By Jernej Polajnar
To American readers, the American grapevine leafhopper (Scaphoideus titanus) is hardly known, even though it lives inconspicuously in a large part of North America, particularly in open woodlands of the eastern U.S. and Canada. It feeds on wild grapevines (genus Vitis) from which it occasionally moves to cultivated vineyards, but is too rare to be noticed.
In Europe, it’s a different story. The leafhoppers arrived here about 150 years ago, probably hitching a ride with American rootstock material that was exported to France to combat the Great French Wine Blight of the 1850s. Since then they have spread throughout many wine-growing regions of Europe, from the Atlantic to the Black Sea.
The root of the problem is actually another blind passenger, a phytoplasma bacterium that is spread by the leafhopper and causes the debilitating grapevine disease Flavescence dorée. The epidemic scale of problems caused by Flavescence dorée mandates multiple pesticide treatments per year against it the leafhoppers in affected areas, which goes against the long-term goal of the European Union to reduce pesticide use in order to protect the environment and human health.
No efficient alternative treatment against the newcomer is yet available. One of the reasons is the vector’s biology — leafhoppers do not use pheromones to find mates, so pheromone traps that proved extremely effective against many moth pests are utterly useless. But an international team of researchers from Italy and Slovenia, led by Dr. Valerio Mazzoni, have offered an odd source of inspiration, focusing on the little-known topic of vibrational communication in insects. Leafhoppers are unfazed by the bodily odors of prospective mates, and instead use a complex repertoire of vibrational signals that propagate through the plants on which they perch and are detected by other animals that happen to be nearby.
Previously, the same team researched in detail the courtship sequence of the American grapevine leafhopper and the various signals used in different steps of this sequence, up until mating occurred. They found that males induce slight trembles in the plant surface to advertise their presence to nearby females, and they locate the females by their responses. A particularly interesting finding was that rival males produce a sort of vibrational noise to interfere with the communication of a couple in order to gain the opportunity to mate with the female themselves.
This finding led to a key question: Can we record and replay this noise to prevent all of the hoppers in a vineyard from finding mates?
Unlike those found in North America, the American grapevine leafhopper is monophagous in Europe. It feeds exclusively on cultivated grapevines, so it cannot spread to other hosts for mating if it is disrupted in vineyards. And if individuals cannot mate, the population should disappear after a few seasons.
Preliminary laboratory trials showed promise, completely preventing pairs from mating on a small grapevine cutting. But transferring an idea to the field is a more complicated issue. In a paper published in the Journal of Pest Science, the researchers report the outcome of a small-scale field study conducted in northern Italy that was designed as a second step in developing a practical solution.
Luckily, distributing vibrations to all of the plants that need protection is not a big problem. Wine growers usually plant grapevines in rows that are trained by metal wires that can serve as convenient conduits for vibrational energy. So a wire in each row was equipped with a specialized “shaker” that played pre-recorded rival noise.
Pairs of leafhoppers were released in cages wrapped around grapevine shoots and left for 24 hours, then recaptured and checked whether they had mated. Noise playback prevented mating in these conditions too, leaving as many as 90 percent of the females virgin, while only 20 percent remained virgin when the noise was not played.
Noise of course gets fainter the further one is from its source, so it ceases to bother the insects at some point. This cutoff value — the threshold — was determined by playback experiments in the laboratory where intensity could be more precisely controlled. Then the transmission along the wire and down the shoots was measured and, as expected, couples in cages far enough from the source — in this case, 10 meters from the shaker or more — were able to mate as if there was no disruption.
The second significant result of the study was that disruption can be switched off between 10:00 in the morning and 6:00 in the evening without losing efficiency because American grapevine leafhoppers are not active during this part of the day.
Together, these findings mean that the shakers would need to be installed every 20 meters along vineyard rows, but it would be possible to conserve energy by switching them off for a few hours around noon without losing efficiency. Of course, transmission can be optimized to increase each shaker’s range, but this is only one of the technical issues that need to be resolved before the method can compete with pesticide treatments in cost, convenience, and efficiency. For now, the new study shows that this approach is promising enough to be tested on a large scale and developed further. But more broadly, it shows that even research on basic topics of insect biology, however obscure they might seem, may be found to be useful.
Jernej Polajnar is a biologist from Slovenia, specializing in reproductive behavior and vibrational communication of insects. He just returned from a postdoc in Italy, where he worked on mating disruption of S. titanus, and is now an assistant at the National Institute of Biology. A geek by heart, he spends his free time editing Wikipedia and devouring science fiction. You can follow him on Twitter at @JPolajnar (sporadically) and at ResearchGate.