Establishment, hybridization, dispersal, impact, and decline of Diorhabda spp. (Coleoptera: Chrysomelidae) released for biological control of tamarisk in Texas and New Mexico

Authors: KNUTSON, ALLEN - Texas A&M Agrilife; TRACY, JAMES - Texas A&M University; RITZI, CHRIS - Sul Ross State University; Moran, Patrick; ROYER, TOM - Oklahoma State University; DE LOACH, CARL - Retired ARS Employee

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2019
Publication Date: 12/1/2019
Citation: Knutson, A.E., Tracy, J.L., Ritzi, C., Moran, P.J., Royer, T., De Loach, C.J. 2019. Establishment, hybridization, dispersal, impact, and decline of Diorhabda spp. (Coleoptera: Chrysomelidae) released for biological control of tamarisk in Texas and New Mexico. Environmental Entomology. 48:1297-1316.

Interpretive Summary: Saltcedar, also known as tamarisk, is one of the most widespread non-native, invasive small trees in the arid western U.S., and thrives in riparian habitats-areas along the shorelines of seasonal or year-round rivers, creeks, and lakes. Saltcedar chokes out native plants, adds salt to the soil, and reduces habitat quality for insects, reptiles, and birds, including endangered or threatened species. Control with chemical herbicides or mechanical equipment is common but not economically or environmentally feasible across the millions of acres of invaded habitat. The USDA-Agricultural Research Service discovered, tested, and released four species of a beetle that were obtained from the native range of saltcedar (Diorhabda carinulata, from China and Kazakhstan; Diorhabda carinata, from Uzbekistan; Diorhabda elongata, from Greece; and Diorhabda sublineata, from Tunisia). In Texas and Oklahoma, over 1 million beetles of three species (D. carinata, D. elongata, and D. sublineata) were released between 2003 and 2010. Different beetle species were released in different regions based on expected climate compatibility with their native homes. All three beetles species established at 50% to 100% of the release sites, formed large populations, and defoliated (removed all of the foliage from the stems ) saltcedar trees. Between 2008 and 2013, beetles continued to feed and reproduce on trees as they regrew foliage, and hungry beetles dispersed up to 100 miles or more per year to find more saltcedar. Beetles defoliated large athel trees, a species of tree that is non-native and closely-related to saltcedar, and is used as a shade and windbreak tree in Mexico and the U.S. Southwest. However, beetle populations did not persist on athel or kill athel trees. Between 2013 and 2016, beetle populations on saltcedar in Texas and Oklahoma declined to low levels or could not be found at most of the sites, and most saltcedar trees remained alive. Scattered beetle populations were continuing to defoliate patches of saltcedar. The Texas beetles dispersed into New Mexico and Mexico, and Oklahoma beetles were found as far north as Kansas. The three beetle species released in Texas formed hybrid populations, and the fourth beetle species, D. carinulata, dispersed into Texas from northern New Mexico. The resulting complex mixing of beetle species may have contributed to their decline in abundance in Texas and Oklahoma, if hybrids had a tougher time finding mates and reproducing. Extreme weather events, especially sudden and severe winter and spring cold snaps, may have also contributed to reductions in beetle populations. Biological control of saltcedar in this region looked promising 5 to 10 years ago, but it now appears that the beetle populations are too small to cause widespread death of invasive saltcedar trees.

Technical Abstract: Three Diorhabda spp. tamarisk beetles (Coleoptera: Chrysomelidae) were introduced and established in Texas from 2003 to 2010 for biological control of tamarisk (Tamarix spp.): the Mediterranean tamarisk beetle, D. elongata (Brullé) from Greece, also introduced and established in New Mexico; the subtropical tamarisk beetle, D. sublineata (Lucas) from Tunisia; and the larger tamarisk beetle, D. carinata (Faldermann) from Uzbekistan. More than one million tamarisk beetles were released at 99 sites, and species establishment success ranged from 50 to 83%. All three species now co-occur in New Mexico with a fourth species that dispersed from Colorado, the northern tamarisk beetle, D. carinulata (Desbrochers), introduced from China and Kazakhstan. Widespread, naturally originating field populations of bispecific hybrids of D. elongata and D. carinata, as well as D. elongata and D. sublineata, rapidly appeared following the overlap of parental species populations. The initial distributions and dispersal of these Diorhabda spp. and their hybrids are mapped for Texas, New Mexico, Oklahoma, and Kansas. The Texas-released Diorhabda spp. and their hybrids produced large-scale tamarisk defoliation and localized dieback for three to four years. However, populations subsequently severely declined and now produce only isolated defoliation, allowing tamarisk to recover. Both D. sublineata and D. elongata temporarily produced non-target spillover defoliation of ornamental athel, Tamarix aphylla (L.) Karst, along the Rio Grande. Further study is needed to determine the extent and stability of Diorhabda spp. hybrid zones, including for potential trispecific, quadrispecific, and D. carinulata hybrids. The potential significance of hybridization for tamarisk biocontrol is discussed.