Meet the World’s Least Picky Insect—A Spreader of Deadly Crop Disease

By Elizabeth Pennisi

Less than a decade ago, olive groves in southern Italy began to die. The culprit? A bacterium, Xylella fastidiosa, spread by an inconspicuous, plant-feeding insect called the meadow spittlebug. An analysis of amateur photographs and other data, published this week in PLOS One, now shows these harbingers of disease are the insect kingdom’s least picky eaters, sucking the sap of more than 1300 plant species, almost twice as many as the insect with the next broadest diet. That could spell bad news for crop farmers. Although only some of these plants are likely to be susceptible to the bacterium, even those protected species could act as reservoirs for the microbe, making it difficult to protect olive trees and other vulnerable species.

The research “teaches us that the spread of Xylella throughout Europe could happen faster than initially believed,” says Claudia Castro, a plant pathologist at the United States Department of Agriculture’s Agricultural Research Service Plant Gene Expression Center who was not involved with the work.

“Spittlebug” is a term that refers to the immature stage of insects often called froghoppers. The bugs are so named because the urine these juveniles excrete forms protective saliva-like masses of tiny bubbles visible on plant stems. Most sap-sucking insects drill into a nutrient-dense plant tissue called phloem, but spittlebugs specialize in the much more dilute sap from another tissue, xylem. This sap differs little between plant species, so, researchers have long suspected the meadow spittlebug (Philaenus spumarius) and its relatives weren’t picky about which plants they fed on. Indeed, when Vinton Thompson, an entomologist at the American Museum of Natural History, began to build a database linking the 2500 spittlebug species to their host plants almost 50 years ago, he ignored the meadow spittlebug because he suspected it just had so many.

But once the spittlebug was discovered to spread Xylella bacterium in olive trees about a decade ago, he joined forces with ecologist Claire Harkin at the University of Sussex to see whether they could pin down the diet of these destructive insects. She helped him sort through the data from a research consortium called BRIGIT, which had recruited members of the U.K. public several years ago to photograph meadow spittlebugs or their spittle relatives on as many different plants as possible. Moreover, Thompson and his colleagues conducted new field studies and pored through often obscure museum records and little-known scientific papers.

Altogether, they found the meadow spittlebug is hosted by 1311 plants belonging to 631 genera and 117 families, particularly the daisy and rose families. But surprisingly, this species also enjoys ferns, grasses, and shrubs as well as trees, they and colleagues report. There is even a new Xylella outbreak in Spanish almond trees attributable to this spittlebug, Thompson says.