Phylloxerans are closely related to the aphids. As such, this insect is sometimes also referred to as the hickory leafstem gall aphid. 29 species of phylloxeran are found in this genus, but P. caryaecaulis is a common species that causes galls that can grow up to almost an inch in diameter. Additional species cause gall formation on hickory. The life cycle of this insect takes approximately one year to complete. Overwintering eggs hatch and the stem mother (fundatrix) emerges. Depending upon geographic location, egg hatch may begin in mid-to-late April, at which point the young stem mothers begin to feed near the swelling buds. Small pits are then formed around each insect, which, by early May, becomes completely enclosed in green gall tissue. By mid-May, galls are fully formed. As galls grow in size, the stem mothers lay 1,000 or more eggs inside each gall cavity. Eggs hatch by the end of May, and galls split open, allowing the now mature, winged insects to emerge. Adults then move to the undersides of host plant leaflets, where yellow eggs that are laid in that location hatch and become males and females that sexually reproduce. Females then lay single amber-brown eggs in or on old galls. These eggs overwinter (Johnson and Lyon, 1991).
Galls are nearly 1 inch in diameter and found on the cambial tissues of shoot bark, petioles, and occasionally midveins of hickory leaflets. Galls are globose in shape and can be identified by their size, location on their host plant, and the location of the exit hole from which the phylloxerans emerge (in the center of the gall, facing away from the plant). Galls eventually dry and become hard and woody, and may remain on the host plant for years. While unsightly and sometimes occurring in large numbers, damage to the overall health of the host plant is not considered to be lethal.
Monitor shoots, petioles, and midveins of leaflets for early gall formation in May. Scout for old, spent, woody galls from previous years at any time.
Pruning off galls before they open and the adults emerge can be a helpful cultural management strategy for the hickory leafstem gall phylloxera.
Little information is available about the effectiveness of the natural enemies of this species.
Not necessary. Dormant oil may be applied to stems to manage the overwintering eggs prior to hatch in April, when temperatures and weather conditions allow.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
