Variation in Attraction to Host Plant Odors in an Invasive Moth Has a Genetic Basis and is Genetically Negatively Correlated with Fecundity

Interpopulational Variations of Odorant-Binding Protein Expression in the Black Cutworm Moth, Agrotis ipsilon

Simple Summary

Odorant-binding proteins (OBPs) are small soluble transporter proteins that are believed to play a key role in insect olfaction. However, there is an emerging set of data that shows a role in insecticide resistance for similar families of binding proteins. The black cutworm Agrotis ipsilon is a migrant species of moth known to feed on multiple types of crops (polyphagous) worldwide. It is therefore likely that the olfactory system of this species can be modulated to adapt to different environments. We compared gene expression between American and European continental populations of the moth. We found continental-specific expression of antennal binding protein X (ABPX) and general odorant-binding protein 2 (GOBP2), suggesting a function of these proteins in migration, environment recognition, crop change and adaptation that are required for a polyphagous species such as A. ipsilon.

Abstract

A long-range migrant species of moth (Agrotis ipsilon) has served as a model to compare the expression profiles of antennal proteins between different continental populations. Our results showed that the American and French populations of the black cutworm moth, A. ipsilon, expressed the same odorant-binding proteins (OBPs), but apparently in different levels. Electrophoretic analysis of antennal protein profiles and reverse transcription polymerase chain reaction using RNA as a template showed significant differences between the two populations in the expression of antennal binding protein-X (ABPX) and general odorant-binding protein-2 (GOBP2). However, the two A. ipsilon populations showed no differences in RNA levels coding for pheromone binding proteins (PBPs), suggesting that the expression of generalist OBPs is population-specific and could be affected by specific odor and/or chemical changes in external environmental conditions. To support the role of ABPX and GOBP2 with expression, the role of ABPX and GOBP2 is discussed in regard to odor detection, memorization and/or degradation of toxic chemical insecticides.

Response profile of pheromone receptor neurons in male Grapholita molesta (Lepidoptera: Tortricidae)

The response profile of olfactory receptor neurons (ORNs) of male Grapholita molesta (Busck) to the three female sex pheromone components [(Z)-8-dodecenyl acetate (Z8-12:Ac), (E)-8-dodecenyl acetate (E8-12:Ac), and (Z)-8-dodecenyl alcohol (Z8-12:OH)] was tested with single sensillum electrophysiology. Sensilla trichodea housed normally one, but sometimes two or three ORNs with distinct action potential amplitudes. One third of the sensilla contacted contained ORNs that were unresponsive to any of the pheromone components tested. The remaining sensilla contained one ORN that responded either to the major pheromone component, Z8-12:Ac ("Z-cells", 63.7% of sensilla), or to its isomer E8-12:Ac ("E-cells", 7.4% of sensilla). 31% of Z- and E-sensilla had 1 or 2 additional cells, but these did not respond to pheromone. None of the 176 sensilla contacted hosted ORNs that responded to Z8-12:OH. The proportion of Z- and E-cells on the antennae (100:11.6, respectively) is similar to the proportion of these compounds in the blend (100:6, respectively). The response of Z-cells was very specific, whereas E-cells also responded to the Z isomer, albeit with lower sensitivity.

High genetic diversity and structured populations of the oriental fruit moth in its range of origin

The oriental fruit moth Grapholita ( = Cydia) molesta is a key fruit pest globally. Despite its economic importance, little is known about its population genetics in its putative native range that includes China. We used five polymorphic microsatellite loci and two mitochondrial gene sequences to characterize the population genetic diversity and genetic structure of G. molesta from nine sublocations in three regions of a major fruit growing area of China. Larval samples were collected throughout the season from peach, and in late season, after host switch by the moth to pome fruit, also from apple and pear. We found high numbers of microsatellite alleles and mitochondrial DNA haplotypes in all regions, together with a high number of private alleles and of haplotypes at all sublocations, providing strong evidence that the sampled area belongs to the origin of this species. Samples collected from peach at all sublocations were geographically structured, and a significant albeit weak pattern of isolation-by-distance was found among populations, likely reflecting the low flight capacity of this moth. Interestingly, populations sampled from apple and pear in the late season showed a structure differing from that of populations sampled from peach throughout the season, indicating a selective host switch of a certain part of the population only. The recently detected various olfactory genotypes in G. molesta may underly this selective host switch. These genetic data yield, for the first time, an understanding of population dynamics of G. molesta in its native range, and of a selective host switch from peach to pome fruit, which may have a broad applicability to other global fruit production areas for designing suitable pest management strategies.

Worldwide population genetic structure of the oriental fruit moth (Grapholita molesta), a globally invasive pest

Background

Invasive pest species have large impacts on agricultural crop yields, and understanding their population dynamics is important for ensuring food security. The oriental fruit moth Grapholita molesta is a cosmopolitan pest of stone and pome fruit species including peach and apple, and historical records indicate that it has invaded North and South America, Europe, Australia and Africa from its putative native range in Asia over the past century.

Results

We used 13 microsatellite loci, including nine newly developed markers, to characterize global population structure of G. molesta. Approximately 15 individuals from each of 26 globally distributed populations were genotyped. A weak but significant global pattern of isolation-by-distance was found, and G. molesta populations were geographically structured on a continental scale. Evidence does not support that G. molesta was introduced to North America from Japan as previously proposed. However, G. molesta was probably introduced from North America to The Azores, South Africa, and Brazil, and from East Asia to Australia. Shared ancestry was inferred between populations from Western Europe and from Brazil, although it remains unresolved whether an introduction occurred from Europe to Brazil, or vice versa. Both genetic diversity and levels of inbreeding were surprisingly high across the range of G. molesta and were not higher or lower overall in introduced areas compared to native areas. There is little evidence for multiple introductions to each continent (except in the case of South America), or for admixture between populations from different origins.

Conclusions

Cross-continental introductions of G. molesta appear to be infrequent, which is surprising given its rapid worldwide expansion over the past century. We suggest that area-wide spread via transport of fruits and other plant materials is a major mechanism of ongoing invasion, and management efforts should therefore target local and regional farming communities and distribution networks.

Season-long volatile emissions from peach and pear trees in situ, overlapping profiles, and olfactory attraction of an oligophagous fruit moth in the laboratory

Insect herbivores that have more than one generation per year and reproduce on different host plants are confronted with substantial seasonal variation in the volatile blends emitted by their hosts. One way to deal with such variation is to respond to a specific set of compounds common to all host plants. The oriental fruit moth Cydia (=Grapholita) molesta is a highly damaging invasive pest. The stone fruit peach (Prunus persica) is its primary host, whereas pome fruits such as pear (Pyrus communis) are considered secondary hosts. In some parts of their geographic range, moth populations switch from stone to pome fruit orchards during the growing season. Here, we tested whether this temporal switch is facilitated by female responses to plant volatiles. We collected volatiles from peach and pear trees in situ and characterized their seasonal dynamics by gas chromatography-mass spectrometry. We also assessed the effects of the natural volatile blends released by the two plant species on female attraction by using Y-tube olfactometry. Finally, we related variations in volatile emissions to female olfactory responses. Our results indicate that the seasonal host switch from peach to pear is facilitated by the changing olfactory effect of the natural volatile blends being emitted. Peach volatiles were only attractive early and mid season, whereas pear volatiles were attractive from mid to late season. Blends from the various attractive stages shared a common set of five aldehydes, which are suggested to play an essential role in female attraction to host plants. Particular attention should be given to these aldehydes when designing candidate attractants for oriental fruit moth females.