One agent, Mediterranean sage root weevil (Phrydiuchus tau, Coleoptera: Curculionidae), has been introduced to the USA and is highly effective in CA. It has one generation per year. Adults are in reproductive diapause during the summer and become active in the fall after rains start. Adults feed on rosette leaves and lay eggs on the undersides of basal leaves and in leaf axils (Fig. 1). Eggs are protected by a fecal covering. Larvae tunnel down the leaf petioles to the center of the root where they feed and develop (Fig. 2). Many larvae can develop inside one plant, and damage tends to reduce or prevent production of flower stems. Larger plants tend to be attacked more than small ones.
All life stages can be present on the plant during the winter. In the spring, mature larvae exit the plant and pupate in the soil. Adults emerge in late spring to early summer, and they feed briefly on the leaves and flower stalks before disappearing for summer aestivation, hiding in the soil under rocks or in other cool moist places. Adults are dark colored and have a small white "T" on their back. Adults can feed on other Salvia species, such as S. sclarea, and S. verbenacea, but larvae are known to complete develop only on Mediterranean sage. Larvae are susceptible to predation by ants when they emerge from the plant to pupate in the soil.
In California, CDFA scientists released a total
of 2,600 weevils at 10 sites in Modoc County during 1976-1980, and 1,500 more
weevils at 2 sites in Modoc County during 2002-2005. Surveys during 2005-2006 indicated low
densities of Mediterranean sage at release sites that previously had high
densities, and the weevils were present at undisturbed sites, but absent at
roadside sites. It has been speculated
that absence of weevils at the latter sites might be related to aggressive
spraying of herbicides on roadside plants.
The weevils were also found at locations far from known release sites,
suggesting widespread dispersion of this agent.
No impacts on nontarget plants have been reported.
Biological Control Agents
|first released in CA in 1976
How the Technique Is Employed
Some biological control agents are likely to already be present at your site. Look for signs of insects: ragged adult feeding holes on leaves in the fall and winter, or late spring and early summer. Presence of larvae and feeding damage inside upper roots during winter and early spring. Collect adult weevils in the field by sweep net or hand collecting. Adults have a white "T" on their back and make a chirping sound when disturbed.
Knowing that insects are present may help you to integrate other management strategies. The adult weevils emerge in the early summer and are inactive during most of summer and fall. This is a good time of year to apply alternate control methods, if desired.
The root weevil will multiply if it has suitable habitat, and adults have safe over-summering sites. Avoid causing drastic fluctuations in the Mediterranean sage population from year-to-year that would cause the weevil population to crash. For example, it would be better to use a treatment such as herbicides or other non-chemical methods to control the periphery of an infestation and let the biological control agents multiply in the center of the weed infestation.
Herbicides that kill Mediterranean sage during the winter or early spring, before larvae emerge, will reduce the weevil population and disrupt biological control.
Areas where herbicides are used regularly, such as road shoulders, appear to prevent establishment of the biological control agent.
Where Can I Get These?
You can collect adult weevils in the field by sweep net or hand collecting. Insects may also be available from your county Agricultural Commissioner.
Andres, L.A., 1966. Host Specificity Studies of Phrydiuchus topiarius and Phrydiuchus sp. Journal of Economic Entomology 59(1): 69-76.
Villegas, B. 2007. Establishment of the crown weevil, Phrydiuchus tau, for the biological control of Mediterranean sage in Northeastern California. In: B. Villegas (ed.), Biological Control Program Annual Summary, 2006. California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, California. pp. 26-28.
Villegas, B. and C. Gibbs. 2010. Mediterranean sage, Salvia aethiopisL., (Lamiaceae). p. 56. In: D.M. Woods (ed.), Biological Control Program 2009 Annual Summary. California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, California. p. 56.
Wilson, L.M. and McCaffrey, J.P. 1993. Bionomics of Phrydiuchus tau (Coleoptera: Curculionidae) associated with Mediterranean sage in Idaho. Environmental entomology 22(3): 704-708.
Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture
Dr. Lincoln Smith, Research Entomologist, USDA