Spittlebug Implicated in Spread of Leaf Scald in Sugarcane

January 24, 2024 Research News 0

Entomology Today

The spittlebug Mahanarva fimbriolata (adult shown here) is already a significant pest of sugarcane in Brazil for the direct damage it causes by feeding, but a new study shows the species may also transmit the bacteria that causes leaf scald disease. (Photo by Carlos Alexandre Mattos Raposo via iNaturalistCC BY-NC 4.0)

By Ed Ricciuti

Ed Ricciuti
Ed Ricciuti

It’s a case of good intentions toward the environment gone more than a bit wrong. Brazilian growers have stopped fouling the air with smoke by burning the leaves and tops of sugarcane, called “trash,” before the sugar-bearing stalk is harvested. Instead, they have been letting trash lie the field for mulch and compost.

But, by doing so, they appear to have created prime habitat for an insect pest that threatens their crops.

The spittlebug Mahanarva fimbriolata wreaks havoc upon sugarcane by injecting toxins that “burn” the leaves as it sucks sap from the plant xylem. A new study published in December 2023 in the Journal of Insect Science, however, shows that M. fimbriolata also spreads a devastating plant disease, leaf scald.

Caused by the bacterium Xanthomonas albilineans, leaf scald is often transmitted by previously infected cane-cutting tools, infected cuttings themselves, and, with heavy rains, through the air. Prior to this new study, there was no known insect vector of the leaf scale bacterium.

Given that sugarcane—for ethanol as well as sweetening coffee—brings Brazil more than $9 billion from exports annually, any potential threat to the crop is bad news. In 2017, the infection of hitherto disease-free sugarcane seedlings in a sugar mill, without exposure to cutting instruments, aroused the suspicions of Brazilian scientists. They hypothesized that the disease could be linked to the spittlebug, since both it and the bacterium abounded in cane fields. While no insect was known to transmit X. albilineans, other spittlebugs transmit other disease bacteria, such as Xylella fastidiosa, which can invade and clog the xylem, causing leaf scorch.

After the leaf scald cropped up mysteriously in the sugar mill, the team of researchers from the Agronomical Institute of Campinas, University of São Paulo, and São Paulo State University went into the fields, caught some spittlebugs and conducted laboratory tests at a molecular level to see if the little suckers—literally—could indeed pick up the bacteria by feeding and then transmit it.

The experiments showed they could. The researchers were able to trace transmission of X. albilineans DNA from sugarcane to spittlebugs and back again. They did so by using a probe, a stretch of labeled DNA that binds to a common sequence in the bacterium, enabling scientists to track its presence.

At first, the probing detected the bacterium in the spittlebug but the insect did not transmit it to sugarcane. After 72 hours had passed, however, researchers observed not only transmission to the plants but symptoms of leaf scald as well. All told, during the course of the experiment, at the end of a 96-hour period, 42 percent of the spittlebugs tested had acquired the bacterium, and 18 percent of the plants had been infected.

The time lag before appearance of the infection may be due to an adjustment in metabolism by the bacterium as it switches from insect to plant host, the researchers explain in their report. Some plants, they note, had higher amounts of bacterial infection and scald symptoms than others. The difference might be due to some individual bugs having a higher ability to transmit the bacterium than others.

Stressing the need for further studies, the researchers nevertheless note that the ability of M. fimbriolata to carry and transmit the leaf scale bacterium may be an important factor in the spread of the disease in Brazil. All the more reason to seek and improve methods for managing M. fimbriolata and its presence among sugarcane.

Read More

Transmission of Xanthomonas albilineans by the spittlebug, Mahanarva fimbriolata (Hemiptera: Cercopidae), in Brazil: first report of an insect vector for the causal agent of sugarcane leaf scald

Journal of Insect Science

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.