IMPACT of INSECTICIDES on PLANTS

Have you ever grown plants?  For sure you have faced the recurrent problem that is… BUGS!

And how do we get rid of bugs? We use insecticides! These products (and other pesticides) are really efficient to kill insects. But are they totally harmless for the plants they are spread on?

Nowadays, you might have heard that insecticides and other pesticides have surely negative impacts on the environment, as they pollute air, soils and water. Worstly, it has been shown that pesticides also alter human’s health; they have been linked to various cancers, neurological diseases and infertility problems.

In order to face all these issues, more and more people try to find natural alternatives to pesticides and insecticides. 

And one of them is… CAFFEINE! Have you ever wondered why you never get a fly in your coffee? Well that is because caffeine is a natural insecticide, that can paralyse and kill insects[1], and so a natural alternative to the ones you find in supermarkets and DIY stores. Wonderful, isn’t it? 

Here is a video showing coffee’s power against insects! If you are more into reading, check out this article and this article. 

But does caffeine have an impact on plants or is it really a good natural insecticide ?

We, a group of bachelor students from Paris Descartes and the CRI, decided to study this question during a scientific interdisciplinary course (the Biosensors!). Since this project lasted only one week, we planned to study the possible effects of caffeine on buckwheat seeds’ growth, as buckwheat grows fast.

So the question we attempted to answer was:

Does caffeine impact buckwheat seeds’ growth?

To answer this question, we performed a short but very cool experiment:

First, we prepared 5 solutions with water and different concentrations of caffeine: 0 mg/mL, 0.2 mg/mL, 0.6 mg/mL, 1 mg/mL and 5 mg/mL (in a concentration of 2 mg/mL of caffeine, most of the insects die[2]). We next poured more than 100 buckwheat seeds in each solutions and let them rest for half an hour[3]. In the same time, we named and prepared 25 Petri dishes (5 Petri dishes for each solutions) with cotton and 15 mL of the respective solutions. Once the half an hour passed, we placed 20 seeds in each Petri dish. This first part of our experiment is represented below:

Then, to keep all the 500 seeds in the same light, temperature and humidity conditions, we placed the Petri dishes in a greenhouse we designed. And to verify if these conditions were perfectly stable, we built and placed an electrical circuit with a temperature and humidity sensor (connected to an Arduino Uno board and a computer), in the greenhouse (Part 2 of our experiment shown below).

If you want to know more about the temperature and humidity sensor (the DHT11, shown on the left) that we used, you can have a look on this page and find our Arduino code here.

This sensor and the Arduino code we used allowed us to read temperature and humidity values on the computer they were connected.

The next day, we remoistened the Petri dishes with 15 mL of the same respective solutions as before and we weighed each seeds with a precise balance (Part 3 shown below).

This was a lot of weighing!

Finally, 2 days after we planted the buckwheat seeds, we cut the germs that grew and took pictures of them to measure their height with a software called ImageJ (much more precise than a simple ruler!). You can see this last part of our experiment below:

So what about our results? This is the exciting part…

The graph below shows the seeds’ weight in mg (after 1 day growing) in function of the caffeine concentration in their medium:

As you can see, it seems that there is no correlation between the weights and caffeine. Also the difference between the weights are so small! So we cannot conclude much from this graph. Maybe repeating the experiment and taking more weight measurements over time could give us better informations.

Now let’s talk about the second graph, below, concerning the size of the germs:

Here, it seems obvious that there is a correlation: The more caffeine concentration increase, the less the germs grew. These results prove that caffeine has definitely a negative impact on buckwheat seeds, as it slows their growth!

So what should we remember from our project?

Caffeine was proved to kill efficiently insects, as a natural insecticide. However, our experiment showed that it slows buckwheat’s growth, meaning that it may has a negative impact on many other plants!

As a result, caffeine does not seem to be a perfect natural pesticide.

For further information about the project, please click there!

Hanaé BRADSHAW, Maryam ARIF, Noémie PAILLON & Nicolas SILVA

Bachelor Students from Paris Descartes and the CRI (Center of Interdisciplinary Research)

Ressources:

[1] Nathanson JA (1984). "Caffeine and related methylxanthines: possible naturally occurring pesticides". Science 226 (4671): 184–7. doi:10.1126/science.6207592.PMID 6207592.

[2] Laranja, A. T., Manzatto, A. J., & Campos Bicudo, H. E. M. D. (2003). Effects of caffeine and used coffee grounds on biological features of Aedes aegypti (Diptera, Culicidae) and their possible use in alternative control. Genetics and molecular biology, 26(4), 419-429.

[3] Partilhe,Toutes les graines que vous pouvez faire germer, online article on mes-grainesgermees.com, posted on 04/04/2014, accessed on 08/02/2016