Oak Marginal Leaf Fold Gall

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So-called marginal leaf fold galls are appearing on oaks in the "red oak group" in southwest Ohio.  The galls appear as rolled or folded leaf margins and are produced by a gall-making midge fly, Macrodiplosis erubescens (Family Cecidomyiidae).  As with the vast majority of oak galls, the leaf fold galls cause no appreciable harm to the overall health of affected oaks.  However, the gall has become notorious in recent years for its connection to a non-native predaceous mite (Pyemotes herfsi) that may feed on the gall-making midge fly larvae (maggots).  The mite has been implicated in itchy outbreaks in past years in Cincinnati, Columbus, and Chicago as well as various locations in Kansas, Nebraska, Oklahoma, and Texas.

The tiny mites use their piercing mouthparts (chelicerae) to inject a powerful neurotoxin to dispatch the midge fly maggots. 

Midge Fly Larvae Inside Oak Marginal Fold Gall

They then feed vampire-like by extracting the essence of maggot.  When all of the maggots have been killed, or when the surviving maggots drop from the galls to the ground to complete their development, the starving mites drop from the trees in search of food.  Unfortunately, the famished mites become "itch mites" if they land on people.  They will bite and their neurotoxin induces itchy skin welts that may last for several days.  Most bites occur beneath shirt collars on the back of the neck.  Unfortunately, research has not revealed conditions that favor high mite populations, so there is no way to predict itchy outbreaks.