The Brown Marmorated Stink Bug (Halyomorpha halys) is a common invasive pest known for its shield-like shape and tendency to invade homes during the cooler months. While this insect is not equipped for aquatic life, its physical adaptations provide a surprising level of defense against drowning. Understanding this biology explains why simple water is ineffective for removal, yet soapy water is a popular and effective control method.
Buoyancy and Behavior in Water
Stink bugs are not true aquatic insects and lack the specialized appendages or streamlined body shape necessary for directed underwater movement. When a stink bug accidentally falls into plain water, it is unable to swim. Instead, the insect flails its legs in an attempt to right itself or reach the edge of the water source.
The insect’s body mass and structure give it a high degree of natural buoyancy, causing it to float on the water’s surface. This flotation is a passive mechanism, meaning the bug rests on the surface film. This allows the bug to survive accidental immersion for extended periods, simply floating until it can be washed ashore or rescue itself.
Physical Characteristics That Influence Floating
The insect’s outer layer, the exoskeleton, is covered in a waxy substance and minute surface structures, which render it highly hydrophobic. This microscopic texture prevents water from adhering to the body, keeping the spiracles, the insect’s breathing pores, clear and above the surface.
This hydrophobic outer layer allows the stink bug to exploit the natural phenomenon of surface tension. The insect’s light weight and the large surface area of its legs distribute its mass, preventing the water’s surface film from breaking. This physical mechanism acts as a protective barrier that prevents submersion.
Water Encounters and Survival
Their encounters with water are typically accidental, such as falling into a sink, a pet’s water dish, or a container used for collection. In clean water, the insect can survive for a significant amount of time, continuing to float and breathe due to its water-repellent shell. The risk of drowning is low as long as the surface tension remains intact.
This survival ability changes when a surfactant, like dish soap, is introduced to the water. The soap molecules immediately disrupt the water’s surface tension, destroying the film the stink bug relies on for flotation. Once the surface tension is broken, the water penetrates the hydrophobic barrier and floods the insect’s spiracles, causing it to sink and drown rapidly. This is the scientific principle behind using soapy water to dispatch the pest.