Heat affecting hoverfly numbers

As I write this, the valley is full of mist and light rain, so seems a bit unusual to make heatwave conclusions right now. But 2018 has been a poor year for hoverflies so far this season and that is due to the unusual heatwave affecting the country. 2017 was typical of the years we have been having lately with wet mild winters and equally wet summers with short intervals of dry warm weather. These conditions worked favourably for hoverflies, allowing the different types of larva feeding to maximize their numbers. 2018 however has reverted back to the weather we used to experience "when we were young", that is extremely cold winters with quite a bit of snow cover. In fact January to May all came in much colder than the average temperatures. June and July on the other hand have been very hot and dry, with drought fears being reported by the national press. Temperatures regularly over 25c have been disastrous for hoverfly larva. The aphid feeders are finding it hard to find food as aphids are the first things to disappear during a drought, The rot-hole larva are just plain dying as all water sources are drying out. This year I have witnessed Myathropa Florea feeding on sap runs and Eristalis nemorum, Xylota segnis and Syrphus torvus all obtaining fluid from exposed mud that is drying out where there should be ponds and streams. Not all hoverflies are affected with Syritta pipiens numbers going through the roof. Below are three graphs of common species, with different larva feeding preferences and how the numbers have been affected so far this year.

Episyrphus balteatus (aphid feeding larva) - is a common European migrant, numbers start arriving in April but pick up greatly in June to August, where the adult lay eggs near large congregations of plant Aphids, then some make it back south, while others try to overwinter. The cold weather from January to May killed off the entire spring brood from the previous summer, so none were recorded until early June. The adults now arriving find that their aphid food source is greatly diminished, so there is no continuation of the life cycle and adult numbers are not inflated by last years brood with numbers crashing. As a migrant this species needs the hot weather to break and the winter to revert back to wet and mild for numbers to recover quickly.

Eristalis pertinax (aquatic type larva) - the larva of this species occur in a wide variety of wet areas, including, streams, silage, manure, wet bogs, damp decaying vegetable matter. The spring brood [from last autumn] hasn't been affected much because the wet habitat had not been overly affected by the cold weather. In fact numbers are higher than normal and looked to have done well in the early part of the year. But as soon as the heatwave kicked in in June numbers plummeted as the larva are dying off in large numbers in the wet places that have dried out quickly because of the drought. In July it should be one of the most numerous of species about, from the graph you can see numbers have plummeted 60%, which is also the number drop for the overall count of the 130 species so far recorded in the valley. Again the weather has to break to see an improvement, as it is multi-brooded this species should be able to recover faster than some other species.

Syritta pipens (aquatic type larva) - this species has seen a dramatic increase in numbers since the drought started and there has to be a reason why. The larva are found in wet decaying organic matter like compost heaps, manure heaps, cow-dung and silage but not in pools or ponds. The reason maybe that as the pools and ponds dry up the silage, compost and manure heaps retain their moisture through the fermentation process within these heaps and are not affected as such by the hot dry weather except to crust on the outside of the heaps, thus the numbers have been less affected. When the larva become adults and fly to feed and breed, their predator numbers are way down and has allowed their numbers to flourish [nature out of balance]. A word of note though, this week I have seen local farmers gathering up their compost and manure heaps and spreading them over their fields to increase grass growth for their cattle and there are few heaps now left. So this species may not be out of the woods yet and a knock-on effect may occur next year unless the weather cools.