Elm breeding for resistance against bark beetles

Abstract

Dutch Elm Disease (DED) forms a complex system of three elements closely interacting: a host, a pathogen and an insect vector. Elm breeding programmes for resistance to DED have been exclusively based on selecting or obtaining pathogen-resistant trees, thus stressing only the host–pathogen side of the interaction and missing the vector component. Resistance against the bark beetle vectors would involve mechanisms other than those implicated in fungus resistance. As a consequence the search for resistance to the vectors would offer more than just a different way of obtaining resistant elms as it would allow incorporation of a new type of resistance into pathogen-resistant trees. Suppressing twig crotch feeding by elm bark beetles would greatly reduce infection on healthy elms. Evidence of preferences by elm bark beetles among different elm species has been documented in the field and in controlled experiments, demonstrating that U. minor is preferred over U. laevis and U. glabra, whereas intraspecific preferences among individual elms or clones remain largely unexplored. The selection of an individual plant by an insect is a two-step process that involves finding and accepting the host. Plant chemicals can play a key role in both of these steps, acting either as long-range stimuli, such as attractants or repellents, or as close-range contact cues, such as feeding stimulants and deterrents, operating after landing on the plant surface. Research on the chemical aspects of elm selection by bark beetles is here reviewed, and prospects for the future are discussed.