Tiny Allies: The Effect of Coprophilous Beetles on Brassica rapa and Lolium perenne Growth and Biomass
Countless shades of green carpet the ground below a sapphire sky stretching to the horizon. Cows graze peacefully nearby. I take a breath of the fresh air wafting through the pasture. With the crisp scent of grass comes the unmistakable odor of manure. The source of this smell is why I am here. Ignoring the stench, I scoop dung into a bucket and continue my quest for the creatures that call this dung home.
According to the United States Department of Agriculture, the number of cattle in the United States last year was estimated to be 93.7 million. Along with the large number of cattle comes a massive amount of excrement. Since a single beef cow produces at least 54 pounds of fresh manure each day (Ohio Livestock Manure Management 2010), the amount of dung deposited daily in the United States is astronomical, totaling almost 2.5 million tons.
With all of this waste left on the pasture, we desperately need something to rescue us from the depths of the dung. Our hero comes in the form of coprophilous (dung-loving) species of beetles. Although these insects are small, size does not matter when one understands the importance of coprophilous beetles. Without these insects in pastures, there would be larger problems than just the smell of manure assaulting one's nostrils, including reduced grazing area, an increased fly population and a loss of nutrients for the soil.
Cattle are not fond of grazing near their own dung, which becomes an issue for farmers when only small patches of grass remain untouched (Thomas 2001). In addition, the fly population multiplies rapidly as dung volume increases. Musca autumnalis (the face fly), Haematobia irritans (the horn fly) and Stomoxys calcitrans (the stable fly) live and reproduce inside dung pats (Floate 2011). Natural fly enemies such as coprophilous beetles compete for their food supplies and can damage the flies' eggs (Nichols 2008). The beetles also improve soil nutrient cycling. In a study done by Marsh and Campling (1970), cattle dung contained 0.8% potassium, 0.36% sodium, 2.4% calcium, 0.7% phosphorus, and 0.8% magnesium. Nitrogen levels ranged from 2.5% to 4.0%. Approximately 80% of the nitrogen found in the dung is lost to the atmosphere if it isn't buried by beetles. When coprophilous beetles are present, more dung is removed from the surface and incorporated into the soil. This process drops the nitrogen loss to between 5% and 15% instead of the usual 80% (Gillard 1967). This is a benefit for both the cattle and the pasture. As nitrogen is incorporated into the soil, it becomes readily accessible for plants. Buried dung has been proven in various studies to improve yield as well as protein levels (Macqueen and Beirne 1975).
After learning about the benefits of coprophilous beetles, I wanted to see them for myself. First I needed an effective way to capture them. I read about pitfall traps at Scarabnet.org. These traps are buried in the ground and attract coprophilous insects with the odor of dung bait. I planned two trap designs based on the materials I had available. One design consisted of a gallon-sized plastic bucket buried to the brim in soil and covered with a half-inch-mesh wire screen. Sitting on top of the wire screen were approximately 250 grams of fresh cow dung. The second trap design consisted of a 16-ounce plastic drinking cup buried in the same way, with a screen and approximately 100 grams of cow dung on top.
On August 4, 2010, I traveled to Michigan State University Lake City Research Center, about two hours north of Grand Rapids, Michigan. This 850-acre farm is used for agricultural studies and is home to 190 mature beef cows. The research farm is currently using a tall-grass grazing method for feeding their cattle, which involves allowing the grass to grow and then introducing the cows to eat half of the vegetation before moving on to another section. One hundred cows are introduced to a one-acre section of pasture and then moved several times a day (Carmichael 2011).
This is a perfect scenario for finding coprophilous beetles because there are so many cows in a small area. While I shoveled dung and set up the traps, the cows gathered curiously around me, constantly bellowing. Somehow I ignored the noise and continued working. I spaced seven traps 10 paces apart and marked them with flags. I collected the traps on August 5 and repeated the process on August 9-10 to identify the coprophilous species at the Lake City Research Center. On August 13, I tried the flotation method, which involved pouring water into a bucket of dung and letting the beetles float unharmed to the top. One of the coprophilous beetle species I found, Onthophagus taurus, is a dung beetle (Coleoptera: Scarabaeidae). According to Michelle Thomas (2001), there are more than 90 species of dung beetles in the United States. Onthophagus taurus is known as the world's strongest insect. It can haul an incredible 1,141 times its own body weight (ScienceDaily 2010). In 1971, it was first recorded in Florida. By 1981, Onthophagus taurus had spread throughout Florida, Georgia, Alabama, Tennessee and Mississippi (Fincher 1983) and is still spreading. This beetle is a paracoprid (tunneling) species of dung beetle. The paracoprid species tunnel beneath dung pats and place several brood masses of dung in the soil. The females then lay their eggs within the brood ball (Thomas 2001).
Another paracoprid dung beetle species I trapped at the MSU Research Center was Onthophagus nuchicornis. This beetle, abundant in North America, is also an introduced species. It was unintentionally brought to North America by European settlers (Floate 2005). This particular beetle has spread across the northern United States over the last 20 to 30 years, and is now common in Michigan (Canadian Forest Service). Like Onthophagus taurus, Onthophagus nuchicornis has an impact on pastures by burying cow manure.
As I was setting the traps in the field, I found a beetle that at first I assumed was a dung beetle. However, it was something very different. With the help of Dr. Kevin Floate and Dr. Patricia Richardson, I identified this insect as Sphaeridium scarabaeoides. This insect is a hydrophillidae beetle, or water beetle. In its larval stage Sphaeridium scarabaeoides feeds on fly larvae, while in its adult stage it feeds on dung (Richardson 2010). Although these insects do not bury dung, they may move manure around enough to make the nutrient-rich dung liquid more available to the soil and plants.
Another observation I made while digging through the dung was that there were mites on the bodies of the dung beetles. These creatures also play a helpful role in pasture well-being. A coprophilous beetle carries the mites to the manure pads, where they drop off and hunt for newly hatched flies or eggs (Nardi 2007). I also found geoptrupes (Geotrupidae) and rove beetles (Staphylinidae) in my traps. I realized that there are more creatures in a single dung pat than I had previously anticipated. Containing everything from fly larva to various species of beetles, the dung seemed almost alive and moving, with many interesting arthropods inside.
As I discovered the variety of coprophilous species in the dung, my appreciation grew for the ecosystem within a dung pat and its beneficial effect on pasture plants. While I watched the insects at work, I wondered: What sort of impact do the different species of coprophilous beetles have on the height and biomass of the pasture grass I observed growing at the research center? How would the beetle species impact the height and biomass of a fast-growing plant species? In addition, how would the impact of different beetle species compare to the impact of a chemical fertilizer on plant growth and biomass?
In order to study the impact of coprophilous beetles on plant growth and biomass, I decided to set up various treatments that would evaluate the effect of Onthophagus taurus, Onthophagus nuchicornis and Sphaeridium scarabaeoides on two species of plants: Brassica rapa and Lolium perenne. According to Fastplants.org, Brassica rapa is a species of plant that grows at a very rapid rate and completes its life cycle within 28 days. It requires 24-hour lighting and plenty of water. Brassica rapa was included in the experiment in order to study the effect of different coprophilous beetle species on a fast-growing plant. To test the impact of different species of coprophilous beetles on a grass grown in pastures, I included Lolium perenne, a grass grown at the MSU Lake City Research Center.
My plan was to set up pots of soil with different combinations of dung and coprophilous beetle species, allowing the insects to work for 10 days. Then I would remove the surface dung, transplant seedlings of Lolium perenne into the beetle-treated soils, and plant Brassica rapa seeds in the reamaining beetle-treated soils. I also wanted to study the impact of coprophilous beetles on plant growth compared with the impact of chemical fertilizer. Chemical fertilizer would be applied once to each type of plant as a separate treatment. I planned to use Miracle Gro Water-Soluble All-Purpose Plant Food, containing 15% nitrogen, 30% phosphate and 15% potash.
Since the size of the coprophilous beetles affects the rate of dung burial (Lastro 2006), my hypothesis was that the largest of the three beetles, Onthophagus taurus, would have the most impact on the growth and biomass of Lolium perenne and Brassica rapa.
MATERIALS AND METHODS
The first step in the experiment was planting Lolium perenne on September 4. Usually grasses in pastures are given time to grow before being fertilized (Carmichael 2010). The Lolium perenne was planted three weeks earlier than the Brassica rapa to simulate what would happen in a pasture setting and because Lolium perenne has a longer life cycle. The Lolium perenne seeds were planted in 48-cell flats, with three seeds in each cell. BACCTO Professional Grower's Mix was used, which is a combination of sphagnum peat moss (75% to 85%) and coarse perlite. The flats were placed near a sunny window, watered as needed, and their positions were rotated for even growth.
On September 10, I collected cow dung at the Lake City Research Center. My goal was to gather the freshest dung possible. Sometimes this meant literally following the cows around with a shovel! All the dung was homogenized and frozen for at least 24 hours to prevent unwanted insect activity.
Next, I needed to collect coprophilous beetles using pitfall traps. Using these traps had resulted in many beetles a few weeks earlier. Unfortunately, once I was ready to begin my research, the weather turned unseasonably cold. The week of August 30 to September 4 was filled with rain and chilly weather. On September 4, the temperature reached a low of 8°C. The cold wet weather provided less-than-ideal conditions for beetle activity. I was unable to catch any beetles in traps set on September 5-6. I also tried using the floatation method, which yielded no results. This worried me, as the end of summer was rapidly approaching. Would I go to all of this work to prepare for my project, only to have the beetles desert me? Finally, the weather warmed up. I set 34 traps on September 8 and again on September 9. This time, I was in luck; the beetles were back at work! I collected them in one-gallon plastic containers, along with a supply of dung for food.
Another difficulty I faced was keeping the beetles alive long enough to collect the amount needed for all of the replicates. About half the beetles I trapped died before I could get the treatment pots set up. This meant a last-minute trip to the Lake City Research Center on the night of September 13. At this point I was so desperate to find coprophilous beetles that I tried digging beneath the dung pats and searching through the soil underneath. I also set 12 pitfall traps. I had difficulty catching the beetles by flashlight, so it took until late into the night to find enough insects.
After obtaining the beetles from my traps the following day, I sorted the beetles into their individual species. Knowing that male Onthophagus taurus and Onthophagus nuchicornis have protruding horns, I used a magnifying glass to identify the males and females, and placed them in separate containers. I was unable to identify the gender of Sphaeridium scarabaeoides. Once I finished sorting, I counted the beetles and realized that even with the late-night trip and the additional traps, I was still lacking a few beetles. Because of this problem, my original plan of having an equal number of insects in all the treatments for my experiment was changed for the Lolium perenne treatments to fit the number of coprophilous beetles available.
Brassica rapa treatments:
1. Six Sphaeridium scarabaeoides with 100 grams of cow dung.
2. One male and five female Onthophagus taurus with 100 grams of cow dung
3. Three male and three female Onthophagus nuchicornis with 100 grams of cow dung
5. 100 grams of cow dung
6. Control (soil only)
Lolium perenne treatments:
1. 10 Sphaeridium scarabaeoides with 250 grams of cow dung
2. Five female and five male Onthophagus nuchicornis with 250 grams of cow dun
3. Five female and three male Onthophagus taurus with 250 grams of cow dung
4. Chemical fertilizer: watered once with Miracle-Gro Water-Soluble All-Purpose Plant Food according to the package directions (after the seedlings were planted)
5. 250 grams of cow dung
6. Control (soil only)
The pots for Brassica rapa were 10 cm-by-10 cm and filled 7 cm deep with BACCTO Professional Grower's Mix because Fastplants.org recommends a peat-and-vermiculite mix. The pots for Lolium perenne were gallon-sized and filled 15 cm deep with sandy loam soil collected from a pasture at the Michigan State University Lake City Research Center. The sandy loam soil was sifted to remove unwanted debris. All treatments for both plant species were replicated five times. The treated soils for Brassica rapa and Lolium perenne were evenly moistened, and the collected beetles and thawed dung were carefully placed on their corresponding pots of soil. To keep the beetles inside, I covered the tops of the pots with nylons secured by rubber bands and the bottom openings with duct tape. I planned to add the plants and seeds after the coprophilous beetles and dung were removed. All pots were set up in the Evart High School greenhouse on September 14, and the beetles were allowed to work for 10 days.
On September 24, I removed the surface dung and allowed the beetles to escape. I saw evidence that the Onthophagus taurus and Onthophagus nuchicornis beetles had tunneled beneath the dung. It was very exciting to see these paracoprid beetles at work! I also observed the tunnels through the dung that Sphaeridium scarabaeoides had created. I then transplanted three Lolium perenne cells, totaling nine seedlings, into each one-gallon pot. At this point the chemical fertilizer was applied to the appropriate Lolium perenne pots until they were completely saturated. The pots were randomized, and an initial height measurement of all the seedlings was taken from the top of the treatment pots. A small piece of wood about 2.5 cm thick was placed on top of the pot before measuring for accuracy, since the level of the soil could change. The Lolium perenne were measured every five days, watered with 250 ml of water, and the pots rotated for even light exposure.
I waited to plant Brassica rapa until September 27 because the lights the plants required were not yet available. Each pot received three seeds. I used two light fixtures, each holding one full-spectrum and one fluorescent bulb. The lights were left on 24 hours a day and were kept 10 cm from the plants, according to the directions at Fastplants.org. After I planted the Brassica rapa, I added chemical fertilizer to the appropriate plants until they were completely saturated, and the pots were randomized.
Every two days, the Brassica rapa plants were measured from the top of their containers to the tip of the highest leaves, then moved to different positions and watered with 100 ml of water. Once Brassica rapa had grown for six days, the excess seedlings were removed, leaving one plant per pot. I pollinated the Brassica rapa flowers with a preserved bee on a stick. This was to simulate the natural pollination they would receive if they were grown outside. The Lolium perenne and Brassica rapa plants were watered, measured and their positions rotated until November 11.
I obtained the final height measurements for both Brassica rapa and Lolium perenne on November 11. I removed the Lolium perenne plants from their pots and soaked them in buckets of water to loosen the soil from the roots. Then I gently rinsed each plant of both species, laying them on a screen and removing the soil. They were patted lightly with paper towels to remove excess water, and their fresh weight (in grams) was taken. Once again, I worked until late at night, this time by the car's headlights.
The plants were placed in individual paper bags to be taken to the Lake City Research Center and dried in its plant dehydrator. After four days I obtained the dry weight of the plants. Then I cut each plant in half at the root line, and the root mass and top mass were measured. I also removed the seeds from each Brassica rapa pod and measured the mass of the seeds. Since the seeds and some root masses were so small, I took them to Ferris State University and used an analytical scale in the Chemistry Department. Values from all measurements were used to obtain a mean and standard deviation for each treatment for comparison.
Brassica rapa RESULTS
During the experiment, one Brassica rapa replicate treated with chemical fertilizer died on October 29. Thus, only four replicates treated with chemical fertilizer were used for my end-of-project measurements. The results show that the chemical fertilizer treatment appeared to be of greater value than all the beetle treatments in terms of height, fresh weight, dry weight, root mass, dry top mass and seed mass (Table 1). In order to compare the biomass of the roots with the biomass of the top of the plant, the root-shoot ratio was calculated. The results show that plants in soil treated by Sphaeridium scarabaeoides appeared to have the highest root-shoot ratio (Table 1). Comparing the control plants with the beetle-treated plants, my results show that the controls appeared larger than the Onthophagus taurus samples in height, and greater than all the beetle treatments in fresh weight, dry weight, root mass, top mass and seed mass (Table 1). The soil samples treated by Onthophagus nuchicornis appeared to have greater height compared to the control (Figure 23).
Lolium perenne RESULTS
In the Lolium perenne experiment, the control soil appeared to have the tallest growth (Figure 25). The control also appeared to have the heaviest fresh weight and root mass. Results show that the soil treated by Onthophagus nuchicornis appeared to have a slightly heavier dry weight. The chemical fertilizer treatment appeared to have the heaviest dry-top mass. The root/shoot ratio was calculated and showed that the control treatment appeared to have the highest root/shoot ratio (Table 2).
In previous studies, coprophilous beetles had had a positive effect on the yield of plants (Lastro 2006). The results of my study showed that soil treated by Onthophagus nuchicornis had a positive effect on the net height of Brassica rapa compared to the control. However, the plants treated with Onthophagus nuchicornis and the other beetles had a lower biomass than the plants treated with chemical fertilizer. With Lolium perenne, the plants in soil treated by Onthophagus nuchicornis were slightly heavier than the others in dry weight. This showed that the treatment with Onthophagus nuchicornis did have some impact on the Brassica rapa and Lolium pernne. However, my results surprised me, because I had expected that the soil treated with the beetles and dung would result in higher values than the control soil. My hypothesis that treatment with Onthophagus taurus would have the most impact on the growth and biomass of Lolium perenne and Brassica rapa was not confirmed.
One possible problem that may have contributed to my inconclusive results was the uneven homogenization of the sandy loam soil collected from the Michigan State University Lake City Research Center. The pasture soils were sifted and mixed, but that may have been insufficient to prevent some pots (and possibly the control soil) from containing more nutrients than other pots. Other experiments have shown that Onthophagus taurus beetles do not like sandy loam soil (Lastro 2006), which also may have affected my results.
Before the beetles were placed in their treatment pots, I watered the soils. I did not water them during the 10 days the beetles were present because I wanted to evaluate the beetles' impact on soil without the addition of water to help nutrients seep down into the soil. Unfortunately, during the time of my experiment, the temperature rose to more than 32°C for a 24-hour period in the greenhouse, which may have dried out the soils to the point that the beetles could not work.
I realized after my experiment concluded that measuring Brassica rapa to the tip of the highest leaf was not precise. At the end of the experiment, height was measured to the top of the plant in order to obtain an accurate measure. Also, at one point in my research I may have begun measuring weed sprouts in two of the Lolium perenne pots (replicate #1 treated with chemical fertilizer and control replicate #2) because of my inexperience with these plants. I removed these replicates from the data used to construct Figure 23. The weed measurements were not included in the net height or any other measurements taken at the end of the experiment.
Since there were only five female and three male Onthophagus taurus to treat the Lolium perenne as opposed to 10 beetles in the other Lolium perenne treatments, the three beetle treatments were hard to compare. Ten Onthophagus taurus would give those beetles an equal opportunity to demonstrate their dung-burying skills. Also, if I were to repeat this experiment, I would increase the number of replicates in order to have enough data for proper statistical analysis.
Onthophagus taurus DISCOVERY
Although the data from my experiment was inconclusive and did not support my hypothesis, my research yielded an unexpected result. Dr. Kevin Floate of the Lethbridge Research Centre in Lethbridge, Alberta, suggested that I might be the first person to recover Onthophagus taurus in Michigan (Floate 2010). He provided a map that showed that the previous northernmost discovery of Onthophagus taurus was in Pennsylvania. To verify my identification, I traveled to the A.J. Arthropod Research Collection at Michigan State University. Gary Parsons, curator of the collection, confirmed my identification and verified that Onthophagus taurus had not been recorded in Michigan before this point (Parsons 2010). Three voucher specimens were added to the A.J. Arthropod Research Collection. Dr. Floate said that this discovery was helpful in his research and that my find suggests that Onthophagus taurus will be strongly established in Canada in the future (Floate 2010). I felt privileged to assist in a scientist's research because he has devoted so much time to my own.
SHARING WHAT I LEARNED
In addition to my research and the discovery of Onthophagus taurus in Michigan, I also started an Insect Club for 15 children aged 5 to 12 in an inner-city neighborhood in Grand Rapids, Michigan. I teach this club every month, providing a short lesson with themed crafts, games, experiments and snacks. My hope is to inspire these 15 kids with my passion for insects and help them learn how to conduct their own research. I also made a pinned coprophilous beetle collection for the Michigan State University Lake City Research Center. This will help farmers in the area learn about the types of helpful beetles that may inhabite their pastures.
Since this research did not fully answer the question I posed about coprophilous beetles and their impact on plant growth, I would like to design another experiment that would create a more accurate test. Another idea for future research is a more comprehensive survey (from May to October) of the coprophilous beetle species at the Michigan State University Lake City Research Center. Maintaining a healthy population of coprophilous beetles could be a less-expensive alternative to the excessive use of chemical fertilizer, and would reduce the harm done to the soil and watersheds (Carmichael 2010). In our efforts to keep our planet clean, these tiny allies in the pasture should not be overlooked.
I would like to thank my mentor, Dr. Kevin Floate, for his support and his invaluable assistance with my project. I have learned so much from him, and I hope to work with him again. I would also like to thank Doug Carmichael for his permission to use the facilities at the Michigan State University Lake City Experiment Station.
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