In this post Gina Angelella discusses the recent paper from Sanford Eigenbrode and colleagues ‘Host-adapted aphid populations differ in their migratory patterns and capacity to colonize crops

When encountering a migratory insect such as a winged aphid, how confidently can one predict its origins and threat to crops? It is tricky enough to track the dispersal of a homogeneous species, but the addition of population-level variation (such as caused by visually-identical biotypes or host-races) makes such predictions extremely difficult. In fact, despite substantial documentation of dispersal behavior and host breadth variation, biotypes and host races within species, it’s notable how underutilized this knowledge is in pest management. Without the means to distinguish among distinct groups and populations, reliance on overall species counts may be masking population-level differences in traits. The ability to distinguish differences in key traits like dispersal behavior and host breadth among populations may significantly impact such predictions and management decisions.

A recent study by Sanford Eigenbrode and colleagues at the University of Idaho and the University of Maryland illustrates how effectively molecular techniques can bring visually unapparent, but consequential, trait variation to light in a regional crop-pest system. Specifically, they use molecular markers to characterize and classify individual pest genotypes into closely related groups. Molecular markers are DNA sequences at specific locations on a species’ chromosome which vary among individuals. Changes in these sequences among individuals—caused by mutation, sexual recombination, etc.—accumulate within populations, enabling inferred population genetic structure. In this case, the focal pest is an aphid with the ability to reproduce clonally (through live birth, no less), and the clones share an identical series of molecular marker sequences, or genotypes.

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Plant specialization among populations of the migratory pea aphid (Acyrthosiphon pisum) generates varying crop damage potential. Characterization of dispersal and traits at the population level could improve management in other migratory species. Photo by Bradley Stokes.

The three-year study tooks place in the Columbia River Basin and Palouse regions of the U.S. Pacific Northwest, where annual pea aphid (Acyrthosiphon pisum) migrations threaten pulse crop production with feeding damage and aphid-vectored plant viruses. The existing risk management and forecasting system in place presumes the aphids migrate from genetically invariable populations within the Columbia River Basin into the Palouse to colonize crops. Yet the pea aphid has well-documented accounts of host specificity in North America and Europe, whereby populations are partially reproductively isolated by adaptations to a specific host or hosts. These host-associated pea aphid populations have divergent traits affecting performance and habitat choice, making them specialists on certain legume species (e.g., Via 1991a,b). Eigenbrode and colleagues thus test the aforementioned presumptions by sampling putative aphid source populations from various legumes in the Columbia River Basin (alfalfa, vetch, red and crimson clover, and pea), and those migrating and settling in pea fields in the Palouse region. They use molecular markers to compare genotypes among aphids sampled and determine both which source population genotypes match those of migrant aphids landing in the pea fields, and which genotypes characterize successful aphids colonizing the peas.

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Bean leaf roll virus (BLRV) symptoms in pea. Pea aphids migrating from multiple legume hosts (including vetch, clover, alfalfa, and pea) are possible vectors. Photo by Bradley Stokes.

They found that although a diversity of migrant genotypes landed in Palouse pea field traps—genotypes matching populations on alfalfa, vetch, and pea in the Columbia River Basin—only one pea-associated genotype colonized abundantly every year. What’s more, lab tests of pea aphids with these plant host-associated genotypes revealed performance tradeoffs on nonhost plants. This suggests that far from being invariable, pea aphids migrating from the Columbia River Basin are host-adapted, and that a single pea-adapted genotype is primarily responsible for colonizing Palouse peas. The results are especially important considering pea aphids spread several potentially devastating viruses to peas, but that these viruses are not all found in the same legumes. Insects migrating from susceptible plants are much more likely to spread infections in the crops they colonize, and therefore more important targets for management. The potential benefits of characterizing host-adapted populations in other crop-vector-virus systems are clear.

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Pea enation mosaic virus (PEMV) symptoms in pea. Pea aphid populations migrating from clover, vetch, and pea are potential vectors; those on alfalfa, which is not a PEMV host, are unlikely virus vectors. Photo by Bradley Stokes.

The utility of molecular markers (and other similar tools) need not be limited to detecting host-adapted populations in migratory pests. Studies have found intraspecific variation in plant pathogen transmission efficiency by vector insects (e.g., Bemisia tabaci biotypes [McGrath & Harrison 1995]). There can likewise be varying degrees of resistance to insecticides among groups (e.g., Bemisia tabaci biotypes [Karunker et al. 2008]), requiring alternative management approaches. The technique can also reveal characteristic dispersal patterns useful for forecast models, as Eigenbrode and colleagues demonstrate. (Early-season migratory pea aphids in the Palouse likely originated from within a 20 km range.) Intraspecific variations in dispersal patterns are equally possible (e.g., fall armyworm, Spodoptera frugiperda [Nagoshi et al. 2012]; sugarcane planthopper, Eumetopina flavipes [Anderson & Congdon 2013]). Linking characteristic population genotypes with consequential traits and enhancing the resolution of dispersal patterns could enable major improvements in pest management.

Eigenbrode and colleagues suggest this technique could be used to create customized thresholds based on the primary genetic groups in trap catch. As with most relatively new technology, molecular marker development and genotyping costs need to be weighed before its adoption for widespread usage; nevertheless, this study illustrates that characterization of regional movement is readily approachable. Perhaps personalized characterization of pest populations will be the way of the future?

References

Anderson, K.L., Congdon, B.C. (2013) Population genetics suggest that multiple invasion processes need to be addressed in the management plan of a plant disease vector. Evolutionary Applications 6, 660-672

Eigenbrode, S.D., Davis, T.S., Adams, J.R., Husebye, D., Waits, L.P., Hawthorne, D. (In press) Not all herbivores are created equal: host-adapted aphid populations differ in their migratory patterns and capacity to colonize crops. Journal of Applied Ecology

Karunker, I., Benting, J., Lueke, B., Ponge, T., Nauen, R., Roditakis, E., Vontas, J., Gorman, K., Denholm, I., Morin, S. (2008) Over-expression of cytochrome P450 CYP6CM1 is associated with high resistance to imidacloprid in the B and Q biotypes of Bemisia tabaci (Hemiptera: Aleyrordidae). Insect Biochemistry and Molecular Biology 38, 634-644

McGrath, P.F., Harrison, B.D. (1995) Transmission of tomato leaf curl geminiviruses by Bemisia tabaci: effects of virus isolate and vector biotype. Annals of Applied Biology 126, 307-316

Nagoshi, R.N., Meagher, R.L., Hay-Roe, M. (2012) Inferring the annual migration patterns of fall armyworm (Lepidoptera: Noctuidae) in the United States from mitochondrial haplotypes. Ecology and Evolution 2, 1458-1467

Via, S. (1991a) The genetic structure of host plant adaptation in a spatial patchwork: demographic variability among reciprocally transplanted pea aphid clones. Evolution 45, 827-852

Via, S. (1991b) Specialized host plant performance of pea aphid clones is not altered by experience. Ecology 72, 1420-1427

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