“Eco-evolutionary theory and insect outbreaks”

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David J. Páez, Vanja Dukic, Jonathan Dushoff, Arietta Fleming-Davies, and Greg Dwyer

Evolutionary change helps drive ecological change in gypsy moth outbreaks

Darwin meets the gypsy moth

Gypsy moth larva eating an oak leaf.
(Credit: Dr. Alison Hunter)

Insect outbreaks devastate forests, killing trees and reducing the value of timber, but the destruction would be worse if outbreaks weren’t terminated by pathogen epidemics. Pathogens kill insects at very high rates, so if disease resistance is heritable, pathogen epidemics could lead to increased resistance through Darwinian evolution. This is important not just because resistance evolution could keep pathogens from controlling pests, but also because outbreaks occur over and over again in irregular but repeated cycles that might be partly due to evolution for increased resistance.

One way to understand how evolution affects pests and population cycles is to construct mathematical models. Models are cheap, and therefore plentiful, but useful applications of the models require estimates of the heritability of disease resistance—and of the extent to which more resistant insects lay fewer eggs, a cost that can prevent the evolution of perfect resistance, in turn preventing forest destruction, but ensuring repeated outbreaks. Estimating these parameters requires field experiments, which are impossible for buffalo and brucellosis, or for fruit bats and Ebola, but which are easy for the gypsy moth and its baculovirus pathogen.

David Páez and colleagues therefore used experiments to show that gypsy moths that are more resistant to the baculovirus have more resistant offspring but lay fewer eggs. When they added these effects to mathematical models, outbreaks kept happening, just as in nature, because evolution alternately favored higher resistance and higher eggs per insect, whereas non-evolutionary models show no outbreaks at all.

Lately, a fungal pathogen has been keeping gypsy moths in check in eastern North America, but the sensitivity of the fungus to climate change means that the virus is likely to make a comeback.

Tired of gypsy moth outbreaks? Darwinian evolution means that they are not going away. Read the Article