Lerp and leaf damage - tricky...

The Lerp damage indicator in the BCM method has presented some difficulties of interpretation in the field. Originally designed to pick up significant infestations of lerp, difficulties arise if you have a good eye, and a good pair of binoculars, and are able to detect low incidence of lerp on leaves. In which case, a tree might score a high incidence of lerp while having only a small trace incidence on each leaf, and a resulting small impact on the tree.

The other difficulty is the interpretation of scars and blemishes on the leaf surface. How do we know if it was due to lerp or due to something else?

There are several theories out there about how lerp works. One is that lerps preferentially attack trees that are stressed for some other reason, e.g. water stress, which somehow reduces their chemical and / or physical protection to insect attack. The other is that lerps bloom in particular areas due to a unique combination of microclimatic conditions, e.g. high moisture and high temperatures, and then set out on their merry path from wherever that happened to be.

In all the sites that I have done over the years, only 2 or 3 times have I come across trees with obvious "blooms" of snow-white crystalline lerps. It was so obvious, and I had been so tortured by the question of how to measure lerp, that all I could say was "ah, now this is lerp!"

Apart from dieback, it would seem to make sense to include a general measure of leaf surface damage on the leaves that remain in a canopy, whether this is due to skeletonising insects, fungus, lerp or other insects. I tend to think of it in terms of photosynthetic capacity, i.e. 100% potential photosynthetic capacity - % dieback - % leaf damage = current photosynthetic capacity. I don't know what the literature would say about such a simple formula, but in terms of general indicators of canopy health, it would seem reasonable.

We are looking at ways to score general leaf surface damage to overcome some of the ambiguity involved with the existing Lerp damage indicator. This might involve estimations of % of leaves in the canopy with "Light" "Heavy" or "Severe" leaf damage, where "Light" = <10% of leaf surface area, "Heavy" = 10-50% of individual leaf surface area, and "Severe" = >50% of individual leaf surface area. You might recognise this as a similar scoring system to Grazing Pressure... We haven't got to the point of proposing benchmarks for a measure such as this, but I think it would be possible.

Again, it would not be our intention to make the field measurements longer or more complicated. Once you have your eye in, it should be fairly quick to put a % on each of these levels of damage for each of the 10 trees in the tree map.

I have found it quite quick to estimate, especially working backwards from "Severe" to "Heavy" to "Light", since "Severe" is really obvious, and then "Heavy" is anything relatively significant looking in what remains, and the rest is usually "Light" by default.

Would be interested in your thoughts. How have you approached measuring "Lerp Damage" in your sites? How important do you think it is an an indicator? Do you have any theories on how lerp infestations arise?