Hoverfly White-clubbed Glasswing (Scaeva pyrastri)

White-clubbed Glasswing (Scaeva pyrastri) Wing length 9.25-11.25. Flight May-Mid August

Species of the week –1st December

The first thing you will notice if you spot this hoverfly, are its abdominal markings. They remind me of hockey sticks, but the shape is often referred to as clubbed or bowed. Some prefer to refer to them as smoothwing, others glasswing due to the lack of microtrichia (tiny hairs) on the wings. There are 5 species listed for the British Isles but there are only 2 that you are likely to come across anytime May to mid-August. These are pryrastri and selentica the main difference being white/cream abdominal markings for the former and yellow for the latter. 

If this hoverfly seems familiar, even if you haven’t found one in the wild, it’s probably because you have seen the 1st edition of Britain’s Hoverflies by S. Ball and R. Morris published by WILDguides, where it is featured on the front cover. 

The White-clubbed Glasswing (Scaeva pyrastri) is a migratory species coming from the continent, so often spotted after a southernly wind has blown them in. They will appear in your garden and places just beyond your garden gate. Its cousin Yellow-clubbed Glasswing (Scaeva selentica) is more often found near heathland and coniferous woodland. Both species have swollen frons, which is the area at the top of the head behind the antennae, not an easily identifiable feature I would say, but with a bit of practice it becomes a little more obvious, it’s especially swollen in males. A dip in the R4+5 vein is also an observable feature, but not species specific.

Interestingly this plate below, plate 66 of 100 from Eleazar Albin’s 'A Natural History of English insects' (London 1720) features our hoverfly, although Eleazar refers to it as a Block-headed bee fly. It was kindly brought to my attention by Dr. J. Whitelock. The book is full of illustrated plants with observed and assumed associated larva/caterpillar, adult, and pupa. The book was dedicated to the Princess of Wales, Caroline of Ansbach and each plate dedicated to other eminent individuals of the day. I am reliably informed this was the ‘done’ thing in those days. But despite most of the book being full of butterflies and moths, this is the only plate to show a hoverfly, a cream hockey stick hoverfly. Interestingly it was only given its scientific name and classification by Swedish biologist Carl Linnaeus 38 years later in 1758. Linnaeus neither classified it as a Block-headed fly nor a Bee fly, as referenced in the index of the book.

The larva of White-clubbed Glasswing, is green with a white dorsal longitudinal stripe, see above label a. What is fascinating is what it is doing in the picture. As it says in the accompanying text ‘Its way of feeding is thus, it having no feet it flattens itself to the leaf by the tail, which is broad and flat like that of a leach, and seizes its prey with its proboscis, then lifting up its head sucks them till there is nothing left but their skins.’ Whether this was behaviour observed or just interpreted from earlier work by Jan Geobert (Dutch Naturalist 1617-1668) is unknown. It has since been proven that hoverflies tend to hold captured aphids aloft to prevent them from sending out chemical signals (Rotheray 2011).This prevents the aphids from using pheromones to alert others, the larva’s hope being that the wind carries the chemical alert away. 

The adult will unlikely take pollen or nectar from the rose, it not being an open flower. It is a female as shown by the separation between the eyes. So, she could be hovering around seeking out a potential spot to lay her eggs near a colony of rose aphids. These eggs are white often laid separately, turning greenish later before hatching. Its cousin selentica lays its eggs arboreally, as the larva prefer to ‘suck’ pine aphids. At the bottom of the page is the hoverfly pupa, suggesting this one had not migrated south before winter. See article Hoverfly Migration: Incredible Journeys for more information about migrating hoverflies.

Table 1 Developmental ages of  S.pyrastri & Graph 1 showing the age specific survival rate.

Research has shown (Palial 2022) that this hoverfly could be used for biological control of Cabbage Aphid (Brevicoryne brassicae). With the developmental time from egg to adult being just under 20 days. The adult lives 15-19 days, enough time for the female to lay 235 eggs and assist in suppressing aphid populations by 84%. The larva eats approximately 377.06+/-26.54 aphids throughout its development.

 
Could it be used to formulate a bio-controlled program and reduce a gardener or farmers dependence on pesticide control? 

The problem might be that females will only select plants infested with high numbers of aphid. 

So how does one encourage egg laying in places where it is needed? 

Recent research shows that chemicals released into the atmosphere from plants under attack, might be the cue for females to lay eggs. So, if those harmless chemicals were sprayed/applied when there is a high risk of aphid infestation, this might give the farmers/gardeners an alternative solution to pesticides. This will need more research but results so far looks promising.

References:

Eleazar Albin’s A natural history of English insects (London 1720)
https://archive.org/details/bp_2904246/page/n173/mode/2up

Rotheray G.E & Gilbert F.S (2011): The Natural History of Hoverflies

Stuart Ball and Roger Morris (2015) 2nd Edition: Britian’s Hoverflies A field Guide 

Palial et al. Biology, predatory potential and growth parameters of the syrphid fly, Scaeva pyrastri (L.)
(Diptera: Syrphidae) feeding on the cabbage aphid, Brevicoryne brassicae (L.)
Egyptian Journal of Biological Pest Control https://doi.org/10.1186/s41938-022-00632-5
(2022) 32:134