Unraveling the Invasion Puzzle: Contrasting Success and Failure in Global Whitefly and Thrips Species

When an invasive insect arrives in a new region, genetic analysis for identification is a crucial step, especially as nuanced variations in populations often contribute to a species’ invasive potential. (Photos by Babu Panthi, Ph.D., University of Florida; Carl Barrentine, Bug Guide; Choi and Lee (2018); Dreamstime Images (204693333, 206675336, 232488730, 285200326, 338987666, 338987666, 338988018); Elijah Talamas, Ph.D., DPI-FDACS; Felipe Soto-Adames, Ph.D., DPI-FDACS; Lance Osborne, Ph.D., University of Florida; Lavender_gooms129 (Reddit); Lyle Buss, University of Florida; Shutterstock Images; Whitney Cranshaw, Ph.D., Colorado State University; Zee Ahmed, Ph.D., Clemson University.)

By Muhammad Z. “Zee” Ahmed, Ph.D.

To implement appropriate preventative measures against invasive species, it is necessary to understand their genetic identities, as well as their native and invasive ranges. This knowledge is essential for enforcing regulatory measures and effectively managing the spread of invasive species into new regions.

Over the past two decades, we have made strides through the use of molecular techniques using polymerase chain reactions (PCR), along with two broad databases for genetic data of invasive species—The Barcode of Life Data Systems (BOLD) and the National Center for Biotechnology Information. These new capabilities have enabled biologists to study the genetic diversity of invasive species and compare it with data from other parts of the world.

By doing this, we can confirm morphological identification of organisms, eliminate doubts about cryptic species, pinpoint haplotypes (genetic variants within species) responsible for invasions, help confirm native and invasive ranges by comparing global genetic diversity data, and trace these species’ invasion routes, particularly those that have recently spread globally.

In recent studies of invasion patterns of whiteflies and thrips, my colleagues and I have found that only a small portion of the genetic diversity (1 to 3 haplotypes) contributed to the global invasions of these two species. The invasive populations or genotypes represented by the predominant haplotypes may possess traits that facilitate their ability to successfully colonize new habitats.

Sweetpotato Whitefly

Our recent study, published in February in the Bulletin of Entomological Research, reveals an intriguing scenario: One of the most invasive global species of whitefly, the sweetpotato whitefly (Bemisia tabaci)—specifically the B biotype, or MEAM1 cryptic species—has recently invaded Australia, China, and the United States over the past three decades but has failed to invade India.

We studied the geographical distribution and genetic diversity of this species in these countries and found higher genetic diversity in India, similar to other countries considered part of its presumed native range (expanding from Asia to Middle Eastern countries, including Iran, Iraq, Israel, Egypt, Pakistan, and Syria), compared to countries in its invasive range (including Australia, China, and the United States).

This suggests that India is part of its native range, as higher genetic diversity is typically found in the native range during recent invasions. This is particularly interesting because several previous studies from other parts of the world have shown that the interaction of invading B. tabaci MEAM1 with indigenous species in Australia, China, and the U.S. has led to the displacement and often extinction of indigenous species. Thus, while the global experience of invasive B. tabaci MEAM1 populations clearly demonstrates their capacity to displace indigenous competitors, India remains an exception. This invasive haplotype of B. tabcai MEAM1 in India most probably came from its native range outside of India. In one of our previous studies published in PLOS One in 2011, we found that the Middle East, most probably Israel, was a possible source of this invasive haplotype of B. tabaci MEAM1.

In one of our other studies published in Pest Management Science in 2011, we found that B. tabaci MEAM1 in Pakistan was genetically different from the globally invasive variant and did not exhibit the same invasive behavior. Further research we published in Pest Management Science in 2013 comparing the biology of Pakistani B. tabaci MEAM1 with published studies revealed evidence strongly suggesting that the Pakistani variant is biologically inferior to the globally invasive one and lacks the same invasive potential. Our studies concluded that genetically different haplotypes within the same species actually have different biological characteristics and varying invasion potential.

However, this was not the case in India in our recent study, as the Indian B. tabaci MEAM1 was identical to the globally invasive variant but failed to invade in India. Natural enemies, especially parasitoid species, might have prevented its invasion, and India also has at least eight native B. tabaci cryptic species, unlike Australia, China, and the U.S., which might have helped prevent the invasion of B. tabaci MEAM1 in India.

This study was conducted in collaboration with researchers from the Punjab Agricultural University in Ludhiana, India; South China Agricultural University in Guangzhou, China; Chongqing Normal University in Chongqing, China; Chinese Academy of Agricultural Sciences in Beijing, China; University of Florida; United States Department of Agriculture; and Clemson University.

“The observed higher genetic diversity of MEAM1 in India, coupled with its lack of spread within the country as indicated by this study, provides valuable insights for developing more effective management strategies against this species,” says co-author Vikas Jindal, Ph.D., a scientist at Punjab Agricultural University.

Thrips parvispinus

In another similar study, published in November 2024 in the Journal of Applied Entomology, we investigated the geographical distribution and genetic diversity of another emerging invasive species, Thrips parvispinus, which recently invaded the U.S. Our geographical and genetic analysis provided evidence of high genetic diversity in India, suggesting that India is part of its native range. However, contrary to B. tabaci MEAM1, which failed to invade its native range (India), T. parvispinus has successfully invaded India.

The pattern here was different from that found in the case of B. tabaci MEAM1. The invasive species T. parvispinus invaded its native range in the same manner as it did outside of its native range. Another example within the same genus is T. palmi, which became a significant issue within its presumed native range due to an intensive crop production system—a situation seemingly applicable to T. parvispinus as well. Additionally, cases of insecticide resistance in T. parvispinus forced growers to use additional insecticides against it, which might have killed its natural enemies and contributed to its successful invasion within its native range. Although our study suggests that India is part of the native range of T. parvispinus—since at least 14 out of the 18 haplotypes identified were reported from India, indicating higher diversity—there is still a possibility that the invasive haplotype of T. parvispinus (Hap1) might have been introduced to India from another part of its native range outside of India. Similarly, our study in PLOS One in 2011 suggested that the invasive B. tabaci MEAM1 was likely introduced to India from another part of its native range, most probably from the Middle East. Still, it failed to establish itself in India, unlike T. parvispinus (Hap1).

This study was conducted in collaboration with Lance Osborne, Ph.D., and John Roberts, Ph.D., from the University of Florida, Felipe Soto-Adames, Ph.D., from the Florida Department of Agriculture and Consumer Services, Cindy McKenzie, Ph.D., from the United States Department of Agriculture, and myself, Zee Ahmed, Ph.D., from Clemson University.

How Better IDs Reveal Better Clues About Invasive Species’ Patterns

Horticultural trade likely drives their global spread, leading to increased international interest in developing policies to prevent or mitigate species invasions. By understanding the native range, we can target phytosanitary efforts and surveillance strategies toward plant materials originating from those regions. Additionally, native range data can be used in models like CLIMEX to predict the species’ potential to establish in new locations.

Unfortunately, existing regulatory and scientific tools are inadequate to address the increasing threat of biological invasions globally. Accurate morphological identification of hemipteran and closely related pests, such as mealybugs, scale insects, whiteflies, and thrips, is challenging due to their small size, cryptic morphology, intraspecific variations in adults, interspecific similarities in immature stages, and the scarcity of taxonomists. For instance, in the case of T. parvispinus, a study demonstrated intraspecific morphological differences in size and color. In the case of B. tabaci, there are at least 42 cryptic species. Morphological identification alone is often insufficient for thrips and whiteflies. Several molecular markers have been developed for species identification, including the mitochondrial cytochrome oxidase I gene (mtCOI), as well as sections of various nuclear genes (18S rRNA and 28S rRNA encoding genes) and internal transcribed spacers (rDNA ITSs). However, mtCOI is regarded as a superior marker for species identification and genetic analyses due to its high nucleotide substitution rates. Proactively developing mtCOI consensus sequences, as produced in these two studies, will help identify accurate species and the haplotypes responsible for their global invasions. This knowledge would be the foundation of pest management against invasive species.

In another related study published this month in Florida Entomologist, we identified the potential presence of at least three cryptic species within the possibly invasive armored scale species Diaspis boisduvalii. Typically, we rely on adult females to identify scale insect species. However, given that adult females can be morphologically identical but genetically distinct, we recommend either analyzing additional life stages morphologically to uncover any unique morphological characteristics for cryptic species delimitation in the future or using genetic tools to avoid misidentification.

Hong Liu, Ph.D., from the Department of Earth and Environment at the International Center for Tropical Botany, Florida International University, Miami, Florida, says, “We speculated that Diaspis boisduvalii is most likely an introduced species, a hypothesis supported by historical specimen records in the collection. However, the presence of cryptic species could render these historical data misleading, as they rely solely on morphological identification. To ensure accuracy in the future, it is crucial to incorporate genetic identification records alongside morphological data in the collection.”

I have been working on the morphomolecular identification for over a decade. I can confidently state that the importance of accurate identification is often underestimated. It is only when others struggle to control these pests that they begin to panic and send samples for species identification, which is often too late, especially for invasive species.

Cindy McKenzie, Ph.D., a research entomologist at the USDA in Fort Pierce, Florida, says, “Our research identified the genetic profiles of invasive whiteflies and thrips, which are linked to traits such as insecticide resistance and host plant specialization. Genetic analysis is a critical tool to confirm invasive species.”

In addition, a study published in Insect Conservation and Diversity in 2018 highlighted that over 98 percent of entomological research papers provide little or no information on how the species in their research were identified. Another study published in Nature in the same year linked this issue to the difficulty in replicating entomological research worldwide.

Qiu Bao-Li, Ph.D., dean of the College of Life Science at Chongqing Normal University, China, says, “Symbiotic bacteria, such as Wolbachia in invasive species, can induce both inter- and intraspecific variations. Therefore, it is crucial to use genetic confirmation to accurately identify invasive species.”

Morphological identification is mandatory before conducting any research on any insect species. However, in the case of invasive species, I would not rely solely on morphological identification but also conduct molecular identification as soon as possible. As evidenced by the cases we’ve studied and shared here, this approach helps to detect complications related to cryptic species and aids in exploring the native range by comparing molecular sequencing with already available sequences in the GenBank from other parts of the world.

Lance Osborne, Ph.D., from the University of Florida in Apopka, Florida, says, “Genetic confirmation plays a crucial role in developing effective management strategies for invasive species.”