OPM – versatile larvae and insecticide inadequacy confound control

OAK processionary moth or OPM (Thaumetopoea processionea) was first found in 2006 on cypress oaks (Quercus robur fastigiata Koster) planted the previous year at amenity sites in the London Boroughs of Richmond and Ealing after being imported from mainland Europe.

Action was taken against the pest by UK plant health authorities, although exactly what that entailed is now difficult to ascertain. However, it clearly did not work, because within just one season OPM was spreading in west London. By 2011, OPM was established as a breeding population in nine contiguous London Boroughs and the Forestry Commission (FC) officially abandoned its policy of control and eradication in favour of one of containment.

That also failed and, by 2019, every single one of London’s 32 boroughs was harbouring a breeding population of OPM. This alien invasive pest has continued to spread further into the Home Counties of Surrey, Kent, Essex, Hertfordshire and Buckinghamshire, and as far out as East Berkshire (Windsor and Maidenhead, Bracknell Forest and Wokingham) although these are not the first OPM findings in the county of Berkshire.

In 2010, OPM was discovered at a new, gated development in Pangbourne (West Berkshire) on newly-planted oak trees imported from Europe. The Forestry Commission subsequently embarked on a programme of insecticide spraying (including aerial application) and nest removal in a concerted effort to eradicate the pest.

Whether or not this was achieved is unclear. No new nests have been found in Pangbourne in recent years, although pheromone traps deployed in summer/early autumn have regularly entrapped a handful of male OPM moths. However, that is now largely academic because at current rates of spread out of London, OPM will be in Pangbourne again within one or two seasons.

The rate of spread since 2006 has averaged 5 km/year despite an extensive and escalating insecticide spray programme. In 2017, Forestry Commission contractors sprayed 17,687 trees across 350 sites. During 2018, 600 sites were sprayed, representing a 71 per cent increase. The latest figures for 2019 show 24,250 trees were sprayed at 1,052 sites, trebling the number of sites requiring treatment and amounting to 37 per cent more trees sprayed annually compared to 2017 (Hoppit, 2019).

However, even this does not give an accurate measure of overall pest numbers, the scale of spraying and the amount of pesticide poured into the environment. Infested trees are given two applications 10 to 14 days apart, so if 20,000 trees are treated in a single year, the number of spray applications is double this figure at 40,000.

Forestry Commission figures for numbers of sites and trees sprayed relate only to the control zone for which statutory plant health notices (SPHNs) are issued. There is a sizable control zone covering the western side of London and North Surrey where SPHNs are no longer issued. Organisations like Royal Botanic Gardens Kew and City of London Corporation do spray, although the majority of other landowners and stakeholders do not.

The logistics and economics of the current OPM situation have forced the Forestry Commission to modify its OPM management policy within the control zone. Spraying is now carried out based on the risk to public health. For instance, public parks areas with a lot of footfall receive priority spraying, while pest-infested woodland inside the park may be left untreated.

The standard recommendation for OPM control using BTK is two sprays applied 10 to 14 days apart, although some sources claim the persistence period of BTK spray deposits on leaves is only three to seven days.

WHY CAN’T OPM BE CONTAINED?

Two basic factors underpin the ongoing failure to control and contain OPM. This is a highly versatile insect pest and the insecticide of choice lacks the necessary levels of potency and persistence to eradicate OPM from affected areas.

Bacillus thuringiensis var. kurstaki (BTK) is the Forestry Commission’s natural insecticide of choice for all the right reasons of safety and environmental protection. BTK is a biological insecticide which comes in an aqueous delivery system for application as a water-based foliar spray. As such, BTK spray deposits are easily washed off of oak leaves by rainfall and, as a living entity, BTK is prone to inactivation by heat and UV light. These pressures will clearly impact on persistence, which is why BTK insecticide product labels recommend a follow-up spray application 10 to 14 days after the first spray, while target larvae are still at the first or second (L1 or L2) instar stage.

The extent of BTK inactivation and loss will clearly depend on environmental conditions (e.g. rainfall, temperature and hours/intensity of sunlight), but figures presented by the USDA Forest Service indicate how a spray interval of 10 to 14 days might be incompatible with a much shorter persistence of BTK on leaf surfaces. Quoting from a manual produced by the manufacturers of two formulations of BTK, they say: “BTK is sensitive to sunlight and heat and will only persist on foliage for 3 to 7 days,” (USDA Forest Service).

BTK activity in relation to the stage of larva development is another factor almost certainly confounding OPM control. Larvae beyond the second instar (L2) stage develop a gut wall which is too tough for the stomach-acting toxin in BTK to punch through and kill the insect.

This is why both applications must be made when larvae are still at the L1 or L2 stages. However, a 10- to 14-day spray interval is sufficiently long for larvae to be at the L2 stage during the first spray application, but at the L3 or even L4 stage when the follow-up application is made, especially if high temperatures prevail during the spray interval period. Larva development is affected by temperature, with warm spring conditions hastening hatching of larvae from the egg stage and accelerating larval growth and development through the six instar stages.

Larvae much beyond the L2 instar stage are too resilient to be controlled using BTK biological (bacterial) insecticide (picture courtesy of Bartlett Tree Experts).

Oak tree budburst and re-foliation is also affected by temperature. BTK acts solely through larval ingestion of sprayed oak leaves so there is no point spraying oak trees (with BTK) until there is sufficient foliar cover to intercept the spray droplets. Research carried out in the Netherlands has established the requirement for a minimum 50 to 60 per cent of full re-foliation to facilitate adequate spray coverage.

These interrelated and confounding factors combine to present spray operators with difficult decisions in relation to the start and end of a spray programme with 2018 as a case in point. Due to unusually cold conditions in early spring, L1 larvae were not recorded hatching in London until 16 April, 2018, at least one week later than in previous years and when trees were nowhere near the 50 to 60 per cent re-foliation requirement. However, due to exceptionally warm conditions thereafter, hatching larvae progressed through the instars much more quickly than usual. The Forestry Commission said it expected the most well-developed larvae to have attained the fourth (L4) of six instar stages during the third week of May.

The OPM spray programme started the week beginning 23 April, 2018, and came to an end on 8 June, 2018. Due to the late spring, a considerable proportion of first sprays would have been applied to trees which had not attained 50 to 60 per cent re-foliation, while many follow-up sprays, and certainly those applied from mid-May onwards, were applied when larvae were beyond the second instar stage (L2) and therefore increasingly difficult if not impossible to kill using the BTK biological insecticide. Every year about one third of OPM-affected sites sprayed with BTK during April to June are recorded with a residual pest population in the summer months after spraying.

Many in UK forestry and arboriculture mistakenly believe Bacillus thuringiensis is a new biological insecticide, but the bacterium, discovered in 1902, was first used as a biological insecticide in 1938 and has been used on a commercial scale in agriculture and horticulture for at least half a century.

BTK ticks most of the boxes for operator safety and maintenance of environmental integrity, but the biological insecticide has its limitations. BTK has proven its worth as a component of IPM (integrated pest management) systems around the world for many years, but is clearly not a pest-eradication tool and does not perform in the classic pest-control context or to an equivalent expectation. The extent to which OPM continues to spread in London and the surrounding Home Counties is testament to that.

OPM larvae have the capacity to cause significant defoliation of the canopy, weakening oak trees and reducing their resilience to a range of abiotic factors (e.g. drought) and other biotic factors (e.g. oak mildew disease). (Picture courtesy of David Humphries, City of London Corporation.)

Aerial spray application in Italy showed the benefits of BTK as a pest management tool but also its limitations for pest-eradication. A trial conducted on Turkey oak (Quercus cerris) woods in Tuscany in early spring 2007 showed it was possible to significantly reduce larva numbers using BTK at a rate of 31.75 BIU ha-1 at the time of bud opening and when L1 and L2 larvae were present. Thirteen days after treatment, larval mortality recorded across all treated plots varied from 75.05 per cent to 96.24 per cent, the insecticide having substantially reduced insect numbers, albeit nowhere near to eradicating the insect pest.

Conventional wisdom on egg hatch is largely based on the concept of ‘temporal synchrony’ whereby an early, warm spring simultaneously hastens larval hatch and oak tree re-foliation so that neonate (newborn) L1 larvae will always have an available food source. But research in Germany has shown how neonate larvae of T.  processionea are generally well adapted to variable between-tree and within-tree budburst phenology and can withstand starvation periods of up to three weeks while searching extensively for suitable buds to feed on. Under both low (4°C) and high (16°C) temperature conditions, the majority of neonate larvae can survive starvation for more than two weeks.

Spring 2018 was measurably late. The oak trees seen here on the northern outskirts of London on 15 April were barely breaking bud. L1 larvae were observed hatching in London on 16th April. The Forestry Commission’s OPM spraying programme commenced seven days later although it is unlikely that most oak trees were sufficiently re-foliated for good interception of spray droplets.

Spray operators clearly need to consider this, especially if electing to use a contact-acting chemical insecticide in response to the appearance of neonate larvae before budburst and re-foliation. They may have reasonably assumed that larvae hatching before budburst will starve to death, but according to German research this is not the case.

Insecticide inadequacy in the face of a highly versatile and resilient insect pest has compromised control. Rash assumptions have clearly been made about the capabilities of BTK biological insecticide as a pest-eradication tool. The Forestry Commission pursued a policy of OPM eradication for five years while using BTK as the insecticide of choice, though in retrospect this was clearly never going to happen.

References
Hoppit, A. (2019) OPM Workshop. ‘Friends House’, Euston, London. 9 October 2019.
Mabbett, T.H. (2015) ‘A watershed year for oak processionary moth’. Forestry & Timber News, August 2015 pp-22–23.
Mabbett, T.H. (2019) ‘OPM is reaching out to touch more arborists’. essentialARB Issue 73, October 2019.
Meurisse, N. Hoch, G. Schopf, A. Battisti, A. Gregoire, J.C. (2012) ‘Low temperature tolerance and starvation ability of the oak processionary moth: implications in a context of increasing epidemics’. Agricultural and Forest Entomology, 14 (3): 239–250.
Roversi, P.F. (2008) ‘Aerial spraying of Bacillus thuringiensis var. kurstaki for the control of Thaumetopoea processionea in Turkey Oak Woods’. Phytoparasitica 36(2):175–186.
USDA Forest Service. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev7_015300.pdf
Wagenhoff, E., Blum, R., Engel, K., Veit, H., Delb, H. (2013) ‘Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst’. Journal of Pest Science 86:193–202.

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