Invasive plants and their negative effects on biodiversity and landscapes are one of the greatest threats to ecosystems - considered second only to habitat destruction. Biological control is one of the few tools capable of controlling widespread plant invasions. However, despite years of successful testing in the laboratory, many biocontrol insects fail to reduce weed populations once they are released in the field. A better understanding of the interactions between biocontrol agents and their weedy hosts is needed to more accurately identify and focus on those insects that are most likely to be effective.
Because plant chemistry regulates plant-insect interactions (for example, by determining how much of the plant insects eat), we can learn much by considering chemical ecology. Most of what is known about plant-insect chemical ecology comes from studying agricultural pests, but this wealth of knowledge has yet to be applied to the biocontrol of weeds. Rocky Mountain Research Station scientists are studying chemical ecology regarding the biocontrol of weeds and discovering that biocontrol insects affect weed chemistry in very different ways. Invasive plant species currently under study include rush skeletonweed (Chondrilla juncea), houndstongue (Cynoglossum officinale), and tansy ragwort (Senecio jacobaea).
The researchers are finding that leaf- and root-chewing biocontrol insects trigger weeds to produce huge amounts of defensive compounds, such as toxic chemicals in leaves, whereas galling insects, which cause and live within abnormal plant growths, have little impact on weed chemistry. Producing these defensive compounds proves costly for the weed because it requires resources that would otherwise be used for growing and producing seeds. For example, in a study on the Gallatin National Forest in Montana, houndstongue plants were tricked into producing these chemicals in the absence of insects by applying a plant hormone. These plants were consequently smaller and thus may not have survived the winter as successfully as unaffected plants.
The importance of plant chemistry in successful biocontrol is becoming evident, and the emerging science on plant-insect chemical ecology should provide valuable information regarding what types of insects are most likely to impact weed populations - saving time and money and maximizing the success of this powerful management tool.