As the earth’s climate warms, pests will cause increased damage to agricultural crops. While one study predicted that crop yield lost to insects increases 10 to 25 percent for every 1 decree Celsius increase, researchers at East Lansing’s Michigan State University think these models are underestimating the losses.
A new study by the MSU researchers show that infested tomato plants don’t adapt well to rising temperatures as they work to fight off caterpillars. The two threats worsen the plants’ productivity. First, as temperatures rise, insect metabolism speeds up, causing insects to eat more. Warmer temperatures could also open a wider range of habitats to insects.
Second, infested plants don’t react as well to the heat as non-infested plants. This is what the current models ignore.
“We know that there are constraints that prevent plants from dealing with two stresses simultaneously,” says Gregg Howe, university distinguished professor at the MSU-DOE Plant Research Laboratory. “In this case, little is known about how plants cope with increased temperature and insect attack at the same time, so we wanted to try and fill that gap.”
Researchers grew tomato plants in hot growth chambers and released caterpillars into the chambers.
“I was shocked when I opened the doors to the growth chamber where the two sets of plants were growing at normal and high temperatures,” says Howe. “The caterpillars in the warmer space were much bigger; they had almost wiped the plant out.”
Plants have systems to deal with both bug attacks and excess heat. If a bug takes a bite out of a leaf, the plant produces a hormone called Jasmonate (JA) that tells the plant to produce defense compounds to thwart the bug. If temperatures are too high, plants lift their leaves away from the hot soil and open their stomata – similar to skin pores – so water can escape, evaporate, and cool the leaves.
“When temperatures are higher, a wounded tomato plant cranks out even more JA, leading to a stronger defense response,” says Nathan Havko, a postdoctoral researcher. “Somehow, that does not deter the caterpillars. Moreover, we found that JA blocks the plant’s ability to cool itself down. It can’t lift its leaves or sweat.”
The plants may close their pores to stop losing water from sites wounded by caterpillars, which causes them to suffer the equivalent of a heat stroke. Caterpillars also may do extra damage that keeps the leaf pores closed and leaf temperatures elevated, which will speed up the insects’ growth and development.
“We see photosynthesis, which is how crops produce biomass, is strongly impaired in these plants,” Havko says. “The resources to produce biomass are there, but somehow they aren’t used properly, and crop productivity decreases.”
While the researchers have more to look into, the study suggests that with global temperatures rising, plants may have too many defense mechanisms to juggle.
“I think we have yet to appreciate the unexpected tradeoffs between defense responses and plant productivity, especially when other types of environmental stress are present,” Howe says. “Turning on the defense response may do more harm than good if the plants face high temperatures or other stresses.”
The study was published in the journal Proceedings of the National Academy of Sciences of the United States of America and is available here.