Insects and hot summers

Melanargia galathea, Marbled White, near Oaks Wood - Brian Eversham

With Britain experiencing a sustained heatwave, our Chief Executive, Brian Eversham, looks at the effect warm summers have on Britain's insect populations.

Direct effects of high temperatures

All other things being equal, the young stages of insects grow faster at higher temperatures. Faster growth, less time spent in their most vulnerable stages, can mean higher numbers of adults. So, this year some grasshopper species became adult 3-4 weeks earlier than usual, and seem to be more abundant than for many years.  It’s also been the best summer I’ve seen for some of our butterflies, such as marbled white which is on the wing right now, in huge numbers. The huge and gorgeous purple emperor is also being seen in record numbers and turning up at new sites.

It’s also the season for flying ants  -  and hot dry weather will suit them very well, so they could also be around in record numbers. They’re harmless but can be mildly irritating if you walk through the clouds of them.

In some species, high temperatures, especially when combined with crowded conditions (which can result from the reduced losses as they grow faster), can lead to longer-winged and more mobile adults.  The hot summers of 1989 and 1990 triggered some bush-cricket species to expand their range, from the south coast right into the midlands, and we may also see new species colonising from mainland Europe.  Most of these, like the tree bumblebee which arrived in 2000 and is now well established as far north as central Scotland, will probably fit it fine, with no impact on existing species.

If foodplant loss is bad news for many insect larvae, many adult insects tend to live longer in dry and warm weather providing they can find food (for many, nectar; for dragonflies, other insects).   

The news stories about horseflies may be due to them moving further from their wetland breeding sites, rather than actual increases in numbers. Unusual reports of horseflies in gardens back this up. 

I recently spent a weekend in the New Forest, walking in boggy and heathy habitats, and didn’t see any more horseflies or mosquitoes than usual. I would definitely NOT suggest anyone should drain their garden pond because of the weather  - they are vital for all sorts of wildlife, not least birds and hedgehogs having a drink  -  and most of the troublesome insects may be breeding in small pockets of water rather than in a healthy pond (which has lots of predators such as dragonfly larvae that keep mosquitoes in check).

The one invertebrate I would warn people to look out for are ticks:  these seem to be more plentiful in some places this year, and a proportion of them carry the infection, Lyme Disease. If you walk in long grass or heather, check yourself carefully for ticks when you get home, and remove them with tweezers or a plastic tick-remover.  If you’re bitten, watch out for a reddish ring developing round the bite, and if you get any flu-like symptoms, mention to your GP that you think you may have been bitten, and ask to be checked if Lyme Disease is a possibility. It’s cleared up by antibiotics if treated early.

Omocestus viridulus, Common Green Grasshopper - Brian Eversham

Omocestus viridulus, Common Green Grasshopper - Brian Eversham

Summer droughts

If the drought continues much longer, we will see ponds, ditches and even rivers, which have low water levels already, actually dry up.  This is obviously bad news for most aquatic life, especially when it happens to water bodies that rarely do dry up.  The beneficiaries include species which need ponds or ditches free of predatory fish. So, although newts, frogs and toads may fail to breed in a dry summer, the loss of fish from their breeding sites may mean the ones who survive will be more successful in future years. 

A few insects are specially adapted to this:  the Willow Emerald Damselfly, recently colonising Britain (arrived in our area in the last 2-3 years) lays its eggs in twigs of bushes over water bodies, often temporary ones which dry up in mid-late summer;  but the eggs do not hatch until the following

Spring, when the larvae wriggle free and drop into the pond, hopefully refilled by winter rain, and devoid of fish and other predators.

In drought years, many land plants flower, set seed and die back early. That way, they can pass the driest weeks as seeds or as small rosettes at ground level which don’t require so much water.  While this is ok for the plants, it may mean that insect foodplants maybe not available, so some butterflies and other plant-feeding insects may suffer. Likewise, during a prolonged drought many plants may finish flowering early, or if they are short of water, the supply of nectar will run low. That’s bad news for bees, hoverflies, butterflies and other nectar-drinking insects. There is some evidence that insects can adapt    -  the ringlet butterfly population crashed after the drought of 1976, when its caterpillars survived only in shady or moist habitats.  More recent droughts have had far less effect on Ringlets, suggesting it’s got better at finding shaded foodplants that survive the heat. 

Chrysops caecutiens, horsefly at Ring Haw, Northamptonshire - Brian Eversham

Chrysops caecutiens, horsefly at Ring Haw, Northamptonshire - Brian Eversham

2018 in particular

One of the oddities of 2018 was the cold winter and fairly late spring – a lot of seasonal wildlife was squeezed together, with early spring flowers running late, and bumping into early summer ones which they usually avoid. 

One consequence of the cold winter is that many insects which gardeners in particular think of as pests -   greenfly and blackfly -  started late and from a low level, so may not reach the abundances we sometimes see.  Plus, ladybirds, lacewings and hoverflies are now building up their numbers and will curtail numbers. A couple of weeks ago, I looked at the local aphid colonies, any every one I found had hoverfly eggs right next to it, so their fate was sealed.