World Wide Fund for Nature (WWF-UK)
Godalming, Surrey GU7 1XR
The activity below is taken from:
Science for Survival: Plants and Rainforests in the Classroom by Adam Cade, published in 1988 by the Richmond Publishing Co. Ltd. Copyright: Adam Cade, Richmond Publishing Co. Ltd., WWF-UK
This publication forms part of the WWF-UK Environmental Education Project. The activity may be reproduced for non-profit, educational purposes provided the source is acknowledged.
Rainforest plants seem to possess many more chemical than physical defences. Their simple, thin leaves and bark offer very little physical protection against grazers such as monkeys or insects. So, more often then not, the plants live by their chemical wits.
Rainforest people have used naturally-occurring herbicides and pesticides for a whole range of uses. Many animals in the rainforest are hunted using poison darts. For example, in Malaysia, the Orang Asli people use the milky sap from the Ipoh tree for their blowpipe darts as it contains a toxin called antiarin. The Punan people of Sarawak, Malaysia, also use this latex to heal festering wounds or snake bites. The antidote to this poisonous plant chemical is obtained from the bark of a local shrub.
In this investigation, you will find out how effective different plants are as pesticides.
You will need
Plastic aquarium with lid (or other containers)
Snails/slugs (unfed for 2 days)
Filter paper squares (2x2 cm)
Plant leaves (lettuce, cabbage, carrot, bracken, rhododendron, rosemary)
Choice of 6
What to do
1. Prepare plant leaf solutions by heating samples of the above leaves separately in beakers of water.
2. Soak a filter paper square in each of the six plant leaf solutions.
3. Dry the squares and write the plant name on each square in pencil.
4. Set up a food preference test in one of the following ways:
a) 6 animals in 1 container with 1 of each of the squares.
b) 1 animal in 1 container with 1 square. Repeat 6 times with different squares.
5. After about 24 hours record qualitatively or quantitatively how much of each of the squares has been eaten.
Discussion of results
Is there any indication that any of the plant solutions used contain chemicals which protect the plant from being grazed? Explain your answer.
1. Devise and carry out investigations to find out if other plants contain natural pesticides.
2. Does concentration of the plant extract have an effect on how useful it is as a pesticide?
3. Try using solvents other than water to prepare plant extracts, e.g. propanone. Does this affect their usefulness as pesticides?
Discussion of results
Write a report of your methods and findings.
Note for teachers
1. Snails and slugs may be able to digest cellulose because they possess commensal bacteria in their gut which produce cellulase enzymes. It may be best to use just one species of similar-sized individuals.
2. The amount of each square that is eaten could be found by weighing dry squares before and after the investigation or by tracing round the eaten squares onto graph paper and calculating the area eaten.
3. Some bracken contains cyanogenic compounds (cyanide), as does rhododendron. Rosemary is an aromatic herb containing an essential oil, bitter chemical and tannin. Different solvents for these chemicals will vary their effectiveness. Other plants such as onion, laurel, sage, mint, ginger etc. could be used.
4. This investigation provides an excellent opportunity for discussion of animal welfare issues and respect for life. Obviously pests compete with humans. Research is needed Into environmentally-acceptable pesticides or, better still, repellents. The plant extracts will not kill the slugs or snails but will only act as repellents.