Juniper twig girdler—Periploca nigra
Larvae of this moth (Cosmopterigidae) are a pest of juniper in the San Francisco Bay Area, Southern California, and warm interior valleys.
Twig girdler damage resembles the foliage browning and dieback of juniper from causes including cypress canker, cypress leaftier and cypress webber, cypress tipminer, minute cypress scale, and root and crown rot diseases. To help distinguish the cause of damage, examine plants closely for insects and the silk webbing produced by certain caterpillars. Look for discolored bark and cambium caused by cypress canker and darkened, soft roots caused by root decay pathogens. Dying branches caused by mice have bark chewed away in bands from lower parts of branches. Twig dieback at the edges of plantings may be the result of dog urine, which sometimes can be diagnosed by the characteristic odor.
To confirm twig girdler as the cause of damage, peel bark from the branch where dead and living tissue meet. Inspect the wood for girdling tunnels containing insect excrement and one or more cream-colored, pink, or yellowish larvae up to 1/4 inch long with a brown head. Shiny, black to brown pupae about 1/5 inch long may also be found in tunnels.
The twig girdler adult (moth) is about 1/4 inch long and shiny with mostly dark brown to blackish forewings. Adults are not often observed, but may be seen taking flight if damaged branches are shaken during late winter through early summer.
Adults are present primarily from May through June in the San Francisco Bay Area and March through May in Southern California. The females lay tiny eggs on woody stems. The hatching larvae tunnel in and feed for about 9 months from spring through winter. There is one generation per year.
Tunneling by juniper twig girdler causes foliage on small limbs to turn yellow, then brown and die. This results in clumps of brown limbs among green foliage by late summer. Twig girdler feeding makes junipers unsightly, but does not kill entire plants in the landscape. Junipers specially cultivated in containers to remain small (bonsai) may be severely damaged.
Prune out and dispose of affected branches to improve plant appearance if their abundance is limited to small portions of the plant. Avoid planting Tam juniper (Juniperus sabina ‘Tamariscifolia’), which is very susceptible to twig girdler. Instead of Tam juniper or Hollywood juniper (J. torulosa or J. chinensis ‘Kaizuka’ or ‘Torulosa’), consider planting cultivars that are less susceptible to the juniper twig girdler.
Effective control with a contact insecticide is difficult. Larvae boring under bark are generally not susceptible to insecticide. Applying insecticide does not restore the appearance of damaged foliage, which remains brown until new growth occurs.
If damage cannot be tolerated, it can possibly be prevented by spraying foliage thoroughly about twice with a broad-spectrum, residual, contact insecticide such as permethrin or other pyrethroid to kill adult moths and prevent them from laying eggs. To help time applications, regularly shake branches of infested hosts from late winter through early summer to observe when the shiny moths take flight and are abundant. Generally, in Southern California spray in late March and early May and in Northern California spray in early May and late June.
Be aware that urban uses of pyrethroids and certain other insecticides are contaminating storm drains and household drains; this causes pyrethroids to pollute surface waters at levels hazardous to aquatic life. Consider tolerating twig girdler damage in landscapes instead of applying insecticide.
For more information, consult Insects Affecting Ornamental Conifers in Southern California and the Juniper Twig Girdler, Capital Bonsai blog.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).