BIOCONTROL (Lab 8)
(Ecol 206: Bonine, Cao, Epps 2009)
Purpose: To understand the basic concepts of biocontrol and use a computer model to simulate the effects of pesticides and biocontrol agents on the size of a pest population.
Website:
Please answer the following questions:
1. What is biocontrol?
Biological control is a method of controlling agriculture pests by way of predation, parasitism, and herbivory.
2. What two types of organisms are used for biocontrol? Provide a specific example and describe its use.
Parasitoids, predators, pathogens, and weed feeders. Any pertinent example would work.
3. What are 3 advantages to using biocontrol agents versus using pesticides?
Nontoxic, environmentally friendly, self-sustaining, low cost, and other logical answers.
Use the information below to run your model simulation:
Flea beetles are one of the most difficult pests to manage! Tomato flea beetles are tiny (~2mm in length) and can jump similarly to fleas (thus the name). These beetles eat the leaves of tomato plants, and if left untreated will kill the affected plants. Female tomato flea beetles lay their eggs (about 25 eggs per oviposition) in thesoil during the spring. The eggs hatch in about 2 weeks. The larvae feed on the roots and shoots of tomato plants for about 2 weeks before pupating in the soil. The adults feed on plant leaves for about 1 month before reproducing. Because of their rapid generation time, there may be up to four generations of mature flea beetles in your garden each growing season! Female flea beetles produce unbiased sex ratios (i.e., equal number of males and females). You will use the computer simulations to determine how tomato flea beetles will affect your wonderful tomato garden! You want to determine the population growth rate of these harmful pests and the best method (pesticides or parasitoid wasps) to control the beetle population and save your tomato plants!
Simulation 1: Population growth
If only 50fecund females were in your garden at the start of the growing season, how many female beetles will you have after the first 5 generations? Play around with the numbers to see how the population can change over time.
Generation / Number of females0 / 50
1 / 625
2 / 7,813
3 / 97,656
4 / 1,220,703
5 / 15,258,789
Draw the graph to show the change in
population size over 5 generations:
Simulation 2: Population growth (taking into consideration birth and mortality rates)
Start with the same number of females (50) and set the birth factor to 25 eggs (same as above). Let’s say that a growing season will consist of 240 days. Now complete the follow table and assessment below:
A: mortality = 25Generation (days) / Population size / r (intrinsic growth rate)
0 / 50 / r = 0
1 / 50
2 / 50
3 / 50
240 / 50
B: mortality = 25.1
0 / 50 / r = -0.1
1 / 45
2 / 41
36
38 / 0
C: mortality = 24
0 / 50 / r = 1
1 / 100
2 / 200
3 / 400
18 / 13,107,200
Based on the results above, how does mortality rate affect population growth?
If birth rate is constant, then changes in mortality rate can have significant effects on population size. As mortality rate decreases, r increases, resulting in significant population growth (and vice versa).
In your tomato garden, what are some factors that could affect the mortality rate of flea beetles?
Predation, resource availability, diseases, temperature, weather, etc…
How does the intrinsic rate of growth (r) affect the population growth rate? What happens when it is a negative value?
When r is zero, the population size is stable. When r is negative, population size decreases. When r is positive, population size increases. Other more technical answers may be acceptable based on:
dN = rN
dt
dN = rN [(k-n)/k]
dt
What is this equation: Nt+1 = Nt + Nt × r? Plug in a set of numbers from above and perform the calculation (show your work). Do you get the same result?
Similar to dN = rN, this equation is used to calculate the change in population size using r (birth - death rate).
dt
Taking over time, this equation can indicate exponential increase or decrease in population size. Plugging in numbers should get the same results.
Simulation 3: Population growth (incorporating K (carrying capacity) into the model)
Because resources and space are not infinite, a population cannot grow exponentially indefinitely. Here we introduce K into the model to see how it affects population growth and stable population size. Similar to before, we will start with 50 females, with each female producing 25 eggs. Keeping these variables constant allows us to see the effects of the manipulated variable on population growth. For this simulation, let’s say that your smalltomato garden has a carrying capacity of 10,000 beetles. Above this number, your garden will be decimated! Run the simulation and complete the following.
Mortality = 24.9Generation (days) / Population size / r / k
0 / 50 / r =0.1 / k =10,000
1 / 55
2 / 60
3 / 66
150 / 10,000
200 / 10,000
Graph the complete data for the results of this simulation:
Now, let’s pretend that some evil neighbor (or classmate) released 20,000 tomato flea beetles into your garden! Remember that your garden cannot sustain more than 10,000 (or K). Keeping everything the same as the previous simulation and only changing the start population size to 20,000, complete the following.
Mortality = 24.9Generation (days) / Population size / r / k
0 / 20,000 / r =0.1 / k =
10,000
1 / 18,000
2 / 16,560
3 / 15,474
87 / 10,000
200 / 10,000
Graph the complete data for the results of this simulation:
Based on the results from simulation 3, describe how K affects population size. What happens when a population starts out at a population size that is greater than K? Explain.
Introducing K into the equation results in logistic growth. As a population grows, its rate of increase is limited by the carrying capacity (K). When a population starts out higher than K, the population size will decrease and levels off at K.
What is this equation: Nt+1 = Nt + Nt × r × (1 - Nt / K)?Plug in a set of numbers from above and perform the calculation (show your work). Do you get the same result?
Equation that depicts logistic growth, incorporating the carrying capacity (K). Show calculations, and the results should match up to the ones above.
Simulation 4: Simulateuse of pesticides
This simulation will allow you to see the effects of pesticide use on the population of flea beetles in your tomato garden. You will be able to test different regimens of spraying and strengths of pesticide to be administered. Start with a beetle population size of 50, birth factor of 25, mortality factor of 24.9 (thus a realistic r of 0.1), and K equals to 10,000 (all the same values used in the earlier simulations). You want to minimize your work effort, so you will only spray your garden once each month (i.e., every 30 days). You first spray your garden 30 days into the season. You will dilute your pesticides so as to minimize harming the environment (and yourself!), but it must also be effective in controlling those darn flea beetles! You don’t want the beetle population to go above 8,000!What is the minimum strength that your pesticide must be (in percentage killed) to maintain the beetle population below 8,000? Draw the graph (below)to illustrate the change in flea beetle population over time when you use this spray regimen at this strength. At what strength will you be able to decimate the entire population in 3 months (using the same schedule of spraying)? If the pesticide is capable of killing this percentage of the population every time that it is used, would you want to eat those tomatoes? Now let’s say that you decide to spray every 3 months, what happens to the beetle population? Summarize by discussing the effectiveness of using pesticides to control pests in your garden. Play around with the simulation some more if you need to.
- Minimum strength to maintain beetle population under 8,000 is 78%.
- To decimate the beetle population in 3 months = 97% efficiency
- Don’t eat the tomatoes (too much pesticide!)
- Spraying every 3 months result in beetles pop going above 8,000 and
reaching its carrying capacity
Pesticides are not effective in controlling the pest population unless:
1. It is strong (high kill percentage)
2. Must be used on a regular basis to suppress pest population
3. Others, may be acceptable
Simulation 7: Simulate biological best control: a parasitoid
After reading some shocking news about the harmful health effects of carbaryl and methoxychlor in Science magazine, you stopped spraying your tomatoes with pesticides. To control the flea beetle population, you decided to introduce parasitoid wasps to your garden! You plan to introduce a species of Microcotonuswasps. These wasps oviposit in adult flea beetles; each adult wasp will lay about 20 eggs inside one beetle. The larvae eat the beetles from the inside out! The larvae then pupate in the soil and emerge as adults from the pupal cocoon in about 15 days.Let’s first simulate how effective these parasitoid wasps will be in controlling the flea beetles. When you stopped using the pesticide, the beetle population climbed until it reached 10,000, its carrying capacity (all of yourtomato plants are in real bad shape!). For the other variables, use the exact same values for the pest that you used in the last simulation. As for the parasitoid, you first introduced 100 wasps. Each wasp typically lays 20 eggs (birth factor). Set the mortality rate to 19.9, so that r equals 0.1 (this is verytypical for many wasp species). The carrying capacity of the wasp will be 10,000 in your garden plot. Initially set the predation factor to 0.1. Run the simulation and complete the table below. Then draw the graph to illustrate the relationship between the population size of the pest and the parasitoid over time.
Population sizeGeneration / Pest / Parasitoid
0 / 10,000 / 100
1 / 9,990 / 110
2 / 9,980 / 121
3 / 9,970 / 133
124 / 6,180 / 6,179
125 / 6,180 / 6,179
126 / 6,180 / 6,180
127 / 6,180 / 6,180
Describe what you observed in this simulation.
The pest population and parasitoid population levels off in a stable equilibrium.
What happens to the population size of both organisms if the parasitoid predation factor is increased? Explain why this pattern would emerge.
The stable equilibrium decreases (pop size decreases for both). Parasitoid kills more of its host, affecting its own population size.
Is it better to introduce a greater number of parasitoid wasps at the start? Explain in terms of the stable population size for both organisms.
No. The stable population size remained the same. But stable equilibrium is reached faster (possible benefit).
Are parasitoid wasps a more effective way to control the pest population and save your tomato plants.
Biocontrol is a safer alternative to pesticides. It does a better job at maintaining the pest population at a particular level, but pesticides (if used frequently and with high potency) may be able to decimate the pest pop entirely. Trade offs…
Bonus. Play around with this last simulation and describe 2 interesting trends that you find.Another logical will do...