MULTIPLE factors related to insect pests and insecticide combine to confound control of OPM, but are there additional factors related to oak tree phenology and, especially, the rapid expansion of newly formed leaves?

Research was carried out in spring 2019 to see whether oak leaf expansion could be diluting deposits of insecticide on leaf surfaces, this contributing to ongoing failures in OPM control. The rate of re-foliation of a mature English oak (Quercus robur) at individual leaf level was monitored during April and early May, 2019, from budburst until the first-formed leaves were fully expanded.

Buds broke on 10 April, with the first leaves appearing as measurable entities by 14 April. Leaf measurements as length (L) and width (W) (at mid-point of the leaf) were taken every three to five days for these first-formed leaves, until they had attained full expansion. Leaf dimensions as L x W; L2 and W2 have been used to construct regression equations which are used as non-destructive methods of leaf area calculation.

The Forestry Commission said spraying would begin during the third week of April 2019 (depending on the stage of re-foliation). The first observed larval hatch date was expected to be earlier than the 16 April recorded in 2018. In the event, it was two weeks earlier, on 1 April, 2019.

18 April, 2019, looked to be a likely start date, with follow-up spray applications on or around 1 May. This turned out to be a time of rapid leaf expansion, with first-formed leaves increasing by a factor of 10 between 18 April and 1 May. Full leaf size was attained by 6 May. More leaves emerged during this period and, by 1 May, new shoots were bearing eight to 10 leaves at various stages of development (Table 1).

If this tree had been sprayed on 18 April with Bacillus thuringiensis subsp kurstaki (BTK), only the first-formed leaves on each shoot would have received insecticide, with surface deposits of BTK decreasing thereafter on a unit leaf area basis corresponding to leaf expansion. That is by a factor of 10 between 18 April and 1 May. As such, OPM larvae would have to consume 10 times an amount of leaf tissue on 1 May to match the insecticide dose ingested on 18 April, assuming no loss or deactivation of BTK deposit had occurred during the intervening period. Larvae feeding on new leaves which appeared after the first spray and before the second spray would ingest no insecticide, unless insecticide had been redistributed (washed off of one leaf and re-deposited on another) during rainfall.

Forestry Journal: First-emerged oak leaves expanded by a factor of 10 during the period from 18 April to 1 May 2019. Any insecticide deposit delivered onto these leaves on 18 April would have been diluted on unit area basis by the same factor of 10.First-emerged oak leaves expanded by a factor of 10 during the period from 18 April to 1 May 2019. Any insecticide deposit delivered onto these leaves on 18 April would have been diluted on unit area basis by the same factor of 10.

Leaf phenology factors may also play a part in the relative resilience of BTK deposits on the upper (adaxial) and lower (abaxial) surfaces of oak leaves. BTK deposits on the lower surface of the leaf will be shielded from the full effects of UV light and the direct wash-off effect of rainfall, compared with BTK deposits on the corresponding upper surface of the leaf.

Stakeholders would appear to be in a ‘Catch-22’ when it comes to deciding the start date of spraying. Spray too early (mid-April) and insecticide deposits may be significantly reduced (on a unit leaf area basis) by the production of new leaves and rapid leaf expansion. Delay spraying until the end of April/early May, when production of new leaves and their expansion to full size is essentially complete, then those larvae which hatched in late March and early April may already be at the L3 (third instar) stage and thus increasingly unaffected by BTK. The first recorded larval hatch in London during spring 2019 was on 1 April, with strong indications that the first recorded hatching date for 2020 will be at least as early.

Oak leaf phenology appears to be compounding an already difficult, if not impossible situation for spray operators trying to control OPM using BTK insecticide, which remains on the leaf surface and is subject to degradation and loss from UV light, heat and rainfall. If nothing else, a 10- to 14-day interval between paired applications of BTK would appear to be at odds with all the evidence, including that from North America, where USDA Forest Service claims BTK activity on the leaf surface persists for no more than seven days after spraying. Indications at the time of writing were for an exceptionally early first hatch date, with the last week of March 2020 strongly on the cards.

Forestry Journal: Oak leaf expansion rate from leaf emergence to full leaf size**Oak leaf expansion rate from leaf emergence to full leaf size**

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