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Abstract

The larch bud moth (LBM) Zeiraphera diniana Guenée causes defoliation on larch in the Alps at 8- to 10-year intervals, after which populations crash. There are two LBM host races, one on larch and the other on cembran pine. These host races are morphologically indistinguishable as adults but they differ genetically in larval color types. Furthermore, females of each host race produce distinct pheromone blends and show oviposition preferences for their respective hosts. It is not clear to what extent host choice contributes to assortative mating in the LBM. Here, we compare the olfactory sensitivities of the two host races to the odors of fresh foliage of the host plants using the electroantennogram (EAG) technique, and the responses of the two host races to volatiles collected from the two host plants as analyzed by gas-chromatography-linked antennographic detection (GC-EAD). Both sexes of the larch and cembran host races show the same EAG responses to vapors of fresh larch and cembran pine foliage. Fifteen plant volatiles identified as chemostimuli by GC-EAD from larch and cembran pine odors elicited the same antennogram responses from the two host races. However, the GC-EAD analyses indicate that the number and quantity of chemostimuli emanating from each host plant is different. It is, therefore, most probably the array of olfactory receptors responding to the bouquet of volatiles unique to each host plant that underlies the host preferences of the two races. What remains open is the extent to which the similarity of the olfactory systems may contribute to cross-attraction. The fact that LBM individuals with intermediate characteristics between the two host races exist, suggests that olfactory perception does not hinder gene flow and contributes to sustained genetic diversity within the species Z. diniana.

The neural bases of host plant selection in a Neuroecology framework

“…The evolutionary processes that cause the diversification of herbivorous insects are not completely understood but host–plant interactions, and in particular plant chemistry, are thought to be a critical factor (Ehrlich and Raven, 1964 ; Jaenike, 1990 ; Whiteman and Jander, 2010 ). Plant volatiles contribute to sympatric speciation and reproductive isolation involving host plant shifts, such as those observed in races of the larch bud moth Zyraphera diniana having different host preferences (Emelianov et al, 2003 ; Syed et al, 2003 ), and in the apple maggot Rhagoletis pomonella (Linn et al, 2003 ; Olsson et al, 2006 ). Changes in host plant preferences can occur very fast, particularly in cases in which few genes participate in mediating host plant selections (Linn et al, 2003 ; Schoonhoven et al, 2005 ).…”