The goldenrod ball gall fly, Eurosta solidaginis (Diptera: Tephritidae), is parasitized by two eurytomid wasps and preyed on by a mordellid beetle, and two bird species. The birds are known to prey most intensively near a forest edge. The proportion of fly larva parasitized and preyed on by insects was examined, and a decline in the proportional abundance of beetle larvae near the forest was noted. The possible role of birds as the selective force for this distribution is considered

Confer, John L

English

ValpoScholar

The Great Lakes Entomologist 
Volume 23 
Number 1 - Spring 1990 Number 1 - Spring 1990 Article 7 
April 1990 
Spatial Distribution of the Goldenrod Ball Gall Insects 
John L. Confer 
Ithaca College 
Follow this and additional works at: https://scholar.valpo.edu/tgle 
 Part of the Entomology Commons 
Recommended Citation 
Confer, John L. 1990. "Spatial Distribution of the Goldenrod Ball Gall Insects," The Great Lakes 
Entomologist, vol 23 (1) 
Available at: https://scholar.valpo.edu/tgle/vol23/iss1/7 
This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. 
It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. 
For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 
1990 THE GREAT LAKES ENTOMOLOGIST 33 
SPATIAL DISTRmUTION OF 

THE GOLDENROD BALL GALL INSECTS 

John L. Confer and Julie Orloffl 
ABSTRACT 
The goldenrod ball gall fly, 
Eurosta so/idaginis (Diptera: Tephritidae), is parasi­
tized by two urytomid wasps and preyed on by a mordellid beetle, and two bird 
species. The birds are known to prey most intensively near a forest edge. The 
proportion of fly larva parasitized and preyed on by insects was examined, and a 
decline in the proportional abundance of beetle larvae near the forest was noted. 
The possible role of birds as the selective force for this distribution is considered. 
The life histories of the ball gall insects of goldenrod2 have been described (Uhler 
1951, 1961). The gall fly, Eurosta solidaginis (Fitch) (Diptera: Tephritidae), emerges 
in spring and lays its eggs in the growing tip of the goldenrod, and its larvae 
stimulate the formation f all ball galls. A f y arva may be parasitized by either of 
two wasps, Eurytoma obtusiventris Gahan or E. gigantea Walsh (Hymenoptera: 
Eurytomidae). Larvae of a beetle, Mord llistena unicolor Lec. (Coleoptera: Mordel­
lidae), are omnivores and may eat any of the above insects or co-develop in the same 
gall with the other insects. Studies of population dynamics showed that ball gall 
density increased linearly with plant density, and ball gall parasitoids increased 
linearly with gall density (Abrahamson et al. 1983). 
Avian predators of ball gall insects include the Downy Woodpecker. Picoides 
pubescens, and the Black-capped Chickadee, Parus atricapillus (Confer and Paicos 
1985). The intensity of predation by these birds is quite variable, but at any given site 
bird predation near the forest edge exceeds that in mid-field. At one site in Ontario 
birds preyed on 81 0J0 f the galls near a forest edge and 60070 of the galls in mid-field 
(Schlicter 1978). For three sites in upstate New York, birds preyed on 9-25070 of the 
galls near a forest edge, but only 0-10070 of the galls in mid-field (Confer and Paicos 
1985). Also from upstate New York, avian predation in mid-field averaged 2.3070 of 
all galls over 14 y ars (Uhler 1961), considerably lower than the rates near a forest 
edge. 
In this study we describe the distribution of ball gall insects relative to a forest 
edge in fields of goldenrod. Some preliminary samples collected with an ecology 
class showed a v ry strong e in the proportion of fly larvae parasitized or 
preyed on by insects at differen lstances from the forest edge, while other samples 
showed no difference at all. Insect distribution in these preliminary samples was 
compatible with the hypothesis that the difference between samples was related to 
lBiology Department, Ithaca College, Ithaca, NY 14850 
1'he 
taxonomy 
of the host genus, Solidago, is complicated by at least three ploidy forms 
(Melville and Morton 1982). Solidago altissima, formerly commonly called S. canadensis, may 
be 
the dominant form in the Ithaca area (P. Marks pers. comm.). 
1
Confer: Spatial Distribution of the Goldenrod Ball Gall Insects
Published by ValpoScholar, 1990
34 THE GREAT LAKES ENTOMOLOGIST Vol. 23, No. 1 
the presence or absence of shade from a contiguous forest edge. We hypothesized 
that the percentage of galls attacked by parasitic and predatory insects would 
increase with distance from a forest into a field when the forest was on the southern 
edge, but not when the forest was on the northern edge. 
We 
did not hypothesize that the fly, itself, would avoid the forest edge. Although 
avian predation 
is reatest close to a forest edge, the preliminary samples had 
suggested that the frequency of parasitism plus predation by the other insects was 
greatest farther from a forest edge. Since avian predation and insect parasitism and 
predation occurred with about equal intensity in our samples, there would be no net 
advantage related to the sum of all predation for the fly o seek or avoid the forest 
edge. 
MATERIALS AND METHODS 
Ten sets of samples were collected, five each from the southern and northern edge 
of 
the goldenrod field with a well-developed and abrupt forest edge. All samples 
were collected in Tompkins County, New York in October, 1988. Four separate 
fields were sampled. For each set 
of samples w  collected all galls from three 
transects parallel to the forest edge. For each set, the transects were located 0-5.5, 
8.2-14, and 16.4-22m from the forest edge. The length of the transect was adjusted 
for differences in gall density and ranged from 7-17m. All gall insects, E. salida­
ginis, E. gigantea and E. abtusiventris, and M. unicaiar, were identified according 
to Uhler (1951). Galls with no insect occupant were omitted from calculations. A 
Chi-square 3 x 4 contingency value was calculated for each set. Within each sample 
set, the proportional abundance of each insect species for the transect closest to the 
forest was compared to the value for the two other transects of that set. Increases or 
decreases in the value with distance from the forest edge were summed for all sets. 
RESULTS AND DISCUSSION 
The first two sets f samples contained 59 and 68 galls. Thereafter, the transect 
length was increased from 7 to 17m and sample size for each set ranged from 105 to 
519 
galls. One 
of the later sets, collected in an area of exceptionally high gall density, 
was also collected from 7 transects. The density of ball galls varied considerably 
from site to site (Table 1). Parasitism plus predation was high with 69-1000/0 of the 
fly larvae attacked in mid-field. Avian predation, which does not begin until late 
October (Confer and Paicos 1985), was noted in only 3 galls. 
Testing if the proportion of fly, wasp, wasp and beetle was equal in each transect 
of 
a set (Table 1), the resultant Chi-square contingency values with 6 
df were 84.9, 
73.2,42.7,33.8,22.9, 
18.0, 18.4, 17.5 (p <0.01), 12.9 
(p < 0.05), and 3.3 (p > 
0.10). Even excluding the two sets with smaller sample size, 7 of 8 sets had signifi­
cant Chi-square values. Clearly the distribution of these insects among the transects 
is 
not random. The one sample set which did not show patchiness was collected 
along the northern edge 
of a field. However, the four other sets along a northern 
edge were not randomly distributed, and patchiness probably regularly occurs 
throughout goldenrod fields. 
The proportion of ball galls occupied by the various insects, as well as the number 
of 
ball galls, varied greatly among the sets (Table 1). For example, the proportion 
of 
ball galls contai' ies for the transects closest to a forest for the shaded, southern 
sample sets rang om 13 to 62%. The environmental factors that caused this 
variation remain unexplained. With such high variation, parametric analyses for a 
difference in the mean proportion of flies attacked at the three distances from a 
forest edge showed no significant difference. 
We 
then compared the proportional abundance 
of each insect species for the 
2
The Great Lakes Entomologist, Vol. 23, No. 1 [1990], Art. 7
https://scholar.valpo.edu/tgle/vol23/iss1/7
1990 THE GREAT LAKES ENTOMOLOGIST 35 
Table I. - The number of ball gaIl insects collected in five sample sets each of three transects 
for the northern and for the southern edge of a goldenrod field with a forest edge. Transects a  
described in Methods. Abbreviations are s follows: Es, Eo, Eg, and Mu for Eu osta solid a­
ginis, Eurytoma obtusiventris, Eurytoma gigantea, and Mordellistena unicolor, respectively. 
Northern Southern 

0-5.5m 8.2-14m 16.4-22m 0-5.Sm 8.2-14m 16.4-22m 

Es 24 22 
7 9 2 4 
Eo 5 
10 9 15 2 2 
Eg 26 18 28 
5 3 0 Mu 25 21 30 
6 8 
12 
Es 18 19 
1 5 1 0 
Eo 
II Il 2 3 2 I 
Eg 25 19 23 
7 4 4 Mu 26 2 6 4 8 20 
Es 63 44 49 7 1 I 
Eo 46 46 63 6 1 6 
Eg 40 23 23 21 9 6 
Mu 25 67 60 20 20 10 
Es 29 47 29 120 37 14 
Eo 21 52 20 31 16 19 
Eg 33 55 26 36 20 33 
Mu 16 33 18 
5 
16 28 
Es 11 19 
9 
71 2 0 
Eo 11 26 5 30 3 4 
Eg 25 16 37 20 16 19 
Mu 
3 
19 33 I 6 9 
transect closest to the forest to its proportional abundance in the other two transects 
of the 
same 
data set. Changes in the proportion were determined for each sample set 
and 
summed 
for all sets (Table 2). For sample sets from the northern edge of a 
goldenrod field, there were no detected trends for any of the insect species. How­
ever, for sample sets from the southern edge, there were two distinct trends. The 
proportion of 
ball galls containing flies 
for the transect closest o the southern forest 
edge was greater th n the proportion for any transect of the same sample farther 
from the forest edge. The proportion of gaIls containing the beetle increased in every 
comparison f the forest edge transect to all other transects of the same set. For two 
representative sample sets, for the transects closest to and farthest from the forest 
edge, the fly proportion declined from 26 to 22070 and 63 to 17%. For the same pairs 
of 
transects, the 
proportion of beetle predation increased from 17 to 66% an  from 
2.6 to 33%. Since all ball galls are initiated by a fly and spontaneously aborted galls 
were excluded from calculations, a decline in the proportion of galls containing a fly 
could only occur if the sum of insect parasitism and predation increased. Since the 
proportional 
abundances 
of the two wasp species did not change systematically with 
increasing distance from the forest edge, the systematic decline in fly proportion was 
caused by the systematic increase in the proportional abunda ce of the beetle. 
This study of the distribution of ball gall insects was inspired by an inte st in 
avian predation. We hypothesized that the parasitic and predacious insects would 
have been selected to avoid the forest edge due to greater avian predation. Only the 
beetle responded as hypothesized, and this occurred only along the southern, for­
ested edges of the fields. In hindsight, we noted that the predation intensity by birds 
3
Confer: Spatial Distribution of the Goldenrod Ball Gall Insects
Published by ValpoScholar, 1990
36 THE GREAT LAKES ENTOMOLOGIST Vol. 23, No. 1 
Table 2.-Changes in the proportional abundance of Eurosta solidaginis, Eurytoma obtu­
siventris, Eurytoma gigantea, and Mordellistena unicolor at three distances from a forest edge 
into a field f goldenrod. The proportion of galls containing each species in a transect close to 
the forest edge was compared to two transects farther into the field. Five sample sets each of 
three transects were collected near both the northern and southern forested edge. Abbrevia­
tions are as in Table 1.
northern southern 
increases decreases increases decreases 
Es 6 4 0 10 
Eo 5 5 3 7 
Eg 4 6 5 5 
Mu 5 5 10 0 
near the forest edge and by beetles in mid-field was similar for the two eurytomid 
wasps, as ell as the fly. Bird predation near the forest edge, as cited earlier, can 
range from 9 to 810/0. In this study, the proportion of galls with beetles for the 
transect farthest from the southern, forest edge ranged from 28 t  800/0. Although 
beetles do not always eat co-inhabiting wasp larva, they usually do so. Thus, preda­
tion intensity by the birds and the beetle is similar, and there may be no gradient in 
predation that would select for wasps that avoid a forest edge. 
In this study we have not experimentally documented either the proximate or 
ultimate factors producing the beetle distribution. The shaded, southern edge of a 
field with a forest boundary is colder and wetter than the northern edge. If avian 
predation is the selective force that led to beetle avoidance of this edge, one can 
wonder if detection of the shadow, the temperature, or the humidity is the proxi­
mate clue. Since avian predation occurs along both the northern and southern forest 
edge of a goldenrod field (pers. obs.), one can wonder why the beetle responded t  a 
forest only on the southern edge. Avian predation is irregular and perhaps has not 
been sufficiently consistent to select for beetles that avoid the forest edge. It is 
possible that humidity or temperature are the ultimate factors that directly affect 
success. Regardless of the uncertainty about cause, the trend is clear. In all sample 
sets along the southern edge of a goldenrod field with a forest boundary, Mordellis­
lena unicolor occurs in lower proportion near the forest and higher proportion out 
from the edge. 
ACKNOWLEDGMENTS 
The encouragement of Lowell Uhler, now deceased, initiated the interest of th  
first author in ball gall insects. We are grateful to Peggy Spofford, who reviewed a 
preliminary draft, and to R. Thaler, who gave permission to sample galls on his 
property. 
LITERATURE CITED 
Abrahamson, W. G., P. O. Armbruster, G. D. Maddox. 1983. Numerical relationships ofthe 
Solidago altissima stern gall insect-parasitoid guild food chain. Oecologia 58:351-357. 
Confer, J. L. and P. Paicos. 1985. Downy Woodpecker predation on goldenrod galls. J. Field. 
Orni. 56:56-64. 
Melville, M. R., and J. K. Morton. 1982. A biosystematic study of the Solidago canadensis 
(Compositae) complex.!. The Ontario populations. Can. J. B t. 60:976-997. 
4
The Great Lakes Entomologist, Vol. 23, No. 1 [1990], Art. 7
https://scholar.valpo.edu/tgle/vol23/iss1/7
1990 THE GREAT LAKES ENTOMOLOGIST 37 
Uhler, L. D. 1951. Biology and ecology of the goldenrod gall fly Eurosta sofidaginis (Fitch). 
Memoir Cornell University Agricultural Experimental Station, Ithaca New York 300:1-51. 
Uhler, L. D. 1961. Mortality of the goldenrod gall fly Eurosta so/idaginis in the vicinity of 
Ithaca, New York. Ecology 42:215-216. 
Schlicter, L. 1978. Winter predation by Black-cappe<:\ Chickadees and Downy Woodpeckers on 
inhabitants of the goldenrod ball gaiL Can. Field-Nat. 92;71-74. 
5
Confer: Spatial Distribution of the Goldenrod Ball Gall Insects
Published by ValpoScholar, 1990


Spatial Distribution of the Goldenrod Ball Gall Insects

Valparaiso University

The Great Lakes Entomologist Volume 23 Number 1 - Spring 1990 Number 1 - Spring 1990 Article 7 April 1990 Spatial Distribution of the Goldenrod Ball Gall Insects John L. Confer Ithaca College Follow this and additional works at: https://scholar.valpo.edu/tgle  Part of the Entomology Commons Recommended Citation Confer, John L. 1990. "Spatial Distribution of the Goldenrod Ball Gall Insects," The Great Lakes Entomologist, vol 23 (1) Available at: https://scholar.valpo.edu/tgle/vol23/iss1/7 This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 1990 THE GREAT LAKES ENTOMOLOGIST 33 SPATIAL DISTRmUTION OF THE GOLDENROD BALL GALL INSECTS John L. Confer and Julie Orloffl ABSTRACT The goldenrod ball gall fly, Eurosta so/idaginis (Diptera: Tephritidae), is parasi­tized by two urytomid wasps and preyed on by a mordellid beetle, and two bird species. The birds are known to prey most intensively near a forest edge. The proportion of fly larva parasitized and preyed on by insects was examined, and a decline in the proportional abundance of beetle larvae near the forest was noted. The possible role of birds as the selective force for this distribution is considered. The life histories of the ball gall insects of goldenrod2 have been described (Uhler 1951, 1961). The gall fly, Eurosta solidaginis (Fitch) (Diptera: Tephritidae), emerges in spring and lays its eggs in the growing tip of the goldenrod, and its larvae stimulate the formation f all ball galls. A f y arva may be parasitized by either of two wasps, Eurytoma obtusiventris Gahan or E. gigantea Walsh (Hymenoptera: Eurytomidae). Larvae of a beetle, Mord llistena unicolor Lec. (Coleoptera: Mordel­lidae), are omnivores and may eat any of the above insects or co-develop in the same gall with the other insects. Studies of population dynamics showed that ball gall density increased linearly with plant density, and ball gall parasitoids increased linearly with gall density (Abrahamson et al. 1983). Avian predators of ball gall insects include the Downy Woodpecker. Picoides pubescens, and the Black-capped Chickadee, Parus atricapillus (Confer and Paicos 1985). The intensity of predation by these birds is quite variable, but at any given site bird predation near the forest edge exceeds that in mid-field. At one site in Ontario birds preyed on 81 0J0 f the galls near a forest edge and 60070 of the galls in mid-field (Schlicter 1978). For three sites in upstate New York, birds preyed on 9-25070 of the galls near a forest edge, but only 0-10070 of the galls in mid-field (Confer and Paicos 1985). Also from upstate New York, avian predation in mid-field averaged 2.3070 of all galls over 14 y ars (Uhler 1961), considerably lower than the rates near a forest edge. In this study we describe the distribution of ball gall insects relative to a forest edge in fields of goldenrod. Some preliminary samples collected with an ecology class showed a v ry strong e in the proportion of fly larvae parasitized or preyed on by insects at differen lstances from the forest edge, while other samples showed no difference at all. Insect distribution in these preliminary samples was compatible with the hypothesis that the difference between samples was related to lBiology Department, Ithaca College, Ithaca, NY 14850 1'he taxonomy of the host genus, Solidago, is complicated by at least three ploidy forms (Melville and Morton 1982). Solidago altissima, formerly commonly called S. canadensis, may be the dominant form in the Ithaca area (P. Marks pers. comm.). 1Confer: Spatial Distribution of the Goldenrod Ball Gall InsectsPublished by ValpoScholar, 199034 THE GREAT LAKES ENTOMOLOGIST Vol. 23, No. 1 the presence or absence of shade from a contiguous forest edge. We hypothesized that the percentage of galls attacked by parasitic and predatory insects would increase with distance from a forest into a field when the forest was on the southern edge, but not when the forest was on the northern edge. We did not hypothesize that the fly, itself, would avoid the forest edge. Although avian predation is reatest close to a forest edge, the preliminary samples had suggested that the frequency of parasitism plus predation by the other insects was greatest farther from a forest edge. Since avian predation and insect parasitism and predation occurred with about equal intensity in our samples, there would be no net advantage related to the sum of all predation for the fly o seek or avoid the forest edge. MATERIALS AND METHODS Ten sets of samples were collected, five each from the southern and northern edge of the goldenrod field with a well-developed and abrupt forest edge. All samples were collected in Tompkins County, New York in October, 1988. Four separate fields were sampled. For each set of samples w  collected all galls from three transects parallel to the forest edge. For each set, the transects were located 0-5.5, 8.2-14, and 16.4-22m from the forest edge. The length of the transect was adjusted for differences in gall density and ranged from 7-17m. All gall insects, E. salida­ginis, E. gigantea and E. abtusiventris, and M. unicaiar, were identified according to Uhler (1951). Galls with no insect occupant were omitted from calculations. A Chi-square 3 x 4 contingency value was calculated for each set. Within each sample set, the proportional abundance of each insect species for the transect closest to the forest was compared to the value for the two other transects of that set. Increases or decreases in the value with distance from the forest edge were summed for all sets. RESULTS AND DISCUSSION The first two sets f samples contained 59 and 68 galls. Thereafter, the transect length was increased from 7 to 17m and sample size for each set ranged from 105 to 519 galls. One of the later sets, collected in an area of exceptionally high gall density, was also collected from 7 transects. The density of ball galls varied considerably from site to site (Table 1). Parasitism plus predation was high with 69-1000/0 of the fly larvae attacked in mid-field. Avian predation, which does not begin until late October (Confer and Paicos 1985), was noted in only 3 galls. Testing if the proportion of fly, wasp, wasp and beetle was equal in each transect of a set (Table 1), the resultant Chi-square contingency values with 6 df were 84.9, 73.2,42.7,33.8,22.9, 18.0, 18.4, 17.5 (p <0.01), 12.9 (p < 0.05), and 3.3 (p > 0.10). Even excluding the two sets with smaller sample size, 7 of 8 sets had signifi­cant Chi-square values. Clearly the distribution of these insects among the transects is not random. The one sample set which did not show patchiness was collected along the northern edge of a field. However, the four other sets along a northern edge were not randomly distributed, and patchiness probably regularly occurs throughout goldenrod fields. The proportion of ball galls occupied by the various insects, as well as the number of ball galls, varied greatly among the sets (Table 1). For example, the proportion of ball galls contai' ies for the transects closest to a forest for the shaded, southern sample sets rang om 13 to 62%. The environmental factors that caused this variation remain unexplained. With such high variation, parametric analyses for a difference in the mean proportion of flies attacked at the three distances from a forest edge showed no significant difference. We then compared the proportional abundance of each insect species for the 2The Great Lakes Entomologist, Vol. 23, No. 1 [1990], Art. 7https://scholar.valpo.edu/tgle/vol23/iss1/71990 THE GREAT LAKES ENTOMOLOGIST 35 Table I. - The number of ball gaIl insects collected in five sample sets each of three transects for the northern and for the southern edge of a goldenrod field with a forest edge. Transects a  described in Methods. Abbreviations are s follows: Es, Eo, Eg, and Mu for Eu osta solid a­ginis, Eurytoma obtusiventris, Eurytoma gigantea, and Mordellistena unicolor, respectively. Northern Southern 0-5.5m 8.2-14m 16.4-22m 0-5.Sm 8.2-14m 16.4-22m Es 24 22 7 9 2 4 Eo 5 10 9 15 2 2 Eg 26 18 28 5 3 0 Mu 25 21 30 6 8 12 Es 18 19 1 5 1 0 Eo II Il 2 3 2 I Eg 25 19 23 7 4 4 Mu 26 2 6 4 8 20 Es 63 44 49 7 1 I Eo 46 46 63 6 1 6 Eg 40 23 23 21 9 6 Mu 25 67 60 20 20 10 Es 29 47 29 120 37 14 Eo 21 52 20 31 16 19 Eg 33 55 26 36 20 33 Mu 16 33 18 5 16 28 Es 11 19 9 71 2 0 Eo 11 26 5 30 3 4 Eg 25 16 37 20 16 19 Mu 3 19 33 I 6 9 transect closest to the forest to its proportional abundance in the other two transects of the same data set. Changes in the proportion were determined for each sample set and summed for all sets (Table 2). For sample sets from the northern edge of a goldenrod field, there were no detected trends for any of the insect species. How­ever, for sample sets from the southern edge, there were two distinct trends. The proportion of ball galls containing flies for the transect closest o the southern forest edge was greater th n the proportion for any transect of the same sample farther from the forest edge. The proportion of gaIls containing the beetle increased in every comparison f the forest edge transect to all other transects of the same set. For two representative sample sets, for the transects closest to and farthest from the forest edge, the fly proportion declined from 26 to 22070 and 63 to 17%. For the same pairs of transects, the proportion of beetle predation increased from 17 to 66% an  from 2.6 to 33%. Since all ball galls are initiated by a fly and spontaneously aborted galls were excluded from calculations, a decline in the proportion of galls containing a fly could only occur if the sum of insect parasitism and predation increased. Since the proportional abundances of the two wasp species did not change systematically with increasing distance from the forest edge, the systematic decline in fly proportion was caused by the systematic increase in the proportional abunda ce of the beetle. This study of the distribution of ball gall insects was inspired by an inte st in avian predation. We hypothesized that the parasitic and predacious insects would have been selected to avoid the forest edge due to greater avian predation. Only the beetle responded as hypothesized, and this occurred only along the southern, for­ested edges of the fields. In hindsight, we noted that the predation intensity by birds 3Confer: Spatial Distribution of the Goldenrod Ball Gall InsectsPublished by ValpoScholar, 199036 THE GREAT LAKES ENTOMOLOGIST Vol. 23, No. 1 Table 2.-Changes in the proportional abundance of Eurosta solidaginis, Eurytoma obtu­siventris, Eurytoma gigantea, and Mordellistena unicolor at three distances from a forest edge into a field f goldenrod. The proportion of galls containing each species in a transect close to the forest edge was compared to two transects farther into the field. Five sample sets each of three transects were collected near both the northern and southern forested edge. Abbrevia­tions are as in Table 1.northern southern increases decreases increases decreases Es 6 4 0 10 Eo 5 5 3 7 Eg 4 6 5 5 Mu 5 5 10 0 near the forest edge and by beetles in mid-field was similar for the two eurytomid wasps, as ell as the fly. Bird predation near the forest edge, as cited earlier, can range from 9 to 810/0. In this study, the proportion of galls with beetles for the transect farthest from the southern, forest edge ranged from 28 t  800/0. Although beetles do not always eat co-inhabiting wasp larva, they usually do so. Thus, preda­tion intensity by the birds and the beetle is similar, and there may be no gradient in predation that would select for wasps that avoid a forest edge. In this study we have not experimentally documented either the proximate or ultimate factors producing the beetle distribution. The shaded, southern edge of a field with a forest boundary is colder and wetter than the northern edge. If avian predation is the selective force that led to beetle avoidance of this edge, one can wonder if detection of the shadow, the temperature, or the humidity is the proxi­mate clue. Since avian predation occurs along both the northern and southern forest edge of a goldenrod field (pers. obs.), one can wonder why the beetle responded t  a forest only on the southern edge. Avian predation is irregular and perhaps has not been sufficiently consistent to select for beetles that avoid the forest edge. It is possible that humidity or temperature are the ultimate factors that directly affect success. Regardless of the uncertainty about cause, the trend is clear. In all sample sets along the southern edge of a goldenrod field with a forest boundary, Mordellis­lena unicolor occurs in lower proportion near the forest and higher proportion out from the edge. ACKNOWLEDGMENTS The encouragement of Lowell Uhler, now deceased, initiated the interest of th  first author in ball gall insects. We are grateful to Peggy Spofford, who reviewed a preliminary draft, and to R. Thaler, who gave permission to sample galls on his property. LITERATURE CITED Abrahamson, W. G., P. O. Armbruster, G. D. Maddox. 1983. Numerical relationships ofthe Solidago altissima stern gall insect-parasitoid guild food chain. Oecologia 58:351-357. Confer, J. L. and P. Paicos. 1985. Downy Woodpecker predation on goldenrod galls. J. Field. Orni. 56:56-64. Melville, M. R., and J. K. Morton. 1982. A biosystematic study of the Solidago canadensis (Compositae) complex.!. The Ontario populations. Can. J. B t. 60:976-997. 4The Great Lakes Entomologist, Vol. 23, No. 1 [1990], Art. 7https://scholar.valpo.edu/tgle/vol23/iss1/71990 THE GREAT LAKES ENTOMOLOGIST 37 Uhler, L. D. 1951. Biology and ecology of the goldenrod gall fly Eurosta sofidaginis (Fitch). Memoir Cornell University Agricultural Experimental Station, Ithaca New York 300:1-51. Uhler, L. D. 1961. Mortality of the goldenrod gall fly Eurosta so/idaginis in the vicinity of Ithaca, New York. Ecology 42:215-216. Schlicter, L. 1978. Winter predation by Black-cappe<:\ Chickadees and Downy Woodpeckers on inhabitants of the goldenrod ball gaiL Can. Field-Nat. 92;71-74. 5Confer: Spatial Distribution of the Goldenrod Ball Gall InsectsPublished by ValpoScholar, 1990

https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1691&amp;context=tgle

Spatial Distribution of the Goldenrod Ball Gall Insects

Abstract

Similar works

Full text

Available Versions

ValpoScholar

Valparaiso University