Biological diversity tends to favour more positive outcomes in terms of the ecosystem services that humans depend on — and two pieces of research from a study group in Switzerland show that this is the case for biological control of insect pests, even when species are competing for the same resources.
The team at ETH Zurich have been exploring how the actions of different organisms, which all colonise and ultimately kill insects to survive, combine to control a range of insect pests.
In one laboratory study, they tested the performance of the bacterium Pseudomonas chlororaphis, the nematode Steinernema feltiae and the fungus Metarhizium brunneum, a combination previously identified by the team as a suitable ‘consortium’ of species, against both a leaf-eating pest, the caterpillar of the large white (Pieris brassicae) and the root-feeding banded cucumber beetle (Diabrotica balteata).
They found that in both settings, all the species can co-infect the same target pest. What’s more, the triple-pronged attack caused higher pest mortality and at a faster speed than would have been possible using one natural enemy in isolation.
However, the combination of the bacterium and the nematode was the particularly-damaging interaction against P. brassicae, while against D. balteata, it was the nematode and fungus that had the main effect.
After the insect died and decay advanced, bacteria became the dominant competitor — which may not be as much interest to growers as the fact that the combination did the job.
Different approaches, same results
In their report in the journal Microbial Ecology, the researchers, led by Anna Spescha and Maria Zwyssig, noted that the three biological controls have different ways of entering the bodies of soft-bodied insects, and produce different insecticidal and antimicrobial chemicals.
“We assume that the different infection pathways and modes of action of the consortium members contribute to the overall activity of the consortium against different insect pests,” they explained.
“An insect is more likely to succumb to infection and might do so faster when challenged with different physical damages and a larger variety of toxic compounds. Moreover, it is highly unlikely that an insect develops resistance against all three simultaneously.”
Success in the field
In a separate series of experiments, the scientists explored the triple combination against the cabbage maggot (Delia radicum), scaling up tests from the lab to potted radishes in greenhouses, and finally, field conditions.
They found that P. chlororaphis is effective in controlling the pest on its own, but also interacts with the fungus and nematode to amplify the effect. When all three were used together, maggot damage in the field was reduced by 50%.
“We propose that unreliable pest control obtained when applying single biological control agents may be overcome by the application of multiple [species],” they wrote in the journal Agriculture, Ecosystems and Environment.
“A successful consortium does not necessarily have to display pronounced synergisms, but should perform better under variable conditions. In case the performance of an individual biological control agent is hampered by adverse environmental conditions, the other consortium members could compensate and provide effective control.”
