The Spatial Distribution and Location of Leaf Mines on Wingstem (Verbesina alternifolia) in Glover-Archbold Park and the Chesapeake and Ohio National Historical Park, Washington, D.C.

Erin M. Scott
Georgetown University
Forest Ecology 355
Fall 2003

Abstract

Objectives of this study are to determine the spatial distribution of leaf mines on Wingstem (Verbesina alternifolia), the location of leaf mines on Wingstem leaves, and the location of herbivory due to external herbivores on Wingstem leaves.  Leaf miners may be more vulnerable to predators and parasitoids because they are relatively immobile on their host plants compared to some other kinds of herbivores that feed externally on the leaves.  Therefore, the location of leaf mines on host plants could be a factor determining the survivorship of leaf-mining larvae.  The spatial distribution of leaf mines could reveal what might be optimal regions on host plants for leaf-mining larvae.  The characteristics of leaf mines were examined on 50 Wingstem plants in October 2003.  Observations indicated that slightly more leaf mines were located above the midpoint of their host plants.  Leaf mines also tended to be located centrally on Wingstem leaves.  Herbivory due to external herbivores were located both centrally and peripherally on leaves containing leaf mines.

Introduction

The objectives of my study are to determine the spatial distribution and location of leaf mines on Wingstem (Verbesina alternifolia) located in Glover-Archbold Park (GAP) and the Chesapeake and Ohio Canal National Historical Park (CONHP) in 2003.  Further, I observe the location of herbivory due to external herbivores on Wingstem leaves.

Wingstem, a native dicot in the Aster family, is found throughout the eastern and central United States (Hilty 2003).  It has several common names, including Golden Honey Plant, Golden Ironweed, and Yellow Ironweed.  It a perennial that inhabits pastures, fencerows, roadsides, moist meadows and woodlands, thickets, openings and edges of floodplain forests, and areas near stream banks ( Haddock 2001, Hagood 2003, Hilty 2003).  This plant grows well in rich soil and partial shade (Haddock 2001).  Wingstem produces yellow flowers that bloom in August and September.  Many species of bees, wasps, flies, butterflies, and beetles visit these flowers for nectar and pollen (Hilty 2003).  This plant is glabrous, and its serrated leaves are alternately arranged along its stem.  Due to the bitterness of its leaves, deer, rabbits, and other mammalian herbivores do not regularly consume Wingstem (Hilty 2003).  However, several phytophagous insects consume this plant.  Caterpillars of the Gold Moth, Silvery Checkerspot Butterfly, and Spring Azure feed on the leaves of Wingstem (Opler and Krizek 1984, Hilty 2003).  Leaf mines are present on this plant in GAP and CONHP, Washington, D.C.

Leaf miners are a specialized subset of chewing insects that are endophytophagous, which means they are internal plant parasites (Weis and Berenbaum 1989).  The larvae of these insects feed on inner leaf material, or lamina, without breaking through the upper and lower cuticle layers of a leaf (Weis and Berenbaum 1989).  Leaf miners produce distinctive damage on leaves, which appear as bloth mines, serpentine mines, or intermediate-type mines.  In this study, I observed serpentine mines, which appear as winding trials on Wingstem leaves.  These mines usually start at the center of a small spiral, where a larva hatched from an egg.  As the larva moves, it consumes the insides of its leaf.  As the larvae grows, it produces a wider mine.  After the larva pupates, it emerges from the inside of the leaf.

Leaf miners are common in the orders Lepidoptera, Hymenoptera, Diptera, and Coleoptera (Weis and Berenbaum 1989).  In this study, I observed leaf-mining flies (Diptera) in the family Agromyzidae.  However, due to time limitations, I was unable to identify the fly species that produces leaf mines on Wingstem in Washington, D.C.

Leaf miners are relatively immobile on a plant, compared to external herbivores.  Leaf-mining insects are, therefore, more vulnerable to predators and parasitoids.  Thus, the location leaf mines on a host plant could be a factor determining the survivorship of leaf-mining larvae.

This study tests the null hypotheses that leaf mines are randomly distributed throughout Wingstem plants, and that leaf mines, as well as herbivory by external herbivores, are equally located on both the central and peripheral regions of Wingstem leaves.  My results indicate that leaf-mine height is positively correlated with plant height.  In addition, more leaf mines were found on the upper halves of Wingstem than on the lower halves.  More leaf mines were also observed to be centrally located, rather than peripherally located on leaves.  Furthermore, most of the herbivory due to external herbivores was centrally located.  The second greatest number of leaves observed contained both central and peripheral herbivory due to external herbivores.  My results suggest that leaf mines tend to be concentrated in the center of leaves and located in the upper regions of Wingstem plants.

Materials and Methods

I collected data in the southern edge of Glover-Archbold Park (GAP), and along the running path in the Chesapeake and Ohio Canal National Historical Park (CONHP) in October 2003.  I examined 50 Wingstem plants in a non-random method while I walked along the footpath in GAP, and the running path in CONHP.  For each of the 50 plants observed, I recorded the height of the plant, the height of each leaf mine on the plant, the number of leaf mines per leaf, the location of leaf mines on a leaf, and the location of herbivory due to external herbivores on the leaves containing leaf mines.  When examining the location of leaf mines, I simply recorded whether the leaf mine was centrally, or peripherally, located within a leaf.  When examining the location of external herbivory on leaves containing leaf mines, I also recorded whether the herbivory was located centrally, or peripherally, on the observed leaves.

The data were analyzed to determine whether leaf mines were high, or low, on the observed plants.  The height of each plant was divided in half.  I designated leaf mines above the midpoint of a plant as "high" and those below the midpoint as "low." Leaf mines located exactly at the midpoint on a plant were ignored for analysis purposes.  The number of leaf mines located in the upper regions of the plants was compared to the number of leaf mines in the lower regions of the observed plants (Figures 3, 4).  In addition, the number of leaf mines centrally located within the examined leaves was compared to the number of leaf mines peripherally located within these leaves (Figure 5).

Herbivory due to external herbivores can be detected by the presence of leaf damage.  I refer to holes in the center of leaves as central herbivory and those around the edge of leaves, as peripheral herbivory.  It appeared that most of the leaf damage on the observed 50 Wingstem plants was due to herbivorous insects, rather than White-tail Deer, falling branches, or other factors.  I compared the number of leaves with central herbivory to the number of leaves with peripheral herbivory, as well as to the number of leaves with both types of herbivory to the number of leaves without external herbivory (Figure 6).

I analyzed my data using graphs of each of the comparisons described above (Figures 3–6).  In order to eliminate biases, I also randomly selected data from my raw data, using a random number table, and displayed these results graphically (Figures 4–6).  In order to observe the possible correlation between plant height and leaf mine height, I constructed a scatter-plot graph for the height of each mine vs. the height of each plant (Figure 1), as well as the average leaf mine height per plant vs. plant height (Figure 2).

Results and Discussion

The observed 50 Wingstem Plants are from 49 through 206 cm tall (Table 1). The number of leaf mines per plant ranged from zero through 13, with an average of 3.7 leaf mines per plant and a mode of 2 leaf mines per plant.

There appears to be a positive correlation between leaf mine height and plant height (Figure 1).  Taller plants have more leaf mines located farther off the ground than shorter plants.  There also appears to be a positive correlation between average leaf mine height and plant height (Table 2, Figure 2).  Taller plants have larger average leaf-mine heights than shorter plants.  One hundred and ten leaf mines had a high location, meaning they were located above the midpoint of the plant (Figure 3).  Seventy-three leaf mines had a low location, and thus were located below the midpoint of the plant.

I randomly selected one leaf mine per plant and compared the location of 48 randomly-selected leaf mines on Wingstem plants.  Twenty-six randomly selected leaf mines were high on the plants, and 22 randomly selected leaf mines were low on the plants (Figure 4).

Most of the observed Wingstem leaves (171 of the 187), contained one leaf mine (Table 1).  A small number of leaves (13 out of the 187 examined) contained two leaf mines per leaf.  Three leaves had three leaf mines per leaf.  Thirty-six leaf mines were centrally located on the leaves, and 12 leaf mines were peripherally located on the leaves (Figure 5).

I compared the location of external herbivory on 48 randomly selected leaves containing leaf mines (Figure 6).  Twenty leaves suffered from central herbivory, which can be detected from chewing holes located in the center of the leaves. Only five leaves contained peripheral herbivory, which appears as chewing damage around the margin of leaves.  Seventeen leaves suffered from both central and peripheral herbivory.  Six leaves were completely intact, and therefore, were not parasitized by external herbivores.

My results suggest that leaf mines tend to be scattered throughout Wingstem plants, with a slightly larger number’s occupying the upper regions of the plants.  It also appears that more leaf mines occupy the center of leaves, rather than the periphery of leaves.  Reasons for this may include food-quality differences in different parts of the leaves and differences in vulnerability to predators and parasitoids.  Further research should investigate why the leaf miners used the central areas of leaves more than peripheral areas for their mines.

Finally, I found that most leaves suffer from central herbivory by defoliating insects.  A slightly smaller number of leaves containing leaf mines suffered from both central and peripheral herbivory by defoliating insects.  This indicates that the most common herbivores on Wingstem are insects that consume the centers of leaves.  Herbivores that feed on the edges of leaves are also common.  Defoliating herbivores do not seem to affect the location of leaf mines on the leaves, because both defoliating and leaf-mining herbivory are more common in the central areas of leaves.

Several biases exist within the sampling methods of this experiment.  Wingstem plants were not chosen at random, but rather by encountering as many as I could along the GAP footpath, and the COCNHP running path.  These transects through the forest are not random, and any unusual habitat features that exist in this area could have biased my data.

Future studies should be performed in order to identify agromizid fly species that produced the Wingstem leaf mines.  Factors that pose possible threats to their survival should also be examined, such as mammalian and insect predators, as well as parasites and parasitoids.  Future experiments should also focus on how leaf miners affect the fitness of Wingstem plants in order to attain a better understanding of the complex interactions between these flies and their host plant.

Acknowledgements

I thank Dr. Edward M. Barrows (Department of Biology, Georgetown University) who helped me to pursue this research project, and who provided helpful comments when editing my drafts.  I thank Dr. Raymond J. Gagné (USDA Systematic Entomology Laboratory) for identifying the leaf miners as agromyzids.  I also thank Nick Dulak for accompanying me in the parks while I collected data.

Literature Cited