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News

Research: Increased Salt Stress Reduces Tomato Pest Activity

21/04/2025 - Sophie Colvine
Increased soil salinity can reduce damage from prominent tomato pests such as the tomato fruitworm, according to researchers at Penn State. They published their findings in the Journal of Plant, Cell and Environment.

The team, comprising entomologists from the College of Agricultural Sciences, found that increased salt content in tomato plants reduces fruitworm larvae — or caterpillar — feeding and limits the number of eggs that moths lay on these plants. However, the researchers also found that salt damages the plants in other ways.

Sahil Pawar, doctoral student in entomology and lead author on the study, said that while some people might wonder how a tomato might get saltier or less salty on a farm, salt is actually a common stressor in the world’s tomato fields.

“The top global tomato producers — like in India, Florida, California and some Mediterranean and African countries — have warm and arid climates with improper irrigation, which causes accumulation of salts in the soil,” Pawar said. “Many poorer countries also resort to irrigating crops with whatever water is available to them, and this causes further increases in soil salinity. All in all, if we want to ensure tomato yields are maintained in these environments, we should be studying the impacts of salinity on plant traits.”

He added that too much salt can have bad outcomes for these plants. In the current study, the salt-treated plants were smaller and visibly stressed. Pawar affirmed that salt stress even can reduce tomato yield at certain levels. But there might be a silver lining to this saltiness: as the salt levels increased, herbivore pest growth rates decreased.

In addition, salty plants sustained less insect feeding damage than normal plants when herbivores were given a choice between the two plants, and moths laid eggs on salty plants less frequently than on normal plants.

To test increased salt levels on plant traits and herbivore responses, the researchers watered tomato plants with either normal water or a salt solution for three days before experiments. They then analyzed plant traits, such as weight, chlorophyll content, nutritional quality, expression of defense genes and amount of defense proteins. They also used the plants to test tomato fruitworm larvae performance and preference.

Pawar said he was surprised that while larvae were inhibited by salty plants, defense proteins that plants often use to suppress herbivores weren’t more active in the salty conditions. To examine what was causing this reduction in feeding, the researchers also reared caterpillars on an artificial diet that was spiked with different concentrations of salt. High salt levels lowered caterpillar survival and growth, and if these caterpillars lived long enough to reach adulthood, moth wing development and ability to lay eggs were stunted. It seems that the salt itself, when at high levels, was toxic to the tomato fruitworm, the team noted.

The researchers found that increased salt also changed how the plants smell to these insects.
 
The study found that increased salt content in tomato plants reduces fruitworm caterpillar feeding and limits
the number of eggs that moths lay on these plants. Credit: Sahil Pawar.

The scent profile of the plants, which is a combination of many different chemical compounds, changed when excess salt was added to the plants. That profile change led to adult tomato fruitworm moths laying eggs less on these plants. Something about the salty plant scents signaled to the moths that these plants aren’t the best place for their offspring to live — which is great for farmers, Pawar said, as a drop in the number of tomato fruitworm eggs means fewer larvae available to eat their crops in the future.

But more salt also means the plants are more stressed and potentially will yield fewer tomatoes, prompting the researchers to wonder whether this salt, on balance, is beneficial.

Pawar said he thinks that perhaps an ideal amount of salt pressure exists, to keep herbivores away without reducing yield. In fact, this “sweet spot” of salt pressure might even make tomatoes tastier to consumers.

“There are other studies out there about how increased salinity makes smaller fruits and lowers water content, and thus it enhances tomato fruit taste by increasing both sugars and acids,” he said. “In addition, higher salinity enhances the toxicity of agricultural pesticides, further lowering pest pressure. So, by having an ‘ideal’ level of salt we could get smaller and sweeter tomatoes, get rid of insects as they would be affected by salinity itself and even use lower amounts of pesticides.”

Pawar said more research could help inform decisions about how low amounts of salt can be used in agricultural settings. He also said future research could focus more on these unseen players in tomato fields.

“There's a lot of focus on bioaccumulation of toxic metals in plants like mercury and lead, and how those compounds affect us,” Pawar said. “However, so-called ‘nontoxic’ metals like sodium and potassium also accumulate, and they affect smaller organisms than us. Investigating the mechanisms behind their effects, plus trying to find the ideal amount of salt we could have in our soils, is one of the best avenues for future research.”

Reference: 
Pawar, S.V., Paranjape, S.M., Kalowsky, G.K., Peiffer, M., McCartney, N., Ali, J.G. and Felton, G.W. (2025), Tomato Defenses Under Stress: The Impact of Salinity on Direct Defenses Against Insect Herbivores. Plant, Cell & Environment, 48: 3647-3659. https://doi.org/10.1111/pce.15353
Full text in free access here

Source: Penn State
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