Insect species estimate triples to 20 million

Earth may be home to as many as 20 million insect species, more than triple the previous estimate of about 6 million, according to a new study published Monday in the Proceedings of the National Academy of Sciences. The work challenges a long-held benchmark used by taxonomists and suggests that roughly 1.5 million species have been described so far — leaving the vast majority unknown.

The team arrived at the total by combining decades of fieldwork, genetic analysis, and a statistical method borrowed from disease tracking.

To get to their number, the researchers leaned heavily on one insect group: parasitoid wasps, which lay eggs inside other creatures. The larvae hatch and burst out in a grisly but ecologically important process.

Overlap between projects was small.

The group needed a way to estimate hidden diversity. They turned to a method developed by coauthor Anne Chao, a statistician who had used similar logic to estimate the true size of a hepatitis B outbreak in Taiwan. In 2015, she compared cases reported by doctors, local hospitals, and student questionnaires to see where they overlapped — and where they did not. The same technique let the team ballpark the true number of parasitoid wasp species in Costa Rica’s Área de Conservación Guanacaste at about 3,400.

They then applied the same ratio to other insects collected in the Malaise traps. The traps held more than 1.6 million bugs. Using a genetic technique called DNA barcoding, they identified nearly 54,000 species among them. Scaling up based on the wasp data, they estimated the park contains about 333,000 insect species.

To go global, the study authors used trees as a guide.

Ecologists have long known that biodiversity rises toward the tropics, a pattern called latitudinal richness. If you know how many tree species live in a tropical forest and how many exist worldwide, you can calculate an “upscaling factor.” They used a global tree survey showing roughly 1,200 to 1,500 tree species in the Costa Rican park and about 73,300 worldwide. Applying that factor to the local insect count gave a global total of about 20 million.

Nigel Stork, an entomologist at Griffith University in Australia who worked on earlier, lower estimates, called the new work “amazing.” He told the study’s authors it was a major advance.

The calculation relies on many assumptions, and the study authors acknowledge the true number could be lower or higher. But they argue it is the best current approximation.

Guzman pointed out that many of those 20 million species likely have remarkable adaptations that scientists have yet to document. With biodiversity under pressure from habitat loss and climate change, the estimate offers a baseline. “It gives us a baseline of how much we stand to lose,” she said.