Alien invasive species are having a major impact on biodiversity, ecosystem functioning, agriculture, and public health (Diagne et al. 2021). Increased international trade and transportation is leading to an increased rate of biological invasions (Seebens et al. 2017). Once an invader has established in a new area, evolutionary processes can affect its further spread and impact on its management (Bock et al. 2015; Estoup et al. 2016; Lee 2002; Ortego et al. 2021).
An invading population often starts out being small compared with populations of related resident species (Estoup et al. 2016; Schmack et al. 2019), increasing the likelihood of genetic drift and loss of alleles (Semenov et al. 2019). In order to eradicate or prevent the spreading of invading populations, intensive control efforts are usually conducted, which further decrease the size of newly colonized populations (Branco et al. 2022) and increase the potential for genetic drift. The resulting loss of genetic diversity may in turn limit the adaptive ability of the newly founded populations because of the possible loss of selectively favoured alleles (Reed, Frankham 2003; Reusch et al. 2005). However, many invading populations successfully colonize new areas despite low genetic variation (Facon et al. 2006; Tsutsui et al. 2000).
Several studies have examined evolutionary processes before and after an invasion, providing insights into the invasion genetics of species (Bock et al. 2015; Estoup et al. 2016; Medley et al. 2019; Sherpa, Després 2021). However, few studies have tracked temporal variation in genetic diversity following invasions. While some species have maintained population genetic diversity during invasion (Brock et al. 2021), others have lost genetic variation (Cao et al. 2016b; Schmack et al. 2019; Xue et al. 2018) or it has been further bolstered by new incursions (Quaresma et al. 2022).
Where low genetic diversity of invading populations in the early stages of introduction is bolstered by ongoing gene flow (Smith et al. 2020; van Boheemen et al. 2017), admixture from different sources may accelerate the rate of invasion by providing rapid adaptive evolution (Qiao et al. 2019; Rius, Darling 2014; Smith et al. 2020). When the colonized environment is different from the native environment, the adaptive benefits of admixture are expected to be high (Verhoeven et al. 2011). Climate change is also expected to pose strong selection on invading populations favouring novel combinations of genes (Chown et al. 2015; Ma, Ma 2022; Ravi et al. 2022). Tracking genetic changes in invasive populations can therefore be useful in understanding likely current and future trajectories of these populations.
The western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is a globally important invasive pest in agriculture and horticulture. The WFT is native to western North America, mainly west of the Rocky Mountains, from Mexico as far north as Alaska. It has spread rapidly worldwide mainly through the movement of horticultural material since the late 1970s (Kirk, Terry 2003). In China, the WFT was first discovered at an international flower show in Kunming, Yunnan Province, in 2000. The first report of the WFT establishment was on greenhouse peppers in the suburbs of Beijing in 2003. WFT rapidly colonized all suitable areas of China in about ten years after the first report and became a common pest on vegetables and flowers. As a small insect, WFT is easily dispersed over long distances by human activities (Cao et al. 2017).
Previous genetic studies suggested bottlenecks and genetic differentiation among populations of WFT in China in the early stages of establishment (Cao et al. 2017; Yang et al. 2015; Yang et al. 2012). Cao et al. (2017) in particular noted strong population structure in populations from across Beijing and suggested multiple introductions of WFT into this region. Here we reconsider these patterns in more recent collections of WFT, including some samples obtained from the same location.
We hypothesized that changes in population genetic structure might occur because of the rapid dispersal of the WFT and therefore potentially high levels of gene flow among populations. However, if new genetic introductions have occurred into China, we expected new patterns of genetic structure to have emerges. We had no clear predictions about levels of genetic variation in the population given that these might be decreased by control measures decreasing population size, whereas gene flow was expected to increase genetic variation through the introduction of new alleles. We also consider insights from our results for invasion genetics generally and for the management of this pest specifcally.