Hoverflies are harmless insects, and many of them are useful in the garden.
Marmalade Hoverfly Larvae Eat Greenfly
The Marmalade Hoverfly (Episyrphus balteatus) is unusual in that the young (larvae) eat greenfly (aphids) – the adult fly eats pollen.
When conditions are right Marmalade Hoverflies can form large swarms, and this often causes concern since they look like wasps! In fact they have no sting and cannot harm humans.
Episyrphus balteatus is quite easy to recognise:
- the body is orange and has large horizontal black bands, and there are also two additional thin black lines
- there is often a faint greyish stripe running along the back
- it is relatively small (9–12 mm)
Hoverfly Swarms
In August 2004 'Fly swarms' were reported on beaches in Essex (UK), and at that time Andy May, of the Essex Wildlife Trust, was reported as saying: "the larvae's main source of food, aphids, had thrived in the warm and wet conditions which in turn has led to an explosion in the hover fly population" and "When the larvae turn into adults they feed off pollen ... We think that this year a lack of flowers in the countryside means the flies are migrating in search of food."
July and August 2011 saw similar large swarms of hoverflies in many parts of the UK, probably fuelled once again by large numbers of greenfly in the spring. Richard Gray, writing in The Telegraph about spring conditions in England, said that: "numbers [of greenfly] have reached 10 times their average levels".
Mark Taylor of Rothamsted Research is quoted as saying: 'When the warm weather came in April the aphids took over and the ladybirds and hoverflies that naturally prey on them are struggling to keep up'. ... The hoverflies certainly managed to 'catch up'!
A reader of the Hoverfly Recording Scheme UK commented on seeing large numbers of these flies this year. The reply attributes this to the north and westward migrations that have occurred as a result of the explosion in numbers.
Animal Swarms
When an unusual year brings unusual conditions many animals can experience a population explosion, but these are normally short-lived (as the current hoverfly swarming is expected to be – as winter comes the adult hoverflies will die off, and next year will impose a different set of conditions).
There have always been, and there will continue to be, years when the weather is either colder or warmer than expected – and these can cause population explasions (as with the hoverflies). But there is an additional phenomenon now. For (whatever the cause) an additional trend has been observed, with average temperatures worldwide slowly increasing. This global climate change is another matter entirely, since it sometimes allows animals to extend their range significantly – jellyfish swarms are becoming more frequent, and the Mediterranean Sea is changing its character.
It is not just hoverfly swarms that cause concern.
Sources:
- 'Fly swarms invade East's beaches', BBC News, August 2004.
- 'Swarms of aphids are swamping British gardens and the countryside in the wake of a heatwave spring.', Richard Gray,The Telegraph, July 2011.
- Hoverfly Recording Scheme
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