Ph.D. student Aaron Dickey checking leaves of a pecan tree for insects at a Brazos County location.
Phylloxera devastatrix found in a gall of a water hickory tree. Click image to watch the video. Video by the Medina Lab.
Researcher’s Study of Aphids In Texas Hickory, Pecan Trees Key to Biological Control
Sometimes scientists have to start with simple to explain complex problems. Developing models is important in shaping our understanding of what happens in nature.
A project in Dr. Raul Medina's lab has been selected as a model system in studying how ecology affects the evolution and the generation of diversity of insects.
Medina's lab is studying a system which consists of two native plants, the pecan and water hickory, and their shared plant-feeding, predatory and parasitic insects that live in both trees. Pecans and water hickories span throughout Brazos, Burleson, Leon and Lee counties.
Studying the interaction of the insects with these two trees, the lab has documented several insect species that show genetic and phenotypic (what the insects look like) differentiation when found on these two different species of plants. This genetic and phenotypic differentiation is generally called host associated differentiation.
Host associated differentiation, or HAD, is the formation of genetically distinct lineages that occur in organisms of the same species that become reproductively isolated by feeding and living on two different host-plant species. For example, an individual aphid may be genetically different on the pecan tree than the same species of aphid on the hickory tree
For parasites, host-associated differentiation of populations is a major mechanism by which specialization might lead to evolutionary diversification. The parasite on the aphid on the pecan tree will be a different species than the parasite of the aphid on the hickory tree.
However, most conceptions of the process of HAD have been limited to a bi-trophic perspective. That is, it has been assumed that the interaction between the parasite and the host plant solely determines the process of differentiation. The purpose of this project was to present HAD in a community context. Medina said they believe the complex interactions between plants, herbivores, and carnivores may be generally important for understanding HAD.
A growing number of studies have shownthat variation in herbivore (insects that eat plants) performance and fitness on particular host plants is highly dependent on interactions with natural enemies. Carnivores, such as parasitoids, depend on plant-herbivore interactions for critical processes such as host location.
Recent studies provide evidence that morphologically cryptic HAD is more frequent than previously thought in plant feeding and parasitic insects. In the sympatric Solidago altissima (Canada goldenrod) and Solidago gigantea (Giant goldenrod) system, HAD has been shown to occur in a diversity of herbivorous and carnivorous insects, suggesting HAD may be an important generator of insect diversity.
Morphologically cryptic HAD in predators and parasitoids has been thought to be crucial in determining the success or failure of biocontrol programs. The presence of host-plant specialized, morphologically cryptic lineages of insects has important implications for ecological research in several key areas. These include fundamental topics such as the macroecological patterns of biodiversity, the roles of plant-related natural selection and ecological speciation in the diversification of insects, the patterns and process of ecological specialization, and the ecological structure of plant and insect communities.
Pecan and water hickory are closely related (both belong to the genus Carya), native trees, which shared a lot of the same insect herbivores between them (so far the trees share 29 species). Several of the most abundant herbivores on these two tree species are parthenogenetic (a type of asexual reproduction) aphids that reproduce clonally. Parthenogenesis has been thought to promote rapid evolution.
Through analysis of DNA fingerprints, the Medina lab has found that 4 out of 6 aphid species are genetically distinct when feeding on pecan and water hickory. Furthermore, evidence from work the lab has completed with the yellow pecan aphid, shows that selection against migrants that end up in the wrong host-plant species occurs. This promotes host-plant fidelity and prevents reproduction among aphids associated with different host-plant species.
Medina anticipates that the pecan-water hickory system will be an important component in the study of the evolution of plant-feeding insects and their natural enemies. The differentiation in these organisms in the model would be key in determining the effectiveness of biological control programs.
These host-associated differentiations, he said, could then help improve programs that require biological control of insects by fine tuning the use of natural enemies of pests. This will in turn allow more effective control of insect pests.