Now is the time to consider the extent of collateral damage to avian predators of non-target Lepidopteran larvae from increasingly widespread and intense insecticide spraying in a largely futile attempt to contain the spread of OPM, writes Dr Terry Mabbett.
AS the London-centric oak processionary moth (OPM) plague continues to colonise the Home Counties, the focus of insecticide application is switching from oak trees in parks and gardens and on commons and golf courses to oak trees in mixed deciduous woodland.
In 2019, 24,250 trees were sprayed across 1,052 sites, trebling the number of number of sites needing treatment and with 37 per cent more trees sprayed annually since 2017. This year’s sponsored spraying by the Forestry Commission started on April 13 at over 1,200 sites identified as requiring treatment.
Alien, invasive OPM was first identified in 2006 on landscaping sites in West London, in the London Boroughs of Richmond and Ealing, on semi-mature oak trees imported from Europe and planted in 2005/6. However, it soon became clear that OPM was already out of control and could not be contained in the longer term. In 2010, Tony Kirkham, head of the arboretum, gardens and horticultural services at the Royal Botanic Gardens Kew, told Horticulture Week: “Within five years it will affect everyone living within the M25.”
His prediction and prophecy were essentially fulfilled.
Ten years on, OPM is way beyond the M25 at all points on the compass with the Forestry Commission’s latest distribution map (September 2019) showing the outer limits of the infestation being connected by a line passing through: Ware and Harpenden in Hertfordshire, Chipping Ongar (Essex), Rochester (Kent), Horley (Surrey), Farnborough (Hampshire) and Reading in Berkshire.
The hardest hit county is Surrey, the most heavily wooded county in England. Not surprisingly, arborists working across much of Surrey are now confronted by OPM during the course of everyday work.
ARBORISTS CONSIDER THE WIDER ENVIRONMENT
This is exactly what happened last year to one unsuspecting arborist who was about to begin a tree-management job involving hundreds of oak trees at a sports facility, only to be faced with scores of OPM nests from previous years. The landowner appears to have known about the problem but was reluctant to take action, apparently deterred by what they considered as the high cost quoted by specialist contractors to spray the trees and physically remove OPM nests. Such services don’t come cheap, especially for OPM nest removal, due in part to the potentially hazardous nature of the work involved.
Faced with this situation, the arborist was essentially left with three options:
- Ask a clearly reluctant and apparently recalcitrant landowner to remove the nests before starting the tree management work.
- Get clued up on OPM, kit out the workforce and remove OPM nests before embarking on any tree management.
- Walk away and give up a large and lucrative tree management contract.
He went middle for diddle, and after receiving expert advice and kitting out his guys with the appropriate PPE managed to safely remove the offending OPM nests.
Such is the escalation of OPM spread across Surrey that the same arborist toyed with the idea of purchasing spray application equipment to be used when OPM situations arose again, which they would surely do with increasing frequency over the coming years.
To this end, I referred the arborist to a specialist contractor with years of experience and sound, up-to-date knowledge of spray machinery, application technique and insecticide required. By the time we spoke again in February this year he had dropped the idea following sound advice indicating a required investment of between £30,000 and £50,000 and certainly not justified unless going into OPM management on an exclusive, full-time basis.
However, he had another reason for not wanting to go down this route. Along with an increasing number of today’s conservation-minded arborists he understands and appreciates the potential impact of insecticides on wildlife, even when applying the bacterial insecticide Bacillus thuringiensis subsp. kurstaki (BTK), the least environmentally damaging of the three insecticides officially and legally on offer to control OPM.
BTK is a highly specific biological insecticide used to target larvae of the Lepidoptera (butterflies and moths) and therefore OPM. As such, it will pose a potential risk to any species of butterfly and moth with a larva-feeding window overlapping that of OPM; and during that period when BTK sprays are targeted at early OPM instar stages, which are most susceptible to BTK insecticide. This can cover a period from mid-April into early June, depending on spring temperature and sunshine conditions, and its effect on the timing and rate of oak tree re-foliation, timing of OPM egg hatch and speed of larval growth and development. OPM feeds on native white oaks but also exotic Quercus species especially Quercus cerris (Turkey oak).
Our native white oaks (Quercus robur – English or pedunculate oak and Quercus petraea – sessile oak) support more wildlife than any other native trees, including over 260 different insect species. Within this arthropod biodiversity are native Lepidopteran insect species including green tortrix moth (Tortrix viridana) and winter moth (Operophtera brumata). Since these two native moths, across all stages of insect development, are smaller and apparently less robust than OPM, it is reasonable to assume they will be more susceptible to insecticide whether it be a chemical insecticide or a biological insecticide such as BTK.
COLLATERAL DAMAGE TO NON-TARGET INSECTS AND BIRDS
What’s the big deal, you may ask, because under favourable conditions the larvae of both moths can assume pest status on native Quercus (oak)? However, larvae (caterpillars) of these moths are a main food resource for nestlings of small native passerine birds belonging to the tit or titmouse family (Paridae), namely blue tit (Cyanistes caeruleus) and great tit (Parus major).
These hole-nesting* native birds have a more varied diet than just green oak tortrix and winter moth larvae and even during the breeding season when they forage for larvae on other native trees such as hawthorn and birch. Be that as it may, anecdotal evidence suggests that in Britain (at least) these native titmouse species, with distributions spanning virtually the whole of Europe and beyond, have co-evolved with our native white oaks. These native oak trees have a huge potential for insect food resources in a relatively short window during April and May, and specifically in the form of lepidopteran larvae of which green oak tortrix larvae and winter moth larvae are the most widespread and frequent.
Evidence supporting such synchrony of breeding with ample lepidopteran food sources on native woodland trees and especially oaks is the titmouse reliance on a single brood featuring a large clutch size averaging 10 eggs for blue tits in an ‘all eggs in one basket’ approach to breeding.
To successfully feed large numbers of nestlings blue tits must collect a correspondingly large number of caterpillars. Ornithologists calculate the requirement as 70 larvae per nestling per day which means 700 larvae per day for the average brood size of 10. And ideally from trees close by because allowable time for foraging is clearly restricted by hungry and vulnerable chicks back in the nest. Blue tits are well known for their territorial behaviour even during the breeding season when they collect larvae on the foliage of the tree in which they are nesting and nearby trees in the woodland, parkland or garden situation.
The blue tit’s finely balanced breeding and feeding requirements have apparently evolved over many millennia so it is easy to imagine how an abrupt change like sudden disappearance of the birds’ main food source for nestlings, due to insecticide spraying against OPM, could have catastrophic consequences on success of a species with just a single precisely-timed brood per year.
BLUE TITS BATTERED IN BERKSHIRE?
So, is there any evidence that native titmouse birds like blue tit and great tit are under pressure from the escalating level of insecticide spraying against OPM within Greater London and increasingly out into the Home Counties?
I am unaware of any area-wide research to determine how insecticide spraying, now in progress for some 15 years, is impacting on non-target lepidopteran larvae, and in turn the bird species which prey on them. However, there was evidence of an impact on insects and birds from an aerial spraying trial using BTK against OPM at Pangbourne in Berkshire during spring 2013.
The Forestry Commission carried out the aerial application of BTK by helicopter in May 2013 on two SSSI (Sites of Special Scientific Interest) woodlands (Herridge’s and Broom copses). Natural England reluctantly went along with the programme on the understanding that environmental assessments would be carried out before and after spraying in the treated woodland and in nearby unsprayed woodland (Horn’s Copse), used as a control comparison.
11 times fewer green oak tortrix larvae were found in Herridge’s and Broom copses after insecticide application (two sprays 14 days apart) compared with the unsprayed, control woodland. Eight blue tit nests were identified in Herridge’s Copse before spraying but only one family of blue tits was recorded post-spraying. Respective figures for the untreated Horn’s Copse were three nests before and seven family groups after spraying.
The real tragedy concerning the insecticide application trials at Pangbourne is that we will never know if aerially applied BTK insecticide actually controlled any OPM. We know that green oak tortrix moths were hit hard because counts of these insects were made before and after spraying. However, what the effect of BTK on OPM was is anyone’s guess because nobody thought to scout Herridge’s Copse prior to spraying to establish whether or not OPM was present in the woodland. My personal view is that the whole thing was an exercise in kite flying but given the extent of uproar in Berkshire, around Pangbourne, Reading and beyond, I doubt whether any more kites will be flown.
The great tit population did not appear to suffer in the same way which may be due to a more varied diet – great tits are known to prey heavily on spiders to feed their nestlings. And perhaps these comparatively larger birds are able to cope with older and more developed larvae. Newly emerged oak leaves progressively produce tannin and by the end of May oak leaves contain a high concentration of this potentially toxic phenolic compound. Tannins are known to deter insect feeding but also affect the growth and development of nestling birds if they are fed insect larvae containing unsuitably high levels of tannin because ingested larvae were feeding on older leaves.
Great tits are known to rip out the insect gut before feeding larvae to their young which means they may be able to feed safely on older and more developed larvae, these having picked up correspondingly higher levels of tannin by feeding on older oak leaves. The same behaviour is not documented for blue tits.
FUTURE FOCUS ON WOODLAND
As OPM moves out of London and into the rural landscape increasing numbers of woodland owners will be presented with SPHNs (Statutory Plant Health Notices) for their woodlands to be sprayed with insecticide during April and May. They should be aware of what is potentially at stake for Lepidopteran (butterfly and moth) species, whose larval periods may span at least part of the period when spraying takes place, and birds which predate the larvae.
Patrick Mannix, owner of Sandhurst Copse and Sheepwalk comprising 84 acres of partly-ancient, mixed-broadleaf woodland in the Surrey Hills Area of Outstanding Natural Beauty, has previously articulated the problems and the impossibility of attempting to spray woodland such as his own. An established cluster of OPM infestation is just a few kilometres north of Patrick’s woodland in the Guildford district of Surrey. Spraying isolated oak trees in parkland is one thing but spraying oak trees in mixed woodland is an entirely different matter, especially in relation to insecticide drifting onto adjacent forest trees. And the redistribution of insecticide either by spray run-off or wash-off by rainfall onto plants below including lower-storey small trees and shrubs and ground-cover plants.
Thus, in addition to oak-feeding larvae like green tortrix moth and winter moth larvae, potentially at risk are larvae of butterflies and moths which feed on high forest tree species, small trees and shrubs in the under-storey and forest-floor plants. Clearly not at risk are those with a larval period which occurs during summer after any spraying has taken place. However, there are a number of moths and butterflies, which due to the specifics of their life cycle, may have larvae feeding on a variety of plants during the mid-April to early June period when spraying may be carried out.
Lepidopteran larvae traditionally feeding during April and May include oak eggar moth (Lasiocampa quercus), which as the name suggests feeds on oak leaves, while the spindle tree (Euonymus europaeus) features among the food plants of the ruby tiger moth (Phragmatobia fuliginosa). Fox moth larvae (Macrothylacia rubi) have a varied diet including bramble (Rubus fruticosus) and sallow (Salix species) depending partly on the environment and ecosystem where found. The large yellow underwing moth (Noctua pronuba) may be found feeding on foxglove (Digitalis purpurea). Feeding by larvae of pearl-bordered fritillary (Boloria Euphrosyne) and brimstone (Gonepteryx rhamni) butterflies on, respectively, common dog violet (Viola riviniana) and alder buckthorn tree (Frangula alnus) may overlap with the period of spraying against OPM; pearl-bordered fritillary in the second half of April and brimstone butterfly in last half of May and into early June.
The situation is clearly complex with around 2,500 species of moth and some 60 species of butterfly in Britain, and complicated by varied life cycles with different species variously overwintering as the egg, larva and pupa, or even the imago (adult) stage in the case of comma (Polygonia c-album) and peacock (Aglais io) butterflies. This means larvae can be present throughout the year.
Be mindful and be warned before you treat the landscape to lashings of insecticide, even with a relatively well-targeted and environmentally benign, biological insecticide like BTK, which, after all, is based on a naturally occurring, soil-borne bacterium. Larvae of any lepidopteran species (butterfly or moth) feeding on the foliage of a designated food plant species at the time of spraying are potentially at risk.
BIRDS WILL FAIL TO CATCH THE EARLIER WORMS
Recent research conducted by scientists from the Royal Society for the Protection of Birds and the Universities of Exeter and Edinburgh strongly suggests British native resident birds in the family Paridae, including blue tit and great tit, could face a famine of the moth larvae that feed on native English oak leaves and on which they in turn rely on to feed their young. This, they claim, will be due to English oak re-foliating increasingly earlier (under the influence of climate warming), causing larva populations to peak before the nestlings hatch, and thus causing ‘late birds failing to catch the earlier worms’.
I always thought that every component of the oak tree ecosystem (e.g. leaf buds, moths and birds) reacted in unison to yearly variations in the timing of spring. However, the authors of this research say the birds may not be able to sufficiently adjust their breeding cycles to match the dynamics of associated plant and insect life as the climate progressively warms. All that said, the impact of insecticide spraying against OPM on native Lepidopteran larvae appears to pose a greater risk and danger to native titmouse birds in the foreseeable future.
*Author’s note:
Entrance to the blue tit nesting hole should be as small as possible to reduce competition from larger birds like tree sparrows and house sparrows. As a child I clearly remember being told the entrance hole of a nest box for blue tits should be the same diameter (28.5 mm) as a florin (2 shilling piece in pre-decimal money), close in size to today’s 2p coin at 25.91 mm.
REFERENCES:
Burgess, M.D. et al. (2018) ‘Tritrophic phenological match-mismatch in space and time’. Nature Ecology & Evolution (2018). doi:10.1038/s41559-018-0543-1
Hoppit, A. (2019) OPM Workshop. ‘Friend’s House’, Euston, London. 9th October 2019. Forestry Commission.
Hoppit, A. (2020) OPM Programme Update 5th May 2020. Forestry Commission. www.forestresearch.gov.uk/opm
Mabbett, T.H. (2015) ‘Oak processionary moth at a crossroads’. Quarterly Journal of Forestry, April 2015 Vol 109 No 2 122–128
Mannix, P. (2018) ‘Oak processionary moth – we need a change of direction says Patrick Mannix’. Forestry Journal, July 2018.
Moxon, G. and Carpenter, B. (2014) Herridge’s Copse and Horn’s Copse (control), near Pangbourne, Berkshire. 2013 monitoring of impact of OPM spraying on bats and birds. Bioscan Report No E1771R1. Commissioned by the Forestry Commission, UK.
Townsend, M. (2014) Interim report on monitoring the impact on non-target Lepidoptera of aerial spraying to control OPM in Pangbourne, Berkshire in 2013–14. Forestry Commission UK, January 2014.
Forestry Journal remains dedicated to bringing you all the latest news and views from across our industry, plus up-to-date information on the impacts of COVID-19.
Please support us by subscribing to our print edition, delivered direct to your door, from as little at £69 for 1 year – or consider a digital subscription from just £1 for 3 months.
To arrange, follow this link: https://www.forestryjournal.co.uk/subscribe/
Thanks – and stay safe.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article