Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...

General Introduction 3
moth in locating the „calling‟ female over long distances. Lepidopteran sex
pheromones are mostly composed of blends of two or more compounds (major and
minor pheromone components) in specific ratios (Butenandt et al., 1961; Arn et al.,
1986). The ratios are species-specific and the pheromone may fail to elicit a response
in males if altered. Sex pheromones are mainly composed of C10 to C18 unsaturated,
straight chain hydrocarbons together with an oxygenated functional group, for
instance an alcohol, acetate ester or aldehyde (Arn et al., 1992; Jurenka, 2003). Sex
pheromone components in Lepidoptera are synthesised from fatty acids via enzymes
such as acetyl-CoA, followed by desaturation whereby double bonds are placed at
specific positions along the carbon chain. Limited chain shortening reactions then
shorten the carbon chain to the appropriate length and finally reductive modification
of the carbonyl carbon generates the functional group (Foster and Roelofs, 1987;
Foster and Dugdale, 1988; Jurenka, 2003).
1.3.2 Other odorants detected by moths
The extreme sensitivity and specificity of sex pheromone reception has made it the
ideal model for study of olfaction in moths. However, other odorants such as plant
volatiles including esters, aldehydes, alcohols, terpenes and carbon dioxide are also
detected by the moth‟s olfactory system (Kaissling, 1971). These odorants are used by
moths to locate host plants, oviposition sites and food sources. For example, linalool,
a plant volatile, is suggested to be used by B. mori, Heliothis virescens, Helicoverpa
armigera and Spodoptera littoralis in host plant location for laying eggs (Heinbockel
and Kaissling, 1996; Angioy et al., 2003; Røstelien et al., 2005; Anderson et al.,
2009). Herbivore-induced volatiles, which are released by plants in response to insect
feeding, result in the recruitment of beneficial insects that parasitise the infesting
insects. One such example is methyl salicylate, which is detected by moths such as
Mamestra brassica, S. littoralis, and Epiphyas postvittana as a warning that a plant is
already occupied and thus females avoid such plants for laying eggs (Suckling et al.,
1996; Jönsson and Anderson, 1999; Ulland et al., 2008). This is a survival mechanism
used by the moths in order to minimise competition once the larvae hatch. Eugenol,
geraniol and citral have also been shown to act as oviposition deterrents for E.
postvittana, while hexanal, linalool, nonanol, octanol and nonanal act as attractants.
Plant volatiles such as citral, nonanol, octanol and n-decylaldehyde have been shown