|Photo credit: Horror by Numbers/ Unsplash
According to a new study by researchers at Cornell University, New York, honeydew- a sugary sticky liquid secreted by aphids can promote the growth of the bacteria that is highly infectious to the pests.
During the study, researchers used Pseudomonas syringae, a type of bacteria that resides on leaves. Pseudomonas syringae is a rod-shaped bacteria that infect a wide range of plant species. It also affects certain infection carriers like Aphids that transfer the infection while feeding on the sap, damaging crops of billions of dollars around the world. The research shows how certain strains or genetic types of Pseudomonas syringae are infectious to aphids and might also be used to control pests.
The research paper, “Context-Dependent Benefits of Aphids for Bacteria in the Phyllosphere,” published in the journal ‘The American Naturalist,’ assessed the virulence (micro-organisms ability to damage the host) of different strains (genetic types) of the bacteria Pseudomonas syringae. The researchers also observed how well the bacteria can survive on leaves without aphids or how their presence affects the growth of the bacteria.
“For one of the experiments, we actually took the aphids out of the picture,” said Melanie Smee, the first author of the paper and a postdoctoral researcher at the lab of co-author Tory Hendry, assistant professor of microbiology in the College of Agriculture and Life Sciences.
Insights into the experiment
The researchers identified 21 different strains or genetic varieties of the bacteria. These strains caused the infection to aphids. Then, they chose 8 of those strains. For every strain, they sprayed bacteria on aphids containing plants. And on other plants having no aphids. They were trying to observe if the presence of aphids affected bacterial growth or not. They discovered that half of the strains increased the bacterial growth on the leaves.
“We literally just sprayed fake honeydew onto leaves, and we still saw the same increase for the bacteria, as when aphids were present, so it’s really just the honeydew that’s helping the bacteria,” said Smee.
The next step was to understand the cause of the growth. Researchers knew that aphids consumed the bacteria, then they grow and excrete out. So, they wanted to see what was inside the aphids was helping bacteria flourish. For this, they set up a new experiment, where they placed aphids above the leaves and sprayed them with bacteria. They fed one group of aphids with an artificial diet containing bacteria, and the other set without bacteria. Then, the aphids excreted the honeydew on the lower surface of the leaves.
In the presence of aphids, the bacterial population increased irrespective of the diet the aphids consumed. It was observed that honeydew was only the factor that boosted the bacterial population. The researchers also discovered that the advantages to bacteria from honeydew were not due to the strain traits. But, their growth was affected by their initial population densities. For example, a smaller bacterial population grew more in the presence of honeydew as compared to larger colonies that grew less.
The researchers need more work to understand the interaction between the bacterial colonies. Also, they want to know which Pseudomonas syringae strains are infectious to plants. Knowing this can help to control pests without the use of fertilizers, pesticides, or insecticides.