Submitted by AJ Cann on Thu, 17/12/2020 - 07:12
    Two Spot Ladybird

    An 11-year study suggests that the invasive Harlequin Ladybird caused the severe decline of the Two Spot Ladybird. More...

    A study conducted in Switzerland records the severe decline of Two Spot Ladybird - Adalia bipunctata, which was the most abundant native ladybird on broadleaved trees in 2006–2009 and has almost disappeared since 2010. In contrast, the Harlequin Ladybird (Harmonia axyridis) became the most abundant species after its arrival in Switzerland in 2004. The first record of the Harlequin Ladybird in Leicestershire and Rutland was on 24/06/2006. So has this invasive species had the same impact in Leicestershire and Rutland? I took a look at 20 years of data (2000-2019) to find out.

    Ladybirds Graph

    The short answer is - we don't know. From the graph there is no obvious decline in Two Spot Ladybird records following the arrival of the Harlequin in 2006, in fact, they could be increasing. The problem is that this is not a systematically collected data and makes no compensation for recording effort. More records are submitted to NatureSpot every year and this needs to be taken into account, as does the number of people submitting records, where they look for ladybirds and how long they spend looking (which we don't know). There's also a substantial flaw in the original paper, the title of which suggests that the Swiss decline in Two Spot Ladybirds is "in response" to the arrival of the Harlequin Ladybird, but the study does not show that the Harlequin causes the decline of the other species. Insect species undergo long term fluctuations and correlation does not prove causation. Did you know that the number of mobile phone masts in the UK parallels the decline in House Sparrow numbers? This isn't a 5G conspiracy theory, it's simply a spurious correlation. In addition, the growth in use of Internet Explorer matches the increase in the murder rate in the USA... 

    What do we know then? Has the arrival of the Harlequin Ladybird in Leicestershire and Rutland been a ecological catastrophe? No. 

    Long Term Monitoring in Switzerland Reveals That Adalia bipunctata Strongly Declines in Response to Harmonia axyridis Invasion. Insects (2020); 11 (12): 883