Decoding Insects' Chemical Communications

Wendell L. Roelofs, professor of insect biochemistry, Department of Entomology, Cornell University, NY.

Insects have survived for millions of years, in part because of their ability to produce and to perceive an amazing variety of volatile chemicals. These chemicals are the basis of extremely efficient communication systems for sexual attraction, alarm behavior, trail-following to food supply, host finding, egg laying, territorial displays, raiding, nest mate recognition, marking, and the mediation of many other social interactions.

In some instances, insects may key on a few specific chemicals emitted by their host plant such as an apple maggot fly being attracted to certain short-chain esters from among the thousands of odors present in an apple orchard. In other instances, they may produce their own unique chemical signals to provide the specificity needed for efficient communication. In either case, decoding the communication system provides us with a potential means of manipulating the chemical signals for monitoring and controlling insect pest populations.

Sex Pheromones

Much of the research on insect chemical signals in the past two decades has been centered on sex pheromones. The term pheromone was coined in 1959 from the Greek pherein, to transfer, and horman, to excite, to refer to substances emitted by one individual and eliciting a specific reaction in a second individual of the same species. Sex pheromones, then, include the chemicals emitted downwind to attract individuals of the other sex or aggregate both sexes for mating. By 1961 German scientists had shown that it indeed was possible to isolate and identify the sex pheromone used by the commercial silkworm. Decoding this signal took over 30 years and the sacrifice of millions of female moths. It, nevertheless, gave impetus to other scientists to attempt the decoding of sex pheromones used by pest species.

Chemical Identifications

Although sex pheromones have been found to exist in species throughout the animal kingdom, efforts to decode the signals were concentrated in the Lepidoptera (moths) and the Coleoptera (beetles) because they comprise the majority of agricultural pests. Chemists at first extracted thousands of female moths to identify the sex pheromone components because of the extremely small amounts of active material in the oily extracts. The responding male's antennae are so sensitive and sharply attuned to their own signals that the female only needs to produce and emit less than a millionth of a gram of pheromone for long-distance attraction.

A monitoring trap in a New York apple orchard is checked for redbanded leaf-roller moths.

The specificity and sensitivity of these antenna] olfactory sensilla provided the basis for an excellent technique called the electroantennogram (EAG) for determining the active chemicals of a certain species by using a male's antenna of that species. This technique, coupled with advances in gas chromatography (GC), which is used to separate mixtures such as crude gland extracts into individual components, allowed chemists to identify the pheromone components of many of the major pest species. For example, by 1980 sex pheromones were identified for several hundred moth species.

Decoding of the chemical signals went through stages: a) with many species, only the most predominant component was identified; b) better GC technology allowed chemists to find specific ratios of geometrical and Positional isomers of the main component; c) sophisticated syntheses and analytical tools provided the means to define (+) and ( ) stereoisomers with components possessing chiral centers; and d) great advances in column technology for capillary GC allowed analysis of single pheromone glands to determine the presence of a number of pheromone components present in trace quantities.

Characterization of trace components has been aided greatly by the development of microtechniques and new computerized instruments interfaced to Fourier transform data systems. These advances have been important because it has become imperative to decode the entire chemical signal.