LD50 Lab Report

Abstract

In the following lab, Hot Shot® pesticide is tested for the concentration at which half of the exposed Lemna minor die over the course of five days.

Introduction

The LD50 test is used to determine the amount (dose) of a substance that causes one half of the organisms it comes in contact with to die. It is expressed as an amount (i.e. grams) per a certain unit of weight (i.e. kilograms). This measurement is useful when manufacturing, handling, or using toxic substances because it assesses the acute toxicity of the substance. The quantitative data helps in calculating risk and can be used to determine insurance rates, necessary warnings, and to whom the substance is made available. The LC50 is the lethal concentration of a substance and is tested in a similar way.

The pesticide used in the following report is called Hot Shot and is manufactured by the Spectrum Group Division Of United Industries Corporation. The active ingredient, lambda-cyhalothrin, comprises .03% of the solution and is a pyrethoid insecticide.

Hypothesis

If the concentration of Hot Shot exceeds 25% (lambda-cyhalothrin .0075%), then 50% of the Lemna minor organisms will die.

Materials and Methods

The following are needed for this experiment: pesticide, petri dishes labeled with different concentration amounts, pipet, spoon, Lemna minor, graduated cylinders, and water.

Mix solutions of the following concentrations of pesticide: 100%, 75%, 50%, 25%, 12%, 6%, 3%, and 1%. Put 30ml of the appropriate solution into a petri dish with corresponding label. To eliminate waste, simply put 30, 22.5, 15, 7.5, 3.6, 1.8, .9, and .3 ml of pesticide into the dishes and then add enough water to make the total, in each dish, 30ml. In the final dish put 30ml of water. Using a spoon, pipet, or tweezers, add twenty Lemna minor to each dish. Record observations and then place the dishes in an area with sunlight and controlled temperature, humidity, etc. After one day, count the number of living organisms in each dish. Repeat the tally the following day and again on the third day.

Calculate the LD 50 by graphing the results and locating the dose at which 50% (10 organisms out of twenty) are killed.

Observations and Results

The Lemna minor came from two different colonies. The first of these had mostly one or two leaves per organism, a relatively small root, and a dull green color. The second colony consisted mostly of individuals with two or three leaves, a root about 1 cm long, and glossy, bright, larger leaves. Organisms from the second tank were used in the following concentrations: 1%, 6%, 12% and 75%. After the first, fourth, and fifth day, a census was taken. Only green Lemna with roots were counted as living. The dead were extremely pale or white or they lacked roots. The number alive was counted instead of the number dead because, it was discovered, the leaves of the dead often broke apart leaving what would have been counted as one alive organism to be counted as two or even three dead. In the case of the 100% concentration, more than twenty organisms were accidentally put into the petri dish. Therefore, all the contents of the dish were tallied and the percent alive was used to calculate how many would have been alive if twenty Lemna were exposed. Because not a single one was considered alive in the final count, this inconsistency did not impact the integrity of the experiment.

Conclusion

From these results, it can be inferred that the LC50 of Hot Shot® is approximately 27% and the LC50 of lambda-cyhalothrin is .0081%. It is difficult to determine the threshold from the data collected because the small sample size magnifies the disadvantages of working with all of the factors that determine an organism’s death. Even if the sample was larger, no smaller increments than whole percents were used. The threshold appears to be at about 6%.

As mentioned before, with no exposure to pesticide, 10% still died, showing that there are other uncontrolled factors affecting them. 10% sounds excessive but, due to the sample size, it is only two individuals who did not survive. The chances are fair that Lemna in all of the concentrations died for reasons other than pesticide exposure. This points to two flaws in the experiment: unaccounted for variables and small and misleading population size.

Nevertheless, this experiment would be a useful tool for someone about to conduct a more accurate assessment of lambda-cyhalothrin. Instead of skipping from 25% to 50%, future tests would use smaller increments between 20 and 35 percent to obtain a more accurate LC50.