How and why do pest populations and behaviours change?

Pest species are well adapted to fitting in to a changing world, that is one of the qualities that lead to them being considered pests – they have adapted to mankind and found ecological niches which happily overlap ours. Recently though there have been a few major changes forcing both human and animal species to adapt. 

Covid, lockdowns and climate change have all had an impact on monitored pest species.

Focusing on climate change, current trends suggest the UK’s climate will get warmer. It is predicted by the HSE that as a result of increasing global temperatures in the next 5-10 years, we might start seeing new pest species such as the Asian Tiger mosquito or Asian Hornet crossing the channel and gaining a foothold in the warmer south of England. There have already been a few incidences since 2016 where these pests have been detected and controlled in the UK.

Focusing on wasps (Vespula vulgaris), one pest controller told us he identified wasp nests as late as December in 2020. The nests were outdoors and, whilst sheltered, were accessible to the elements.

How and why were the nests able to have survived until December? 

For nests to survive until November they must have had a plentiful food source. A nest will consume approximately 250,000 aphids in a year and will quickly succumb to starvation if there is a lack of prey.

Situated by a lake they would have had an abundant food source and a slightly warmer microclimate which would buffer sudden chills in the run up to winter. There weren’t many winter chills according to the MetOffice, who stated that 2020 was going the be the 3rd or 4th warmest year on record, even before the year had completely ended.

This explains how they could survive until December, the why might be slightly more complicated.

The simplest explanation is the queen survived longer than she would have been expected to. While an adult wasp can survive on nectar and the sugars from fallen fruit, an adult worker will only live for 12-22 days, which means that the queen must have survived and continued laying into the beginning of October.

Ordinarily all wasp nests in an area tend to lay their queen and drone cells all at the same time, to allow the best opportunities to find mates. Following this exertion, the current queen generally gets sick and dies, leading to the steady collapse of the nest. This clearly didn’t happen.

The possible reasons are:

• Warmer climate at the end of 2020; the queen was able to stay healthy due to a warmer climate and a steady food source.
• Starvation within the nest did not occur until significantly later in the year; the average temperature stayed above 10oC until January 2021. Wasps are unable to fly if the temperature falls below 10oC, a situation that rapidly leads to starvation, sickness and death.
  
  

What does this mean for those managing pests?

• Accepted behaviours may change; In milder climates (such as Florida, USA) – vespid wasps have been known to overwinter rather than die off, with nests growing significantly larger as a result.
• If insect populations are surviving until later in the year, this will have a cascade effect for other species that rely on insects as a food source, ie: birds, amphibians, small mammals. 
• Professionals will need to keep up to date with industry research into new and changing pest behaviours and related issues. These will influence treatment approaches and products used.
• Pest managers will need to ensure they don’t fall into the habit of making assumptions - reading the evidence of what is happening onsite correctly, before deciding treatment.  It is all too easy to fall into the trap of making the evidence fit the scenarios because ‘that is the way it has always been’.