Psyllids are tiny, leaf-sucking insects that attack a wide range of host plants throughout North America. Often called "jumping plant lice," The University of California states more than 100 different species occur in the U.S.
Boxwood Psyllid (Psylla buxi) adults lay eggs under leaf buds at the tips of branches in summer. The eggs overwinter and hatch in spring. The psyllid nymphs begin sucking on newly-forming leaves, causing them to deform into cupped shapes. Pysllids do not kill boxwood plants; they only cause ugly leaves.
Psyllid eggs are protected throughout the winter until they hatch, enclosed within the parts of the plant where leaf buds will eventually emerge (bud scales). If you choose to apply a horticultural oil for psyllid control, watch closely in early spring. You must spray the oil just as the bud begins to open in order for the treatment to have a chance at being effective. Shortly after the nymphs begin feeding, the new leaves begin to cup and protect the psyllids from sprays.
While feeding, pysllid nymphs are concealed within the cupped leaves caused by their sucking of juices. Most insecticides sprayed on the leaves have little effect at this stage of damage as well. Systemic insecticides, which are absorbed into the plant through the roots or leaves, are best for psyllid control. Choose an appropriate product, and apply as a soil drench or spray, per label instructions, shortly after new leaves emerge.
Boxwood pysllids only lay eggs once a year. This means only new boxwood leaves are affected by nymph feeding. The nymphs become adults by late spring, and the damage is purely aesthetic. Most new growth, particularly on shrubs grown as hedges, gets pruned off eventually. The best control of boxwood psyllids may be to accept their presence and do nothing at all.