Chemical ecology of the Insecta, comprises an invisible environment wh
ere semiochemicals are the principal factors regulating the mating and
host-selection of the one million or more species. Biologically effec
tive concentrations of these semiochemicals range from 10(-10) to 10(-
3) mu g at the insect antennal receptors. These levels are virtually u
ndetectable by conventional microchemistry, and can be measured only b
y behavioral bioassay and by electrophysiology which are about 10,000
and 100-fold more sensitive than gas chromatography. Despite more than
40 years of study, the sex pheromones have been identified from only
about 1300 species of insects (0.1%). The dearth of information about
kairomones from host plants for phytophagous insects is even more asto
nishing, and only about 400 plant species have been studied extensivel
y (ca. 0.2%), and the odorant spectra characterized in only 10 importa
nt crop plants. These odorants are chemically complex and their action
as semiochemicals for insect herbivores, involves degrees of receptor
specificity and synergism that are virtually unstudied. These lacunae
in our knowledge of chemical ecology are fundamental to the study of
ecology, behavior, and evolution of insects; but also are of vital imp
ortance in applied entomology. There is much societal pressure to prog
ress from broadcast application of insecticides, to specific and innov
ative methods for insect control that are non-hazardous to human healt
h and to environmental quality. Pest management strategies involving t
he use of semiochemicals are essential to reach this goal. These inclu
de monitoring insect populations vis-a-vis the economic threshold, the
use of attracticide baits, and strategies for mating confusion and be
havioral confusion of insect pest populations in row and orchard crops
. The immensity of the semiochemical environments of plant and insect
communities, the intellectual challenges for understanding ecology and
evolution, and the immediate need for application to applied entomolo
gy provide compelling reasons for enhanced study of the microchemistry
of semiochemicals.