By Xavier Carroll

Entomologists have been aware of insect-electrostatic interactions for decades. By the beginning of the 1960s, researchers were using electrostatic measurements to track the flight activities of flies in blind enclosures. Toward the end of the 1960s, scientists were already describing the role of static charges in pollination and sensory reception. From obscure beginnings, this body of research has grown into an entire field of study.

Electric Entomology

This realm of research, which I like to call “electric entomology,” encourages us to consider insects by their electrical properties in addition to their physiology and ecology. Insect matter, like all matter, is composed of atoms and is therefore subject to electrostatic phenomena. Insect cuticle (exoskeleton) acts as a dielectric material, meaning it conducts electricity poorly. Pure chitin, a primary component of insect cuticle, has an experimentally determined dielectric constant ranging from 5.2 to 7.0. For context, insulators display dielectric constants ranging from 1 to 10. An ideal conductor would have an infinitely large dielectric constant.

As a dielectric, we anticipate insect cuticles to have a high resistance to electric currents. The ohm-meter (Ω*m) is a standard scientific measure of electrical resistivity, and the resistivity of the chitin is indeed high, ranging from 2×108 to 3.57×106 Ω*m. This makes purified chitin a semiconductor, being more resistive than silicon but less resistive than glass. It is these attributes that allow charges to aggregate on the insect’s surface.

In reality, though, the electrical qualities of living cuticles are likely as diverse as insects themselves. We know that water content and even temperature can affect the resistivity of live cuticles. However, the effects of other phenomena like surface structures and cuticle chemistry still need to be researched.

Even with the suspected diversity, certain trends have emerged in the study of insect surface charges. Net charges on insect surfaces form via motion. During flight, for instance, many insects accumulate electric surface charges through frictional interactions with the particles in the air. In these interactions, electrons are transferred from one surface to another, producing a positive charge on one and a negative charge on the other. Think of a balloon rubbed against the hair of a child. Because there is a difference in the electron affinity of the balloon’s material and the hair, a demonstrable net charge is produced on both surfaces. For insects moving through their environment, this movement of electrons from the insect’s surface generally results in a net positive charge for the insect.

Due to their reduced scale, smaller insects are governed by a different suite of forces than we as humans are normally aware of. The net charges of tested insects are relatively small when applied to human scales, typically around 10-12 Coulombs (several orders of magnitude smaller than the charge from the balloon rubbed on hair). However, these tiny charges can facilitate critical interactions for the insects carrying them. Insect surface potentials have been proven to enable electroreception, resource assessment, and pollen transfer.

Electroreception describes the ability of an organism to detect electric fields and is understood to be an extension of mechanoreception in insects. In this system, charged mechanosensory hairs are deflected by the attractive or repulsive electrostatic forces of charges in the environment. The insect sensory system is sensitive enough to detect these very small deflections. This enables bees to detect the presence of flowers through electroreception over short distances.

Due to their vertical orientation relative to the atmospheric potential gradient, flowers accumulate negative charges. This also provides the flower’s pollen with a negative charge. Visitation of positively charged bees creates an attractive force on the pollen which is strong enough to lift the grains from the floral surface. By increasing the amount of pollen deposited on the pollinator, electrostatic forces can significantly improve pollination efficiency. The contact also momentarily neutralizes the charge on the flower’s surface, allowing other bees to assess the visitation rate, and therefore the resource availability, of a flower based on the presence of an attractive force.

Surface charge also affects ecto-parasitic interactions for arthropods. Researchers have found that ticks can be passively pulled toward their host surface using electrostatic forces. While this effect is reliant on relatively strong surface potentials and short distances, it still aids the tick in locating suitable hosts.

Electrostatic cues received by arthropod parasites don’t always need to be strong enough to resist gravity. Varroa mites, a major honey bee parasite, have been shown to increase activity in the presence of negative electric fields too weak to lift them. This increased excitability may have some influence on the mites’ success in using flowers to travel from one colony to another. Since flowers aggregate a negative charge at their apex, the resulting electric field could act as an additional sensory signal to attract mites to the site of pollination. Additionally, this field could serve as a signal of recent floral visitation as it does for honey bees, increasing the mites’ odds of interception. In hummingbird flower mites, modulating electric fields associated with the landing of hummingbirds are detected using sensory organs on the mites’ front legs. These commensal mites then move into close enough proximity to be electrostatically transferred to the bird’s surface.

Applications and Future Directions

A lot of the research in electric entomology is interested in the importance of electrostatic forces in insect pollination. By understanding the relationship between surface charge and pollen transfer, we can better understand the qualities that make an insect effective as a pollinator. Through this relationship, the scope of pollination-relevant qualities grows exponentially larger.

Electric entomology also has many exciting technological applications. Recently, this research has inspired the engineering of electric field barriers that use the electroreceptive ability of insects to behaviorally repel them from windows and doors. By exploiting the naturally occurring charge on insects, this technology reduces the presence of flying insects in people’s homes.

For me, viewing insects through the context of electrostatic forces is an opportunity to appreciate the nuance of insect’s physical interactions with their environment. Even after decades of research, there is still much to learn about how insects navigate these obscure and dynamic forces in their environment.

Glossary of Relevant Terms in Electric Entomology

• Atmospheric potential gradient: A vertical electric field in the atmosphere.
• Charge: An essential property of matter which governs how particles are affected by electric fields. Charges are classified as either negative or positive. Negative charges are attributed to the presence of electrons while positive charges are attributed to protons. Measured in Coulombs (C).
• Charge induction: The redistribution of charges on a conductive, grounded object induced by close proximity to a charged surface. Charges of the same sign are repelled and driven to ground, resulting in a net charge on the conductive object.
• Conductivity: The opposition of an object to an electrical current. The reciprocal of resistivity.
• Current: The rate at which charges flow in an electrical circuit. Measured in Amps (A).
• Dielectric: A material with high electrical resistivity and therefore low conductivity. Dielectrics are composed of electrical dipoles which can be polarized by external fields.
• Electric fields: A physical field surrounding a charge, dictating the electrostatic force exerted per unit of charge. Any object with a net charge produces a measurable electric field. Measured in volts per meter (V/m).
• Triboelectrification: The separation of formerly touching objects, resulting in an asymmetric deposition of charge on their surfaces. The effect is often thought of as “frictional charging.”

Xavier Carroll recently completed his master’s degree in entomology as a member of the Alleyne Bioinspiration Collaborative in the Department of Entomology at the University of Illinois at Urbana-Champaign. Email: xcarroll@illinois.edu.