HILO — County, state and federal agencies are still seeking solutions to an invasive insect that threatens much of the Big Island’s livestock industry.
The two-lined spittle bug is originally native to the southern United States but was first discovered on the Big Island in 2016. Since then, the insects have affected hundreds of thousands of acres of cattle grazing land on the island, but no effective solution has been proven.
Franny Brewer, communications director for the Big Island Invasive Species Committee, said the insect was originally found in South Kona, but there are signs that populations of the bug are moving northward. This year, the insects were reported as far north as Puu Waawaa.
The two-lined spittle bug is attracted to high-nitrogen grass species, which are unfortunately the same kinds of grasses preferred by ranchers for grazing cattle, Brewer said. An unchecked infestation of the insects can reduce an entire pasture to dust, to be replaced by weeds that cattle cannot eat.
“The first sign is that it looks as if someone sprayed Roundup all over the grass,” Brewer said. “If you see a patch of grass that looks dead and you wonder, ‘Huh, why is that happening,’ you should check for spittle bugs.”
Like other species of spittle bugs on the island, two-lined spittle bugs encase their nymphs in a protective mass of foam — which resembles saliva, giving the insects their name — placed on a plant. Unlike other spittle bugs, the foam is located close to the ground, which is another indicator of infestation.
Brewer said the nymphs are less of a problem than the adults. The nymphs within the foam do little damage to the plant as they feed, but the adults suck nutrients from surrounding plants and can drain entire pastures of nutrients if the bug population gets too large. And, with no known predators on the island, and no mainland winters to keep their numbers in check, their population is growing.
So far, the bugs have maintained seasonal breeding patterns consistent with those of their mainland counterparts: they have two generational peaks in the late spring and late summer, going into a dormant state, or diapause, in the winter. But Brewer said the insects may adapt to the less severe weather of the island and begin breeding year-round, which will only worsen the situation.
Currently, there are only a handful of tools available to ranchers for managing spittle bug infestations, said Mark Thorne, pasture and livestock management specialist for the University of Hawaii at Manoa’s Tropical Pasture &Livestock Management Academy. However, each of those tools are imperfect or unproven.
Contact pesticides are ineffective against the nymphs, which are protected by foam and located too close to the ground to be consistently affected by standard spraying, Thorne said. While spraying pesticides can and will kill the adults, it is a wasted effort if the adults have already laid their eggs.
There is a systemic pesticide — a pesticide that is absorbed by the plant’s tissues — that can kill the spittle bug, but its use is restricted on the island. Thorne added that all pesticides are expensive at the scales necessary for treating pasture land.
Thorne said researchers’ best guess for how to deal with infestations is with a well-timed period of intensive grazing, although he emphasized that there is not enough data yet to determine whether such a strategy is effective. This strategy would have cattle graze a pasture down to just a few inches of grass, before the nymphs leave their protective foam.
In theory, the high grazing would alter the nymphs’ environment enough to reduce their survivability, Thorne said, while also trampling thousands of nymphs. With fewer nymphs surviving into adulthood, adult populations would theoretically not overwhelm a pasture, allowing the grasses to recover during the insects’ diapause during winter.
Of course, Thorne said, that practice will only cause the surviving insects to move to the next available pasture, so ranchers will have to continually shuffle cattle between pastures, taking care not to overgraze.
“So, the question is, how long can you keep that up?” Thorne said. “Because the bug’s not going anywhere. It’s here to stay, I’m sorry to say.”
Thorne said the high-intensity grazing strategy is not known to be effective yet and is contrary to how ranchers are recommended to manage their land. But, he added, ranchers are caught between a rock and a hard place.
“There’s the rub: if they don’t do something, they’ll lose the pasture anyway,” Thorne said. “If you don’t graze it, you lose it.”
Thorne said there are some species of grass that are being bred to resist spittle bug infestations, which show some promise. Until those are available on the Big Island, those who suspect a two-lined spittle bug infestation on their property should contact the Big Island Invasive Species Committee or the UH Manoa College of Tropical Agriculture and Human Resources.
