Hi Curt,

Thanks for contacting Ask Extension. The bumps on the lower surface of hackberry leaves are due to feeding by the hackberry nipple gall maker (Pachypsylla), a very small psyllid. Nipple galls appear as 1/8 to 1/4 inch swellings of tissue on Hackberry leaves or petioles

The adult psyllids, which resemble very small cicadas, overwinter in the bark of the hackberry and once the new leaves appear migrate out to them and lay eggs. Once the eggs hatch, the young nymphs begin feeding and while doing so inject a substance into the leaf which stimulates the formation of a gall to enclose and protect the insect. Hackberry psyllids are protected inside the gall from insecticides sprayed on the leaves so foliar treatments won't be effective when sprayed in the summer. It is too late to treat the gall for this season.

The nymphs hatch out from these galls as adults in late summer and either fly about often collecting on window screens and even plugging up the intakes on air conditioners before settling in for the winter on the hackberries. Caulking cracks around windows and doors and repairing holes in screens can reduce the number that enter the house though the adults, at 1/10-inch, can often squeeze through screen mesh. Hackberry psyllids are not harmful to people or pets and will not attack house plants. Once the psyllids get indoors they will die in your home - even if you do absolutely nothing.

While the galls do detract from the appearance of the tree, they cause no significant injury to their host. Some heavily infested leaves will fall prematurely but generally the tree capability to manufacture food is not reduced by the formation of these galls. Hackberries also continue to produce new foliage as the season progresses.

In late winter you can use a systemic insecticide, Bayer Advanced Garden Tree & Shrub Control, which contains imidacloprid to provide long control of sucking insects, like psyllids. Determining the amount needed (based on the diameter of the tree), mix the liquid insecticide in water and pour around the base of the tree. When using any insecticide product, be sure to read and follow all label directions.

For the insects that spend the winter on the hackberry tree, apply horticultural oil before insect activity begins in the spring.

Here is some additional information:

https://insectlab.russell.wisc.edu/2020/11/09/tiny-jumping-flying-biting-psyllids/

https://lancaster.unl.edu/pest/resources/hackberrypsyllids.shtml

https://aspenarbo.com/homeowners/diagnosis-treatment/hackberry-nipple-gall/

https://extension.colostate.edu/topic-areas/insects/insect-and-mite-galls-5-577/?_gl=1*1hup68s*_ga*Mzc2OTg4MDg4LjE3MDQ5MDMxMDk.*_ga_ZZ27K7HW51*MTcyMjI2NDAwMC4zMi4xLjE3MjIyNjUzMTkuMC4wLjA.

Jeffco Master Gardener Clinician Nancy