New estimate: Earth has 14 to 20 million insect species

Earth is truly a planet of insects. There are “thousands of billions” of ants crawling around, and there could be as many as 2,400 species of firefly alone. While those numbers may seem like a lot, a new study estimates up to 20 million insect species living on Earth, instead of six million as previously believed. Only a fraction of these species have even been discovered, according to a study published today in the journal Proceedings of the National Academy of Sciences (PNAS).

Insects are diverse for several reasons. Many species undergo metamorphoses during their life cycles, which means that they can live in different habitats based on their life stages. For instance, caterpillars eat plants earlier in life, but when they become butterflies or moths, they feed primarily on flower nectar. Additionally, since insects are mostly small, they can keep populations in more hidden and restricted areas that are harder for scientists to access.

The estimate that the planet is home to about six million insect species has stood for 40 years. Given that it is so difficult to sample these tiny organisms that are really good at hiding, getting an accurate count of insect species is a Herculean task.

“The core problem is that most insects are rare, and even with enormous samples you keep finding new species,” Robert Colwell, a study co-author and entomologist at Museum of Natural History at the University of Colorado and the University of Connecticut, tells Popular Science. “We collected over 1.6 million individuals from traps running for 69 trap-years in Costa Rica and still hadn’t captured everything living right there. The statistical challenge is estimating not just what you’ve seen, but how much you’re missing.”

To get around this problem, Colwell, University of Kentucky professor emeritus Michael Sharkey, and Cornell University entomologist and biodiversity scientist Laura Melissa Guzman looked deep on one well-studied subgroup of parasitic wasps called Microgastrinae. These wasps lay their eggs inside of caterpillars. When the eggs hatch, the larvae then eat the insides of the caterpillar, grow, and eventually emerge. The team studied the wasps found at the Área de Conservación Guanacaste (ACG) in northwestern Costa Rica.

They used the flying insects as a “measuring stick” for how incomplete their bigger sample was. Two of the team’s sampling methods used tent-like traps called Malaise traps, including a core set of traps and a peripheral set, and the third set that collected caterpillars and analyzed the wasp species that emerged from them.

The team used several statistical models to determine a ratio of the number of these wasps compared to how many are living undetected. They found that there may be eight to 14 million additional species.

“Most are almost certainly small, rare, and highly specialized,” Guzman tells Popular Science.

According to Guzman, 75 percent of the parasitic wasp species that the team found were captured by only one of our three sampling protocols even within the same forest. This indicates that these species have specific and narrow enough niches or behaviors that they can be missed by a single trap type.

“Many will probably turn out to be parasitoids, insects that complete their life cycle inside other insects, which means they’re embedded in food webs we don’t yet understand,” she explains. “Some may only exist in the forest canopy, which we barely sampled at all.”

A doubling or tripling of estimated insect species, which are already the most diverse group of animals, could have profound implications for understanding the scale, richness, and future of our planet’s biodiversity. Human activities are leading to global insect die offs, and a new estimate could help protect those that remain.

“We cannot protect species if we don’t know that they exist, and so to be able to understand the biodiversity on our planet, it’s important to know how many there are,” Guzman added.

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