Hickory Galls Induced by Phylloxera

Published: May 24, 2012

Stem galls on shagbark hickory. Photo credit: Chris Starbuck

Phylloxera nymphs and alates inside a shagbark hickory gall. Photo credit: Michele Warmund

Bright red galls, as large as one inch in diameter, are prevalent this season on hickory (Carya) trees. An easy way to spot the galls is to find dead terminal branches. At the base of these branches, there will be one or many clustered galls. One of the largest galls is the hickory pouch gall, which is induced by Phylloxera caryaecaulis. Galls first appear pale yellowish green and then turn red before opening (in early May this year). At maturity, galls become leathery and black. Not only does Phylloxera caryaecaulis affect many native hickory species, but also pecan. This insect produces smooth galls on some hickories, including shaglbark (C. ovata), shellbark (C. laciniosa), pignut (C. glabra), bitternut (C. cordiformis), and pecan (C. illinoensis). However, it has also been reported that P. caryaecaulis induces spiny galls on mockernut (C. tomentosa) and bitternut (C. cordiformis). More recently, at least twenty-nine species of Phylloxera have been identified that cause galls on Carya species. When infestations of Phylloxera are severe for multiple years, the defoliation occurs on affected limbs and entire branches may die.

In early spring, the Phylloxera fundatrix (wingless female) emerges from an overwintering egg and feeds on new leaves and petioles, which induces gall formation. As feeding continues, the gall encloses the fundatrix. Eggs deposited inside the gall hatch in six days and become the alate (winged) form. Nymphs of the alate form have three instar stages in fifteen days while feeding on the inner gall tissue. The bottom portion of the gall splits open and alates leave the leathery galls and deposit amber-colored eggs on the underside of nearby leaflets adjacent to the veins. Male and female sexuales hatch from these eggs and mate. Each female deposits one overwintering egg in an old gall or in a bark crevice from which a fundatrix develops in the spring and the life cycle continues. Affected wood with new galls can be pruned from the tree for suppression of this insect.