The effect of plant chemicals on animals
World Wide Fund for Nature (WWF-UK)
Weyside
Park,
Godalming, Surrey GU7 1XR
ENGLAND
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)
6 beakers
Choice of 6
No choice
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.
Extension
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.
