What Is the Purpose of Detergent in Sugar Water for Insect Traps?

An insect lands on water.

A mixture of sugar water and detergent is commonly used to trap insects. The sugar attracts the bugs, and the detergent creates a sinking water trap for the insects.

Landing

Sugar water has a skin-like surface tension. Insects are light enough that they can land on this watery film without sinking, and they can then fly away unharmed.

Surface Tension

Water molecules are composed of two hydrogen atoms and one oxygen atom. The surface tension of water is caused by a process known as hydrogen bonding, or what happens when the positively charged hydrogen atoms in each water molecule is attracted to the negatively charged oxygen atoms of other water molecules.

Detergent

Detergent is a surfactant, a substance that disturbs or breaks the hydrogen bonds between water molecules.

Broken Bonds

With the hydrogen bonds broken by detergent, water loses its surface tension. Insects can no longer land on the water's surface, and will sink into the liquid instead.

Fun Fact

Sugar water and detergent traps are also used by entomologists to catch and count bees for insect population surveys.

References

• Biology 110
• Sticky Water
• Surface Tension