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Phylloxera caryaecaulis

Hickory Gall Phylloxera (Phylloxera caryaecaulis). Photo: Lacy L. Hyche, Auburn University, Bugwood.
Scientific Name: 
Phylloxera caryaecaulis
Common Name: 
Hickory Leafstem Gall Phylloxera
Growing Degree Days (GDD's): 
91–246 GDD's, Base 50F, March 1st Start Date (Source: Cornell Cooperative Extension.)
Host Plant(s) Common Name (Scientific Name): 
Hickory (Carya spp.) 
Pignut Hickory (Carya glabra)
Shagbark Hickory (Carya ovata)
Insect Description: 

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). 

Damage to Host: 

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.

Cultural Management: 

Pruning off galls before they open and the adults emerge can be a helpful cultural management strategy for the hickory leafstem gall phylloxera. 

Natural Enemies & Biological Control: 

Little information is available about the effectiveness of the natural enemies of this species. 

Chemical Management: 

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.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: January 2024. To check current product registrations in Massachusetts, please visit: .