Monday, May 26, 2014

A Neighbor With a Lot of Gall

Growing next to the sidewalk just around the corner is an eyecatching oddity. These should be leaflets of an ash tree's leaf, but instead they bear an uncanny resemblance to a butterfly's chrysalis.

This is a normal ash leaf, with a bunch of leaflets.

Here, the development of each leaflet has been altered to make the chrysalis-like structure. Sure looks like something should be living inside.

Open it up to find the midges living the good life, well protected by the distorted growth of the leaf, and with ample food to reach maturity in a month or so. The larvae drop to the ground, overwinter in the soil, then emerge in the spring as adults to climb up the ash tree in time to lay eggs in the emerging leaves. The midge injects the rapidly growing leaf with a hormone-like chemical that causes the leaf to grow in a conveniently distorted manner. (Nice description found here.)

The latin name for the midge, a kind of fly, is Dasineura tumidosae. Considerable internet searching yielded no common name other than "ash gall". Here's some more info.

One of the more common galls on ash is the ash flower gall, which can be seen in the canopy in the spring, before leaves emerge. Thinking back to our hike up Baldpate Mountain a month ago, I realized we had been looking up at an ash that looked like it had growths on the twigs high up, wondering what it was. Probably ash flower gall.

A question to be asked is how closely tied is the fate of all of these organisms to their host plant. If our ash species succumb over the next ten or so years to emerald ash borer, whose presence in New Jersey was documented for the first time this past week (more on this later), will all of these insect species find other hosts, or disappear along with the ash? A tree like the ash, which can develop considerable stature and grandeur, feeds an entourage of less charismatic creatures. At least one of them I can now count as a neighbor. 

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