Arundo wasp, a biocontrol agent, is established and spreading in Delta watersheds

Arundo or giant reed (Arundo donax) is invasive in riparian areas in much of central and southern California, as well as other parts of the U.S. In the Sacramento-San Joaquin Delta, arundo grows on islands and along the edges of sloughs and canals. It is also common along the water's edge in the watersheds upstream of the Delta. This giant grass can grow to 20 ft tall or more and from a distance might be mistaken for corn. Arundo stems act like giant straws, wasting Delta water as the stems rapidly grow during the spring and summer. Dense mature patches of arundo block access to water, destabilize flood levees and constitute a fire hazard. For these reasons, the U.S. Army Corps of Engineers and various state and local agencies have spent a lot of time and money to control arundo in the Delta with herbicides.

Biological control of arundo using insects as natural enemies may be a more sustainable long-term management approach. The arundo wasp Tetramesa romana was discovered by USDA-ARS about 15 years ago, feeding on arundo in its native range in Mediterranean southern France and Spain. The arundo wasp is a small (< 1 cm) black wasp that is harmless to humans. It can feed and reproduce only on arundo, not on any crops or California native plants. Adult female wasps lay eggs in the tip of the main shoot and side shoots of arundo, leading to the formation of a gall in the shoot tip. Larvae feed on the gall tissue, pupate, then emerge 1 to 2 months later during the warm spring and summer months or overwinter inside galls. When a new generation of wasps emerge from shoots, they chew distinctive round exit holes which can be observed to track wasp populations in the field. Since the first U.S releases of the wasp, in the Rio Grande Basin of Texas and Mexico in 2009, the arundo wasp has decreased live arundo shoot biomass in that region by 30 to 40%.

 

The California Department of Food and Agriculture first released the arundo wasp in northern California in 2010. The arundo wasp was released more widely by USDA-ARS scientists in support of the USDA Delta Region Areawide Aquatic Weed Project (DRAAWP) upstream of the Delta in the spring and summer of 2017. Wasps were released at three sites near Orland, CA in the northern Sacramento River watershed, and at three sites near Madera, CA in the southern San Joaquin River watershed. Now, two years later, surveys by USDA-ARS scientists have found that the arundo wasp has established vigorous populations at two of the sites (one in each region). In a survey of 99 sample points at one site near Orland, 66% of the points had exit holes or galls. In a survey of 30 points at one site near Madera, 90% of the points had galls or exit holes. At the Orland site, large galls were seen on young main stems at sampling points up to 100 meters from the original 2017 release plots. These stems typically do not survive galling, as the gall stunts growth and increases susceptibility to stem breakage. Surveys are ongoing at four other sites (two per region) at which smaller wasp populations may be present. Releases and surveys are also ongoing at sites in the western and southern Delta. The arundo wasp is showing promise as a biocontrol agent of arundo in northern California.

Patrick Moran is a Research Entomologist with the USDA-ARS Invasive Species and Pollinator Health Research Unit.