Some of the leaf is brown

(And the sky is not always gray)

Are there brown or gray patches on the leaves of your plants?  Does the leaf appear intact, but no longer green?  And is there no external sign of any insects?

Your plants might have leaf miners.  These are insects that have small and/or flattened larvae that live between the upper and lower epidermis of the leaf, mining (eating) the tissue in between.  It is an impressive adaptation, and there are several kinds of insects that do it (some flies, moths, beetles, and sawflies).

If the miners are still living in the leaf, the larvae might be obvious between the translucent layers of cells, or you can pry the leaf open and find them (what you do with them is up to you).  But it is quite likely that the miners have left the mine.  They might have pupated in the mine and emerged as adults.  Or they might have emerged as larvae and sought a place to pupate elsewhere.  

There are several species of leaf miners that feed on goldenrods, but one of the most common is the beetle Microrhopala vittata, the goldenrod leaf miner (in the family Chrysomelidae, the leaf beetles).  I found some of them in my examination of goldenrods back in the 1970s.  Naomi Cappuccino, studying goldenrods at multiple sites, including some near where I worked, often (but not always) found many of these beetles.  They were spotty, and their numbers fluctuated widely from year to year, for reasons that were hard to track down.  But sometimes, they were really abundant!

Walter Carson experimentally reduced beetle abundance in places where there were a lot of beetles.  When he did so, the goldenrods grew much better.  Where he left the beetles in place, the goldenrods grew poorly.  So these beetles matter, at least sometimes and in some places, to goldenrods.  

These mines on Solidago juncea (early goldenrod) certainly resemble those of the goldenrod leaf miner, though I’m not sure how distinctive the mines of the different species are.  I found them only on this tiny patch of stems, even though there are quite a few others within about 100 meters (though down a shady road – does shade affect the movement of the adults?).

I haven’t noticed any adults of the leaf miners, but it’s only June.  Stay tuned.

Further reading:

Naomi Cappuccino 1991

Mortality of Microrhopala vittata (Coleoptera: Chrysomelidae) in Outbreak and Nonoutbreak Sites 

Environmental Entomology 20 (3): 865–871

Walter P. Carson, Richard B. Root 2000

Herbivory and Plant Species Coexistence: Community Regulation by an Outbreaking Phytophagous Insect

Ecological Monographs 70 (1): 73-99