Imagine you’re standing in a sun-drenched field full of lettuce plants. There’s a gentle breeze and a smattering of tiny insects flitting about. It’s a pleasant scene, right?
Now let’s say one corner of the plot is contaminated with the deadly human food-borne pathogen Salmonella enterica, due to dirty irrigation water.
Could the insects, which hop from leaf to leaf feeding on plant sugars, play a role in spreading the contamination further afield?
“As insects feed and wander around on contaminated plant surfaces, it’s possible that they pick up Salmonella, so we decided to ask if they are playing a role in food safety,” says Jeri Barak, a CALS professor of plant pathology.
It’s an important question. Salmonellosis is one of the leading causes of acute bacterial gastroenteritis in the United States, responsible for an estimated 1.4 million illnesses, 15,000 hospitalizations and 400 deaths annually. In recent years, there have been more salmonellosis illnesses linked to fresh produce than to animal products. Yet very little is understood about produce-associated outbreaks, including basic information about how human pathogens survive and spread on plants.
To test whether plant-eating insects play a role, Barak teamed up with CALS entomology professor Russell Groves to design experiments assessing the ability of two common crop insect pests—the aster leafhopper and the green peach aphid—to pick up and spread Salmonella.
“Because we work with human pathogens in my lab, we never get to do research outside, so it was important to have someone like Russ Groves on board,” notes Barak. “He’s a UW–Extension entomologist, so he has a lot of experience in the field, and he pushed us to be as practical as possible.”
In one set of experiments, insects were given a piece of Salmonella-contaminated leaf material to munch on for 24 hours. In another, they spent the day drinking Salmonella-laced sugar water through a protective barrier that prevented physical contact between Salmonella and insect. In both setups, the insects were then transferred into a series of clean environments over the next 48 hours to see if—and how long—the contamination lasted.
In both cases, the insects readily picked up Salmonella, and once contaminated, most stayed contaminated.
Those that ate tainted leaves became contaminated inside and out, harboring Salmonella in their guts as well as on their feet and antennae, and when they were transferred to clean tubes, they spread the bacteria to fresh leaf material.
For insects that drank the Salmonella-laced sugar water, the bacteria got into their guts—and also found a way out.
“They excreted the Salmonella through honeydew—that’s a nice word for insect poop. Even after 48 hours, they were still pooping it out,” says Barak.
Honeydew, they also found, serves as a nutrient-rich fertilizer that helps Salmonella grow on plant surfaces that would otherwise be inhospitable.
On the practical side, notes Barak, farmers can now add these insects to the list of risk factors they consider when making crop management decisions.
“Now when a raw-produce grower looks out and it’s been a bad year for insect infestation, it might signal to them that they may have a higher food safety risk,” she says.