements were taken from the base of the plant to the base of the gall.  Diameters of each gall were also taken.



The holes of 30 galls eaten out by birds were measured at the widest part of the hole and averaged 7.58 mm (SD=2.16).  The holes of thirty successful galls were also measured and averaged 2.54 mm (SD=0.44).  The holes of galls that were parasitized averaged 1.57 mm (SD= 0.26). 


Forty elliptical galls exhibited a mean of 13.6 mm in diameter.  (SD=2.48).  Four hundred and sixty-seven ball galls averaged 20.3 mm (SD=3.78).  (During the first year of study some gall diameters were not taken).  Averages for diameters of galls classified by fates are shown in Figure 1.  (The diameters of the unknown galls were not taken.)  There is a notable difference in the size of eaten out galls and the other types of galls.  Eaten out galls tend to be larger than all other types of galls.  Figure 2 compares the percentages of the different fates of ball galls of those taken in 1990 to those taken  in 1991.  There was an increase in the percentage of eaten out galls in 1991 (27%), compared to 8% in 1990.  The percentage of successful galls decreased from 43% in 1990 to 23% in 1991. 


In 1990 five types of inhabitants were collected after the fly had hatched (or should have hatched).  These included twenty-one ant colonies, an individual ant (unidentified), eight wasps, two solitary bees, and three spring tails.  In 1991 four different wasps were collected along with four spring tails, three spiders, a solitary bee, and eighteen ant colonies.


In the summer of 1990, twenty-one ant colonies were discovered out of the 251 galls collected (8.4%).   In 1991 the adults of fourteen ant colonies were collected and counted from July 7 through August 9, and averaged 144 per colony (SD=106.58).  These colonies were found in thirteen ball galls and one elliptical gall.  The ball galls which they inhabited averaged 23.5 mm in diameter (SD 2.89).  The elliptical gall was 16.6 mm.  The ants inhabited seven successful galls, three eaten out galls, two gall with multiple holes (probably eaten out), and two parasitized galls.


Actively growing plants with galls averaged a height of 667 mm while those without galls average 691 mm.  The average height from the base of the plant to the point just beneath the gall was 595 mm, and the average diameter of the galls on these plants was 14.4 mm.




Downy woodpeckers and sometimes black-capped chickadees use the fly larvae as a food source in winter (Weis and Abrahamson, 1986).  Moths hatch in the fall.  Therefore, woodpeckers cannot use the moth larva as a winter food source.  Predictably, no elliptical gall has been found with peck marks.  Woodpeckers are able to distinguish between a ball and elliptical gall (Weis and Abrahamson, 1985).  Additionally, eaten-out galls were found on the upper end of the size range of ball galls.  Both year of data support the conclusions of Confer and Paicos (1985) and Abrahamson, (1989) that birds prey on the larger galls.  It seems to be advantageous to the gall larvae to produce a gall with a diameter in the middle size range, indicating a type of stabilizing selection.  Larger galls had a higher incidence of bird predation, and smaller galls had a higher incidence of parastitism.  E. gigantea, one type of parasite, attacks the gall after it has reached its maximum size.  The E. gigantea female can only inject eggs when her ovipositor is longer than the thickness of the gall wall.  Therefore, the parasitoid is limited to small galls (Weis and Abrahamson, 1985).  Weis and Abrahamson (1985) found that larger galls contain larger larvae.  I have yet to determine if this relationship between gall size and larva size exists in my study area.  Unsuccessful galls were found to be smaller in diameter than successful galls.  Thus, by choosing a larger gall, a woodpecker would perhaps increase his chances of selecting a gall with a fly larva and increase his chances of getting a larger larva.


From 1990 to 1991 there was an increase in the percentage of eaten-out galls from 8% to 27%.  Interestingly enough, the percentage of successful galls decreased from 43% in 1990 to 23% in 1991.  There could easily be a direct correlation between the two percentages.


The ant colonies reveal some striking information.  They have a fairly high appearance rate (8.4% from 1990).  Furthermore, of the fourteen ant colonies that were counted, an average of 144 adults per colony was discovered.  However, a surprisingly high standard deviation was found (106.58).  The colonies were collected in a time frame of less than three weeks.  Therefore, colony size does not seem to correlate with time of year.  It should be noted that the average size of galls is on the upper end of the size range.   Ants scouting for homes appear to look for larger habitat, but do not seem particular to the type of hole present on the gall.


Besides the ant colonies of Crematogaster lineolata, inquilines inhabiting the used gall have not yet been identified according to species.  However, the variety of secondary inhabitants is high.  Besides the initial insect (gall fly or moth) and the parasites, the gall plays host to a variety of inhabitants reusing the gall as a microhabitat.  Additionally, woodpeckers and chickadees use the fly larvae as a food source (Weis and Abrahamson, 1986).  Therefore, within a seemingly simple system, lies not one system, but a whole community.  Each individual system intertwines with the system revlolving around the Canada goldenrod.  Seemingly, the gall fly and moth are considered to be parasites.  Having negative effects on their host plants (Stinner and Abrahamson, 1979; Hartnett and Abrahamson, 1979; McCrea and Abrahamson, 1985; McCrea et al, Abrahamson and McCrea, 1986).  It was discovered that “Gallmakers may have a number of negative impacts on host plants…” (McCrea and Abrahamson, 1986) and “..the gall insects can lower total ramet production and, perhaps more importantly, decrease reproductive allocation in the gall-bearing ramets, most probably having an appreciable effect on plant fitness” (Stinner and Abrahamson, 1979).  Yet, this idea must be reconsidered when one looks at the overall effect.  The gall is utilized by such a variety of insects that it plays an important role within the community by promoting diversity.  Quantifying the overall effect that galls have upon the community of Canada goldenrod is a lengthy and immense study.  Much research has been done focusing on gall and host interaction.  However, each result seems to add many more questions than answers.




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