2019: Summer of the Shieldbug

Guest blog by Alison Robertson

As someone interested in moths, particularly micro-moths, I spend a lot of time staring at trees.  Sometimes a slight movement or a flash of colour will draw my eye to a creature very much larger and more robust than any micro-moth: a shieldbug.  This year has been a particularly lucky one and they’ve become an interesting sideline, combining well with mothing.

Although they can be found all year round, late summer and autumn are good times to look for shieldbugs as many species start to reach maturity around August so there are fewer nymphs to worry about (nymphs often don’t look anything like the adult). Like many insect groups, they are under-recorded in Dumfries and Galloway but that’s just because no-one has ever looked for them; once you get your eye in they are not so hard to find, often to be seen sitting on top of a leaf in broad daylight, either in a tree (binoculars are useful) or on low vegetation.  Venturing out with a torch after dark is another good way of finding them.

The two species I’ve seen most often this year are Red-legged Shieldbug (aka Forest Bug) Pentatoma rufipes, found on many deciduous trees, and Spiked Shieldbug (aka Spined or Spiny Shieldbug) Picromerus  bidens, found in many habitats but I’ve mostly found them on roadside verges.  The two species look superficially similar (both have red legs) until you notice the thorn-like projections on the Spiked Shieldbug’s pronotum.  Red-legged Shieldbug is occasionally predatory but mostly a sap-sucker, piercing the leaves with its mouthparts.  Spiked Shieldbug, by contrast, is a predatory beast, feeding entirely on other invertebrates, particularly caterpillars.  These two differ from the other species in this blog in that they overwinter as nymphs and eggs respectively rather than as adults.  Despite its “Forest Bug” appellation, the P. rufipes found on 25 August was not in a forest at all but in the middle of Dumfries, caught in a spider web on the suspension bridge high above the River Nith.

Hawthorn Shieldbug Acanthsoma haemorrhoidale and its smaller relative Birch Shieldbug Elasmostethus interstinctus were among the first in the 2019 notebook, both species an eye-catching combination of red and bright green.  Castle Wood, Caerlaverock, was the location; later in the year, on 5 September, Castle Wood also provided a Bronze Shieldbug Troilus luridus.  Luridus just means “pale yellow” in Latin; there’s certainly nothing lurid about it – apart, that is, from its feeding habits.  Another carnivorous one, this particular individual was sucking a juicy caterpillar, its mouthparts sunk into the body of the living, writhing caterpillar.  Hawthorn, Birch and Bronze Shieldbugs all inhabit a range of deciduous trees so any area of woodland would be a good place to look.  They’re often on the vegetation underneath the trees so check this out as well.

The Hairy Shieldbug (aka Sloe Shieldbug) Dolycoris baccarum found on 25 July was an extremely hairy nymph clambering through sparse vegetation near an old quarry.  Its hirsuteness will be carried forward to adulthood and can be seen under a hand lens.  This one doesn’t have a specific habitat or food plant – though neither adults nor nymphs feed on sloes.

Gorse Shieldbug Peizodorus lituratus has two colour forms, one red and green, the other subtle and muted, seeming to disappear among the spines and seed-pods of its host.  At least 40 (13 adults and 27 nymphs of varying instars) were present on the Gorse bushes at Castle Corner car park, Caerlaverock, on 18 August.  Broom is an alternative foodplant and on 2 August I found a nymph, so worth checking both plants.

A pleasant surprise was finding a nymph of the Common Green Shieldbug Palomena prasina on a Stinging Nettle at Scottish Wildlife Trust’s Southwick Coast reserve on 27 August.  There are few Scottish records so that’s another dot on the map.

Another species with few Scottish records is Juniper Shieldbug Cyphostethus tristriatus.  Admittedly more in hope than expectation, I headed for Tynron Juniper Wood on 7 September – and found a nymph on only the second tree (how lucky was that).  There are not many Scottish records – but then, there are not many Scottish Junipers.  This species is being found increasingly in gardens on junipers and Lawson’s Cypress.

[SWSEIC note: the only other record we have of Juniper Shieldbug from SW Scotland is from a garden in Dumfries.]

After the Tynron success, confidence was high.  However, a search for Heather Shieldbug Rhacognathus punctatus on Longbridge Muir proved fruitless: an afternoon spent crouched in the heather surrounded by whining mosquitos produced only a Spiked Shieldbug, this time sucking the juice out of a Beautiful Yellow Underwing caterpillar.  A search for Parent Bug in Castle Wood was similarly unsuccessful, even though I’d found one there four years ago so they’re definitely around.  It’s still only mid-September so plenty of time for a return visit to both locations.

A final word about identification: just as caterpillars often look totally different every time they shed their skin, so do shieldbugs.  The British Bugs website has an excellent gallery of the various instar nymphs for each species, as does the Nottinghamshire Eakring Birds website (scroll down the page).  There is also an FSC laminated chart.  English names vary with each website or publication so always check the Latin.  And shieldbugs become darker as winter approaches, brightening up again in spring (the Castle Corner Gorse Shieldbugs were noticeably darker on 10 September than they had been a month earlier).  Finally, just because it has a tree in its name doesn’t mean that’s the only tree it’s found on.  Hawthorn Shieldbug larvae, for example, can also be found on oak, hazel and birch, and the adults can be seen anywhere.

All the records mentioned in this Blog have been put on iRecord.

 

SWSEIC addendum

The following species of shieldbug have been recorded in SW Scotland:

Birch Shieldbug Elasmostethus interstinctus
Blue Shieldbug Zicrona caerulea
Bronze Shieldbug Troilus luridus
Gorse Shieldbug Piezodorus lituratus
Green Shieldbug Palomena prasina
Hairy Shieldbug Dolycoris baccarum
Hawthorn Shieldbug Acanthosoma haemorrhoidale
Heather Shieldbug Rhacognathus punctatus
Juniper Shieldbug Cyphostethus tristriatus
Parent Bug Elasmucha grisea
Red-legged Shieldbug Pentatoma rufipes
Southern Green Shieldbug Nezara viridula
Spiked Shieldbug Picromerus bidens

This map shows the distribution of all the Shieldbug records held by SWSEIC as of September 2019. This is derived from just over 630 records. Filled circles show records 2000 onwards, hollow circles pre-2000. The map does not include 2019 records, so Alison’s records above will no doubt add a few more dots to the map.