Invasive species are those that are not native to a particular area and have the potential to have severe economic and environmental impacts. The Brown Widow spider, Latrodectus geometricus, is arguably one of the most successful invasive spider species in the world. The Brown Widow is thought to have originated in Africa, but the species has since seen a drastic spread across the world. This cousin of the Black Widows can now be found in North and South America, the Middle East, Australia, and Asia. In fact, in some areas, Brown Widows are outcompeting Black Widows for the same kinds of habitats. Understanding the mechanisms of invasion can be extremely useful for management strategies, particularly with a medically relevant pest, like the Brown Widow.  

 

 

There are several factors that can dictate the success of an invasive species. Some factors of success are related to the species itself- for the Brown Widow, part of the success is due to this species having a very high reproductive potential. This species has been known to lay approximately 120-150 eggs per egg sac and can lay up to 20 egg sacs in a lifetime!  Other factors of success are related to the new environment, such as the ability to find prey and shelter. However, one of the factors that we often don’t consider on a species’ path to invasion is the tiny hitchhikers, like endosymbionts, that may accompany these invaders on their journeys. 

Endosymbionts are bacteria and viruses that live inside the cells of their arthropod hosts. Endosymbionts can play a wide variety of roles in arthropods, ranging from the good, the bad, and the neutral. As such, the presence of these endosymbionts can either be beneficial or can be detrimental to an invasive species trying to make it in a new environment. But, before we can determine the role of these endosymbionts in this invasive spider, we first need to document the diversity and how prevalent they are in Brown Widow populations around the world.  

My collaborators collected Brown Widows from populations found around the world, representing two areas where Brown Widows are invasive, with six locations in Israel and four locations in the United States, and an area where Brown Widows are considered to be native, with seven locations in South Africa. We then analyzed the microbiome, or bacterial diversity, to see how the infections differed across the world.  

We found that Brown Widows generally harbor two main endosymbionts: Wolbachia and Rhabdochlamydia. Wolbachia is a familiar face to the arthropod endosymbiont world, as it is well-known for its ability to reproductively manipulate its hosts to ensure that the bacteria lives on in the next generation. However, more work is needed to determine if it is performing that same role in Brown Widows. The other symbiont, Rhabdochlamydia, was detected in every single population of Brown Widows that were sampled, but further work is needed to determine what this endosymbiont is doing in these spiders. In addition, both Wolbachia and Rhabdochlamydia were found in the eggs of the Brown Widows that were collected, which is an indication that these bacteria are passed on through the mother spider. Maternal inheritance of a bacteria is often an indicator that these bacteria may be performing some kind of reproductive manipulation, like cytoplasmic incompatibility or feminization, in Brown Widows.  

Across the board, Brown Widows have mostly kept their symbionts as they have spread across the world. However, when we look a little deeper, we can see some interesting patterns emerge. For example, Wolbachia was not found in populations that were more recently established- such as Southern Israel and Los Angeles, California. Also interestingly, the populations from Israel featured a higher proportion of Rhabdochlamydia compared to other populations.   This suggests that loss and gain of these endosymbionts through a population can be common in the invasion process based on who gets there first and establishes, but it could also be an indicator that the new environment hit these Brown Widows a little fast and hard before their population took off in those areas. 

While this is just one species in a small snapshot look, it helps us understand how these species (and their hitchhikers) are affected by drastic changes in the environment, like the invasion process.  This can not only help us understand how these kinds of pests spread, but also provide context on how to manage them. Species that are well-adapted to environmental change are likely to be those pests that feel like they’re one step ahead of the game in terms of your management strategy. By having one more piece of knowledge to help us potentially stop invasive species in their tracks, we can have one less pest to worry about.  

If you’d like to learn more about the common widows found in North America, please check out the latest library update from NPMA, which can be found here.  

Article by: Laura Rosenwald, BCE

References

Sadir, Melissa, Marske, Katherine A. 2021. Urban Environments Aid Invasion of Brown Widows (Theridiidae: Latrodectus geometricus) in North America, Constraining Regions of Overlap and Mitigating Potential Impact on Native Widows. Frontiers in Ecology and Evolution. Volume 9. https://doi.org/10.3389/fevo.2021.757902 

Podcast