What it shows
Natural sprays can reduce insect damage on crops without strong pesticides. Counting damaged leaves on sprayed and unsprayed plants gives data on which home-made spray works best.
The science
Neem contains compounds that make leaves taste bad to many insects and slow their growth. Garlic gives a strong smell that repels some pests, and soapy water breaks the outer coating of soft-bodied insects such as aphids. Their effect varies with the pest and the weather, so repeated counts are needed.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four similar potted plants such as okra, brinjal or cabbage
- Neem leaves, garlic and mild soap
- Four spray bottles and a muslin cloth
- Water
- Labels, gloves and a hand lens
- A notebook for weekly counts
Safety
For this project: An adult must crush garlic and neem and mix the sprays. Wear gloves and do not touch eyes. Never spray onto food that will be eaten, keep sprays away from pets, and wash hands after use.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Soak 50 g crushed neem leaves in 1 litre of water overnight and strain through muslin.
Crush 5 garlic cloves in 1 litre of water, soak for a day and strain.
Make a mild soap spray with 1 teaspoon of soap in 1 litre of water.
Label four plants as neem, garlic, soap and control, with the control sprayed with plain water.
Count the leaves with holes or aphids on each plant before starting.
Spray each plant lightly every three days in the evening.
Count damaged leaves and pests once a week for three weeks.
Plot damage against weeks for each spray and decide which worked best.
What you should see
The graph shows damage on each plant over three weeks. Sprayed plants usually show less damage than the control, and the best spray is the one with the lowest curve.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is a control plant needed?
To show damage without any spray.
2Why are natural pesticides useful?
They are less harmful to people and soil life.
3Why spray in the evening?
Strong sun can burn wet leaves.
4Why is one count not enough?
Pest numbers vary from day to day.
Take it further
Combine two sprays and test whether the mix works better than either alone.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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