Ashe juniper (commonly called cedar) is a trademark of the Texas Hill Country. This tree is loved by some and hated by many others. It has been said that through transpiration, Ashe junipers remove untold thousands of gallons of water from the rocky, Central Texas soil. The trees occupy valuable space that could harbor native grasses that would be grazed by deer and cattle. The bark of older Ashe juniper trees is used by the golden-checked warbler (an endangered bird) for nesting material. In some areas, there is no other vegetation present. It is apparently the only food in Central Texas for the juniper budworm, the larva or caterpillar of a small moth.
During the spring and early summer of 2002, an unusual outbreak of the juniper budworm was documented across Travis, Hayes, Comal, and Blanco counties (additional adjacent counties may also be infested). Although this native insect has been reported in these areas before, this seems to be the worst outbreak in recent history. The small, green larvae attack the tips of Ashe juniper trees. As they feed, they web the foliage together and much of it will turn red or brown. The actual impact of this little caterpillar to the juniper trees is unknown.
The current outbreak was first reported in April 2002 as “juniper webworm” in Travis County. At that time, the identity of the insect was not known. Only after considerable effort was it finally identified by entomologists John Jackman and Ed Riley at Texas A&M University. It is a moth (Lepidoptera) in the family Tortricidae and has the Latin name Cudonigera houstonana (Grote). It is closely related and similar in appearance to the spruce budworm, a major defoliating insect pest of true firs in the eastern and western forests of the United States and Canada. Interestingly, the juniper budworm was first described by A. R. Grote over 125 years ago (1873) from a specimen collected on “cedar” in Texas. It is known to occur in Kansas, and is distributed throughout the pinyon-juniper woodlands of the southwestern United States.
The larva of juniper budworm goes through 8-11 instars, or caterpillar stages, as it grows. Then it changes to a pupa from which the adult moth emerges. Life history studies on this insect were done in Kansas where one generation per year was reported. However, records of adult moths in the Texas A&M University insect collection indicate there are probably two generations per year in Texas (labeled specimens showed moths were collected in June and October).
As the larvae feed in the juniper foliage, they construct silken tubes and pupation occurs in the shelter where the larvae fed. Adult moths appear shortly after pupation occurs. In Kansas, larvae overwinter in the infested juniper trees, pupate in late June and July, and emerge as adult moths in July. In Texas, however, the overwintering stage is not known, but it is probably the egg stage or as very young larvae.
The adult moths are about ¼-inch long and are similar to the color of dead Ashe juniper foliage. They have a mottled tan and brown color pattern on their wings. They are active mostly at night and are attracted to lights. They generally remain at rest on the host plant during the day and only fly when disturbed. Unless they fly, they are difficult to detect.
As stated above, it is not known exactly what impact the juniper webworm will have on the Ashe juniper in Central Texas. It is not known if defoliation by this insect will cause juniper mortality. It is not known if feeding by a second generation of larvae will cause additional damage to trees that have already been impacted. So, that raises an important question, “What should landowners do about this pest?” In rural, unpopulated areas, control for the juniper budworm is not recommended and probably not feasible. However, landowners might want to consider applying direct control when the pest occurs on high value landscape or yard trees. It should be remembered that natural controls in the form of parasites and predators currently are present. These include wasps, flies, spiders, birds, and others. Removing infested branch tips by hand could be effective on individual trees.
Some recommended insecticides for juniper budworm include Bacillus thuringiensis variety kurstaki (a commercially available bacteria known as Btk), Mimic® (tebufenozide), and Orthene® (acephate). Btk and Orthene® should be applied when the larvae are young (1st through 3rd instars). Mimic® would probably be more effective when the larvae are about half grown (4th and 5th instars). Mimic® and Orthene® have systemic qualities (similar to a vaccine in humans) and Mimic® will persist on the foliage for several weeks or longer. Btk is a bacteria that only affects larva of Lepidoptera (butterflies and moths), so it would have no impact on non-target insects, in particular beneficial parasites and predators (primarily wasps and flies). Mimic® causes premature molting (shedding of the exoskeleton or skin) of lepidopterous larvae and has no affect on other groups of insects. Mimic® is registered for and used to combat spruce budworm, among other insects. If an insecticide is used, read and follow all label instructions – it is the law.
The author thanks Carrie Burns, forester for the City of Lakeway; Eric Beckers, oak wilt forester for the Texas A&M Forest Service; and Ron Billings, principal entomologist with the Texas A&M Forest Service for information and photos included in this report.