Herbs vs.Bugs

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THE CASTLES of medieval and Renaissance Europe must have been rich in sights, sounds, and smells. I imagine the inhabitants tossing bones and other scraps for the dogs on the floor, and the heaps that would accumulate. It was probably helpful in those times to own at least two castles so that when “the middens became stinking”, the royalty could move on and the floors could finally be swept out. Just imagine the populations of fleas, ticks, and lice evicted along with the detritus! The practice of strewing herbs on the floor to repel vermin and freshen the air between cleanings dates back at least to this era. Leaves of sweet flag, flowers of lavender, and leafy stems of pennyroyal were among the herbs commonly used for this purpose.

These days, few of us have spare castles to retire to when the vermin take over. We have not only higher standards of sanitation, but also more effective ways of controlling populations of insect pests. Herbs still can play a part, though, particularly as we search for “natural” solutions from the garden. Countless plants have been used throughout the ages or may have potential for use as insect repellents.

Insects and scents

Insects as well as other arthropods have an extremely acute sensitivity to odors. For example, tiny amounts of chemicals called pheromones produced by an insect can elicit sexual and other behavioral responses from others of its species; a male gypsy moth will react to a single molecule of pheromone from a female, even at a distance of several miles. Insects are likewise capable of detecting chemical scents from plants and other animals.

Insects and plants evolved together, and complex interactions have developed between them. Just as many plants can use odors to attract pollinators, they can also produce scents to ward off insects that might eat them. Paradoxically, certain substances that act as insect attractants at low concentrations will repel the same insects at higher concentrations. Olfactory repellents will work only if in sufficient concentration, especially if they are competing with such strongly attractive odors as that of human sweat. In the final analysis, repellents sometimes do not repel at all but rather counteract attractive odors by either masking them, altering them chemically, absorbing them, or inhibiting their formation. Each herb thus has a constellation of mechanisms by which it may “repel” insects.

The principal component of most leading mosquito and tick repellents sold today is N,N-diethyl-meta-toluamide (DEET). An effective repellent of biting flies, mosquitoes, ticks, and chiggers, DEET is recognized as the standard against which other repellents are compared, yet today it carries a warning: it should not be ingested, inhaled, or used in contact with skin. DEET has been shown to be toxic to humans and domestic animals, and ­reactions can be severe, rapid, and sometimes fatal. The compound ­directly affects the central nervous system, causing seizures and coma; children are particularly susceptible to its effects in concentrated formulations.