How a heatwave affects our bees, butterflies and flies

 My interest in how the heatwave would affect our bees, butterflies and other insects was sparked by Roger Morris's post (16 July 2021) in the UK Hoverflies Facebook group.  He suggested that anyone interested could do a survey to help see how the heatwave would affect numbers of insects as follows:

"Chose a consistent route around your garden ... Walk the route and count the numbers of insects you see on an hourly or bi-hourly basis from as early as possible. Breaking numbers down into bees and wasps/Flies/butterflies and moths/Beetles/true bugs would help but lets start simple this weekend. For each trip record the numbers or if nothing was seen."

I did this on Sunday (July 18th) and then repeated the survey on Saturday the 24th to see what the numbers looked like on a more normal day.

A "normal" summer day

With the English weather, deciding "normal" is tricky but, for my purposes, a dull, damp start drifting into a sunny but not hot afternoon will do just fine.  

6-spot Burnet moth (Zygaena filipendulae) at 8am

I did my first count at 8am and found a total of 25 insects, mostly hoverflies with a few moths and one honey bee.  

Picture Fly - Urophora quadrifasciata

As the day wore one numbers increased until about 4pm the garden was loud with the buzzing of bees and hoverflies. I counted a total of 91 insects and nearly half of these were bees.

Insect numbers over a "Normal" day - 24 July 2021

Heatwave

So how did numbers change during the heatwave?

Wool carder bee (Anthidium manicatum) - archive photo

There was a faster start with more bees already active - a total of 33 insects compared to 25 on the "normal day". Wool carder bees were already active around our lambs ears plant, the males tussling for prime position in case a female turned up.

Insect numbers over a Heatwave day - 18 July 2021

Numbers peaked early, at 10am with 63 insects, after this numbers drifted slowly downwards.  Although there were insects around in the afternoon, there was no "summer buzz". At 4pm there were only 53 insects compare to 91 on the "normal" day.

Impact of the heat

So the main differences were that on the heatwave day:

• Total insect numbers were reduced from 353 to 315, about 10% down
• There were more insects at 8am - 35 compared to 25
• The peak was much earlier - 10am rather than 4pm
• The balance between bees and flies had changed - from slightly more flies to slightly more bees
• Although numbers of insects were otherwise depressed, numbers of butterflies and moths rose by about 20%.

What was missing?

During both counts, there were:

• very few beetles - I would expect to find a number of ladybirds and some Common Soldier beetles
• no bugs - I was surprised not to see any shield bugs as they are usually common in our garden
• very few craneflies - they were not in the official count but I only saw one, on the "normal" day - Mum has remarked that, in the past, we had been unable to keep windows open during heatwaves due to the sheer number of craneflies that used to come in.