Mites in the genus Tropilaelaps (such as T. clareae shown here) could follow in the footsteps of Varroa mites as significant threats to honey bees if they were to expand their range. A new test using analysis of the “melt curves” of DNA samples can discern the four known Tropilaelaps species and could be an important tool in surveillance for the mite pests in apicultural settings. (Photo by Pest and Diseases Image Library, Bugwood.org)

By Andrew Porterfield

Honeybees on nearly every continent face threats from parasites. In North America and Europe, the mite Varroa destructor has been well-studied as a problem for the European honeybee (Apis mellifera). Over the past few decades, another variety of mites, those in the genus Tropilaelaps, have plagued bees in tropical and subtropical Asia, and have been moving out of their native range.

Andrew Porterfield

The four known species of Tropilaelaps—clareae, koenigerum, mercedesae and thaii—were described as early as 1961, as parasites of bees Apis dorsata and Apis laboriosa, mainly in the Philippines, Thailand, Vietnam, China, as well as in India, Sri Lanka, Afghanistan, and Pakistan. The mite also has invaded European honeybee hives where their habitats overlap, feeding on developing bees and killing them. They are considered a major economic threat to the beekeeping industry, and European and U.S. authorities are also preparing for an eventual invasive threat from the pest.

Of the four mite species, two—T. clareae and T. mercedesae—infest Apis mellifera hives. The other two have only been seen impacting A. dorsata and A. laboriosa. Therefore, a precise way to identify the mites specifically responsible for invading A. mellifera hives is needed. Current identification methods consist of cruder techniques like the “bump” method, which can detect presence of mites (but not species), or morphological examination, which is more precise for species identification but is laborious and requires expertise in mite examination.

A more precise identification is important because only two species have been shown to harm A. mellifera, while the Asian honey bees have developed relatively effective defenses to reduce severe infestations of Tropilaelaps, defenses that European honeybees lack.

Mites in the genus Tropilaelaps (such as T. mercedesae shown here) could follow in the footsteps of Varroa mites as significant threats to honey bees if they were to expand their range. A new test using analysis of the “melt curves” of DNA samples can discern the four known Tropilaelaps species and could be an important tool in surveillance for the mite pests in apicultural settings. (Photo by Pest and Diseases Image Library, Bugwood.org)

To find this rapid, precise identification method, scientists with the honey bee pathology unit at ANSES, the French Agency for Food, Environmental and Occupational Health & Safety, turned to the polymerase chain reaction (PCR), a trusted method for the precise identification of organisms based on their DNA sequence. The researchers used a high-resolution melting analysis (HRM) on the PCR-amplified DNA to find a unique molecular signature for each species of mite. The researchers found that the PCR/HRM test could rapidly and easily identify each of the four species of mite, introducing a new method for identifying them. Their results—the first to show a PCR/HRM test for a bee mite genome—were published in January in the Journal of Economic Entomology.

Real-time PCR testing produces a “melt curve,” a graph that measures the temperature at which the two strands of double-stranded DNA denatures, or “melts” apart, during the test. Very often, these “melt curves” are specific to a species and can differentiate small variations between closely related organisms, like Tropilaelaps.

Mites in the genus Tropilaelaps could follow in the footsteps of Varroa mites as significant threats to honey bees if they were to expand their range. A new test using analysis of the “melt curves” of DNA samples can discern the four known Tropilaelaps species and could be an important tool in surveillance for the mite pests in apicultural settings. A technique known as high-resolution melting analysis can be used on DNA samples amplified via polymerase chain reaction. The analysis measures the temperature at which the two strands of double-stranded DNA denatures, or “melts” apart, during the test. The charts shown here display the melt curves for the four Tropilaelaps species analyzed in a new study by researchers at ANSES, the French Agency for Food, Environmental and Occupational Health & Safety. (Image originally published in Del Cont et al 2021, Journal of Economic Entomology)

The researchers focused on regions of a gene coding for an enzyme common in animals, the cytochrome c oxidase subunit I (COI), which is often used for DNA amplification in PCR tests and classifying animal species.

Their research showed that four distinct melt curves could be determined for the four mite species. The method was also shown to be sensitive in small samples and specific to the four mites. The specific test did not produce results when used against other mite species, including Varroa destructor.

“Rapid detection and identification of the Tropilaelaps mite can contribute to improved surveillance and monitoring in countries free of this parasite, such as in Europe,” the authors write in their report. “Like the Varroa destructor mite, Tropilaelaps could also become a major threat to A. mellifera colonies in Europe and could cause even greater damage.”

“A distribution study of this parasite shows that T. mercedesae is widespread and could become an important pest in future years. However, despite a lower spread of the three other species, the importance of their surveillance on A. mellifera remains essential,” the researchers add, noting that T. koenigerum was reported in a A. mellifera hive in Thailand. “The potential adaptation along with climate change and importations explains why Tropilaelaps mites are considered an important emerging threat in countries currently free of these parasites.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.