Leaf Miners in the Understory

by Carl Strang

Yesterday I reported on one of my herbivory studies at Maple and Meacham Grove Forest Preserves. Today I have the data for the first part of the other study, a decades-long following of 4 leaf miner  genera in sugar and black maples in the understories of the two forests. While attempting to photograph confused ground crickets at Warrenville Grove, I had noticed a high incidence of tent mines, produced by the micro moth Phyllonorycter clemensella.

This photo from Warrenville Grove shows many leaves with one or more Phyllonorycter mines.

Consequently I was wondering if I would find a lot of mines at my study preserves this year. In fact, Phyllonorycter incidences were relatively high in both forests, in 15 percent of understory leaves at Maple Grove and 4 percent at Meacham. Statistically there were more at Maple than at Meacham, which has been true over the years, probably because of more intensive management at the latter site (controlled burning, and culling of maple saplings). Numbers were not different from last year at Maple Grove, but there was a statistically significant increase at Meacham for this species, possibly because there was no burn last year.

The other leaf miners were present in lower numbers that were indistinguishable from last year’s values. The two species of moths in genus Caloptilia, which leave their mines early and construct little cones or boxes in the leaf lobe tips for most of their development, were more abundant at Maple Grove (8 percent incidence) than at Meacham Grove (2 percent of leaves had them). While 3 percent of leaves at Maple Grove had blotch mines of Cameraria saccharella (another tiny moth), none of the 300 leaves in the Meacham Grove sample had any (only one had a mine last year). The fourth mine is distinctive in having a winding linear form.

The linear mine is visible in the lower part of this maple leaf at Warrenville Grove. I have not reared this one; it probably is produced by a caterpillar of the non-native moth Stigmella aceris.

This one was present in low numbers that statistically were indistinguishable between the preserves (8 leaves at Maple, 1 at Meacham). In November I’ll return to assess canopy incidence of these moths.

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Leaf Miners 3

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

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.

Maple Leaf Miners: Introduction

by Carl Strang

 

Leaf miners are amazing insects, so tiny that they live between the upper and lower surfaces of leaves. They eat the succulent interior leaf tissues, and as they grow, the area they have depleted of these tissues expands to form a shape that is easy to see and is distinctive according to the insect species. Here are two common forms, linear mines in a cow parsnip (Heracleum) leaf, and blotch mines in a leaf for you to identify (note: this is a trick question! Plant identification given below).

 

Linear mines in Heracleum leaf

Linear mines in Heracleum leaf

 

Blotch mines in mystery leaf

Blotch mines in mystery leaf

 

 

There are flies and beetles that are leaf miners, but the ones I have been studying for more than two decades are minute moths whose caterpillars do the mining in maple leaves. The trees, at Maple Grove and Meacham Grove Forest Preserves, are either black maples (Acer nigrum), sugar maples (A. saccharum), or hybrids of the two. In the mid-1980’s, when I began studying leaf eating insects in these two forests, my attention was drawn by the large numbers of leaf miners in the understory maples on both study areas.

 

I was able to sort out five species of insects doing the damage. They include Caloptilia bimaculatella and C. packardella, of the moth family Gracilariidae. Their larvae begin with tiny mines which they soon leave, and their major growth takes place in folded, cone-shaped shelters (called here “boxfolds”) which the larvae construct at the tips of leaf lobes.

 

maple-leaf-mines-b

 

In the photograph, the boxfolds are unrolled and flattened to show how the green tissue has been scraped away by the tiny caterpillars’ mandibles. I have found both species on both study areas, and also two more members of the same family, Cameraria saccharella and Phyllonorycter clemensella. Cameraria form irregular blotch mines on the upper surfaces of the leaves, and Phyllonorycter produce tent-like mines on the under surfaces. I have not reared adults or identified larvae from the fourth mine type, a distinctive linear mine form, but it likely is the exotic (European) Stigmella aceris, of the moth family Nepticulidae.

 

There are a lot of questions that can be addressed in a system like this. For example:

  • Do the leaf mining species interact in such a way that they either avoid one another, occupying separate leaves as a rule, or do they congregate, clustering onto particularly attractive leaves?
  • 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)?
  • How does restoration management practice affect these organisms?
  • Do the moths have the same impact on the canopies of large trees as they have in the understory?

 

I’ll address these questions in future posts.

 

Mystery plant with blotch mine: did you recognize it as a rare, 4-leafleted poison ivy leaf? Probably most would regard this as less lucky than a four-leaf clover.

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