Physico-Chemical Aspects of the Effects of Certain Phytochemicals on Insect Gustation

Abstract

The chemical bases for host tree acceptance and non-host-tree rejection by the smaller European elm bark beetle, Scolytus muUistriatus, were studied. Chemical messengers were shown to interact with chemosensory neurons in the insect's antennae through formation of an energy-transfer complex. Oxidation-reduction interactions also may occur. Hydroquinone is the most potent known stimulant for S. muUistriatus feeding, and p-benzoquinone is inhibitory to feeding. Thus, the difference between stimulation and inhibition of feeding is the loss or gain of one or more electrons. Relative feeding inhibition among a group of variously substituted 1,4-naphthoquinones was positively correlated with the combined redox potential and intra-molecular hydrogen bonding capabilities of each molecule. This group of 1,4-naphthoquinones showed the same relative order of feeding inhibition in Periplaneta americana as in the elm bark beetle. A receptor protein for quinone feeding inhibitors was isolated from nerve membranes in P. americana an-tennae. This receptor protein changes shape in response w messengers. Its change in shape is attributable to the making or breaking of disulfide bridges. Stimulatory quinols promote the reduction of disulfides; whereas the inhibitory quinones oxidize sulfhydryls of cysteines and promote the formation of disulfides. This folding and unfolding of the membrane protein in response to stimuli regulate the cationic permeabil-ities of the dondritic membranes of the sensory neuron. Our total findings allow us to present a functional energy-transfer mechanism for this chemoreception.