by Carl Strang
A few weeks ago I introduced my ongoing study of leaf miners in maple leaves, moths so tiny that their caterpillars live between the upper and lower surfaces of leaves. I left a few questions unanswered, and I want to return to what I have learned so far in attempting to answer them.

Cameraria saccharella adult specimen
- These two study areas are separated by many miles of suburbs. Do the populations of the leaf mining species go up and down together on the different study areas (which might reflect responses to climate as it varies between years), or do they fluctuate independently (which might indicate biological regulation of populations)?
Numbers of these insects have been so low since my return to this study in 1996 that drawing conclusions is problematic. However, there have been several occasions when proportions of leaves with one type of mine have been significantly higher on one preserve than the other. In the case of the boxfold species, Caloptilia, there were appreciably more at Meacham Grove in 1996, and at Maple Grove in 2004 and 2008. The tent-mine-forming Phyllonorycter has consistently been more abundant in Maple Grove samples over the years, and this difference has been statistically significant several times. Other miners have been present at such low levels that for all practical purposes they have been the same.
- How does restoration management practice affect these organisms? These maples are the main target of management in these forests, because they are so shade tolerant that they can push out all the other plant species over time. Once they were kept in check by the rare fires that reached even the moister woodlands. Now, selective removal of maple saplings and controlled burns are used to restore higher diversities of other plants, of the many insects that depend on those plants, and the consumers of those insects.
The potential impact of management on these leaf miners has to be speculative, as little is known about their natural history. Meacham Grove has been managed more intensively than Maple Grove, though both preserves have received some attention. Is this why Phyllonorycter has remained so low in numbers at Meacham? Though the differences are not statistically significant, in almost every year the numbers are slightly smaller at Meacham than at Maple Grove for Cameraria blotch mines and (presumed) Stigmella linear mines, too.
A controlled burn of the leaf litter at Meacham Grove in the fall of 2007 was followed by practically no leaf miners of any kind in understory maples in 2008, but their numbers were so low in 2007 that any connection to the burn would be hard to argue. If the moths overwinter in that litter, the burn may have knocked them down enough that I may see very low numbers there for several years.
- Do the moths have the same impact on the canopies of large trees as they have in the understory?
All of my 1980’s focus was on saplings in the understory. In recent years I have attempted to compare the understory and canopy trees, first by looking at leaves on the branches of large trees that could be reached from the ground, and second by measuring incidence of mines on fallen leaves (the vast majority of which come from the canopies of the larger trees; also, the larger trees lose most of their leaves before the saplings do, so I can improve the distinction with careful timing).
In 2005 I looked at leaves on low branches of larger trees, and found no difference in any of the leaf miners from what I measured in understory plants. This is not necessarily a trivial comparison; large trees have greater resources with which to create chemical or physical defenses, and might have had fewer miners.
In 2006-8 I have been looking at fallen leaves as a measure of leaf miners throughout the forest. After three years I have only tentative conclusions. It is beginning to appear that tent-mining Phyllonorycter prefers the understory, supporting the possibility that management removal of maple saplings has had an impact on that species at Meacham Grove (some do live in the larger trees, however, and the number of leaves in those trees is so huge that a 1% incidence level still represents a lot of moths). The other miners have not shown differences between canopy and understory, though it should be said that blotch-mining Cameraria and linear-mining presumed Stigmella are at such low levels at Meacham Grove that nothing really can be said about them from these few years.
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