Scientists have described – meaning they have named and characterized insects so others can identify them – about 1.2 million insect species so far. “We know there are many more to go, and one of the challenges is the more we sample, the more we discover,” Guzman said. “It’s a question of trying to estimate what is unobserved based on what we know.”

Insects are so diverse for a few reasons: Many undergo metamorphoses during their life cycles, which allows them to exploit different habitats based on their life stages. For example, caterpillars feed on plants earlier in life, and then when they become butterflies or moths, they feed primarily on flower nectar. Also, insects are mostly small, enabling them to maintain populations in very restricted areas. 

In the study, the team of researchers took advantage of intense insect sampling at the Área de Conservación Guanacaste (ACG) protected area, encompassing 169,000 hectares in northwestern Costa Rica. They first used three methods to conduct a deep sample of Microgastinae – an extremely diverse subfamily of small parasitoid wasps – in the ACG. The wasps lay their eggs inside caterpillars, and when hatched, the larvae consume the insides of the caterpillar, grow and eventually emerge. 

Two of the sampling methods involved tent-like traps called Malaise traps, including a core set of traps and a peripheral set, and the third involved collecting caterpillars and analyzing the wasp species that emerged from them. Fifteen core Malaise traps captured more than 1.6 million insect specimens, and all of these were barcoded – a technique where a small segment of DNA is sequenced to determine unique species. The core Malaise traps yielded a total of close to 54,000 insect species. In the end, the three sample sets yielded 1,414 Microgastrinae wasp species. 

The team used statistical techniques to determine a ratio of the number of Microgastrinae wasps detected in the ACG compared to how many more were potentially undetected. This ratio was then applied to the larger number of close to 54,000 total insects to determine the estimated true species count of all insects in the ACG, which equaled close to 333,000. 

To estimate how many global insect species there might be, they determined the ratio of estimated global tree species (around 73,000) compared to the estimated number of ACG tree species (1,200-1,500). They also calculated this ratio for mammals, amphibians and saturnid moths. By applying that tree ratio to the estimated 333,000 insect species in ACG, the research team ultimately estimated a range of total insect species globally of 14 million to 20 million. 

Recent reports have warned of human activities leading to a dramatic die-off of global insects, dubbed the “insect apocalypse.” The new global insect estimate may be a step toward protecting those that remain.   

“Our results point to a large number of undescribed insects, those without a name,” Guzman said. “With recent reports of insect declines, there could be many species that are declining that we haven’t even discovered.”  

Of the 15 study co-authors, Robert Colwell, distinguished research professor at the University of Connecticut and entomologist at the Museum of Natural History at the University of Colorado; Michael Sharkey, a professor emeritus at the University of Kentucky; and Guzman contributed equally to the work.

The study was supported by Canada’s New Frontiers in Research Fund, the Canada Foundation for Innovation’s Major Science Infrastructure program, the Walder Foundation of Chicago, the University of Southern California and Cornell.