The Effect of the Environment on Eurosta Solidagins Oviposition and Larva's Survival

The effect of Solidago canadensis environment on Eurosta solidagins larva survival rate is tested by collecting 200 galls from four different sites along the Fanshawe Road West. We predicted Eurosta solidagins larva survival rate will be low on Solidago canadensis galls that are in close proximity to the road compare to the galls that are far from road. The galls were taken to the lab, dissected and the fate of the larva was determined as dead or alive. The result showed that as the distance between the road and E. solidagins increased, the larva survival rate increased as well. So we can conclude from the study that E. solidagins environment is correlated to the larva survival rate of S. canadensis that winters inside the plant.

Introduction

The Canada goldenrod (Solidago canadensis) is parasitized by the gall fly (Eurosta solidagins) that lays its egg inside the terminal bud of the goldenrod. The larva hatches out from the egg within seven to ten days and chews its way to the stem, and stimulates the formation of a gall in the plant using its saliva. The gall serves as a source of food and shelter for the larva during winter and in spring it emerges as a mature adult (Abrahamson and Weis, 1998). While the gall fly larva is developing in the gall, it is susceptible to predation from birds (downy woodpecker (Picoides pubescens) and blacked-capped chickadee (Parus carolinensis)), wasp (Eurytoma gigantea and Eurystoma obtusiventris), and the beetle larva (Mordellistena unicolor); these predators can have a huge influence on the population of gall flies (Shealer et al. 1999).

Before depositing their eggs, female Eurosta solidagins climbs to the apices of the Solidago Canadensis buds and rubs the apex rapidly with their forelegs, after finding a suitable golden rod plant, she searches for a good oviposition site to deposit their egg. (Abrahamson and Weis, 1998). In a related specie to the gall fly, spittlebug (Philaenus spumarius), the environment of the host plant and oviposition preference can have great impact on the survival rate of the larva (Wise et al. 2008). The same trend could be possible for the gall fly, since the female spends a great amount of time on host-evaluation and oviposition screening process (Horner and Abrahamson, 1992). This process of evaluating the plant could be used to determine the environment of the plant; the disturbance level a experiences could be a important factor in determining the plant's environment, so a good mother would deposit her egg in a golden rod with the least disturbance, therefore, increasing the survival rate of its progeny (Horner and Abrahamson, 1992). So the purpose of this study is to test the hypothesis that goldenrod's environment plays an important role in larva survival. I predict that larva survival rate will be low on golden rod galls that are in close proximity to the road compare to golden rods galls that are far from road; due to the fact that being closer to the road makes the golden rods more susceptible to disturbance.

Methods

For the experiment, four study sites, which were located on different distance from Fanshawe Road West in London, Ontario, were used. All the sites used were abandoned fields along the same road so indirect disturbance (snow, predator) could be controlled for. The distance from the road to each site was measured to estimate the disturbance level caused by the road traffic for the goldenrods. Three transects (20 m long), running parallel to each other were set up in the middle of the sites and the galls located on and within 1 meter of the line transect were removed from the field. Approximately 50 galls from each site and 200 in total were collected and dissected in the lab to determine whether the larva is dead or alive; the results were recorded. Analysis of Variance (ANOVA) was used to determine significant difference among sites, followed by bar graphs to compare the results between different sites.

Results

A total of 200 galls were collected from four different sites, which were dissected and examined for larva survival. In general, there was a significant increase in the larva survival rate (F = 4.152 df = 3, P = .055) as the distance from the road increase (Figure 1; Figure 2). The number of larva found dead in dissected galls went down and number of larva found alive in galls increased as the distance from the road increased.

Discussion

Our results show that as the distance from the road to the sites increased the larva survival rate increased as well. The result supports our hypothesis that goldenrod's environment plays an important role in larva survival; and agrees with our prediction that larva survival rate will be low on golden rod galls that are in close proximity to the road compare to golden rod galls that are far from road; due to the fact that being closer to the road makes the golden rods more prune to disturbance.

In a similar experiment done by Wise et al. (2007), there was a strong correlation found between the larva survival rate and disturbance level experienced by the larva while they wintered inside the galls. The experiment was carried out in a open field, in which different plants were planted in selected position to control for indirect variables; the results from the experiment showed that "good mothers" that took more time screening golden rod, deposited their egg in golden rods further away from the source of disturbance; while "bad mothers" that didn't spend a lot of time screening golden rods deposited their eggs in golden rods close to the source of disturbance. The larva of the good mother had considerably high success rate, while the larva from the bad mothers did not, so the results showed that the environment of the golden rod played an important role in larva survival rate.

Another experiment done by Craig et al. (1999), no correlation was found between larva survival rate and amount of disturbance the larva encountered while it was inside the gall. The experiment was carried out for 2 years in a garden; the data obtained from the experiment did not support the hypothesis but it showed a correlation between the larva survival rate and growth rate of the golden rods; Golden rods with medium growth rate had the highest survival rate. The difference in the results could be due to the difference in location; Craig et al. (1999) experiment was done in a garden with external disturbance, while our experiment and Wise et al. (2007) was done on open field sites where the external disturbance was considerably higher.

There could be other reason why we got these result, it could be possible that the soil further away from the road was healthier, less soil erosion occurred so it retained more nutrient compared to the soil near the road where constant wind flow and water runoff resulted in a loss of nutrient. The golden rods that were in better soil condition grew well, so the larva had more nutrients available while it was wintering inside the gall (Craig et al. 1999). It is also possible that golden rod galls that were located further away from the road had higher amount of vegetative cover in the surrounding which helped the larva better protect itself from different predator, while golden rod galls that were close to the road had less cover (Weis et al. 1985). The result could also be related to genetic variability, if the golden rod located further away from the road were older plant and they had a history of being attacked by a certain insect, they would be better resistant against predators compared to new plants that didn't have previous insect attack history (McCrea and Abrahamson. 1987). This variability could be what the female gall fly screens for before depositing her egg, the golden rod that is able to protect itself better by having a stronger cell wall will give gall fly larva better shelter, this could be the reason for the increase in the larva survival rate in sites far away road.

Our result show that the environment of the S. canadensis is an important factor in determining E. solidagins larva survival rate, larva wintering in golden rod gals that are away from disturbance have a higher chance of surviving compare to the larva wintering in golden rod galls closer to disturbances. This experiment shows the effect human growth and construction has on the environment and ecosystems; clearing area to build road can not only affect the original area but also the surrounding area. This study helps us indentify the impact on gall fly life cycle and reproductive success; with the increase in human population more natural land is being converted into roads and building to fuel human civilization. The study also shows how some impacts might not be visible to the naked eye or they can be long term so the study is very significant in being a stepping stone for future studies that involve testing the human impact on small insects and micro ecologically important environments.

We can conclude from the study that human construction like roads can affect the environment of Solidago canadensis that can have a negative impact on Eurosta solidagins larva survival rate, so as the distance from the road increases the larva survival rate increases as well.

Literature Cited

ABRAHAMSON, W. G., and WEIS, A. E. 1997. Evolutionary ecology across three trophic levels: Goldenrods, gallmakers, and natural enemies. Princeton University Press. Princeton, New Jersey.

Horner, J. D., and Abrahamson, W. G. 1992. Influence of plant genotype and environment on oviposition preference and offspring survival in a gallmaking herbivore. Oecologia. 90: 323-332.

Shealer, D. A., Snyder, J. P., Dreisbach, V. C., Sunderlin, D. F., and Novak, J. A. 1997. Foraging Patterns of Eastern Grey Squirrels (Sciurus carolinenis) on Goldenrod Gall Insects, a Potentially Important Winter Food Source. Am. Midl. Nat, 142(1): 102-109.

Wise, M. J., Partelow, J. M., Everson, K. J., Anselmo, M. K., and Abrahamson W. G. 2008. Good mothers, bad mothers, and the nature of resistance to herbivory in Solidago altissima. Oecologia. 155:257-266

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