Establishment, Hybridization, Dispersal, Impact, and Decline of Diorhabda spp. (Coleoptera: Chrysomelidae) Released for Biological Control of Tamarisk in Texas and New Mexico

Abstract

Three Diorhabda spp. tamarisk beetles (Coleoptera: Chrysomelidae) were established in Texas from 2003 to 2010 for biological control of tamarisk (Tamarix spp.): Mediterranean tamarisk beetles, D. elongata (Brullé) from Greece, also established in New Mexico; subtropical tamarisk beetles, D. sublineata (Lucas) from Tunisia; and larger tamarisk beetles, D. carinata (Faldermann) from Uzbekistan. More than one million tamarisk beetles were released at 99 sites. Species establishment success ranged from 52 to 83%. All three species now co-occur in New Mexico with the northern tamarisk beetles, D. carinulata (Desbrochers). A phenotypic hybrid scoring system was developed to assess Diorhabda phenotype distributions and character mixing in hybrid zones. Widespread field populations of bispecific hybrid phenotypes for D. carinata/D. elongata and D. sublineata/D. elongata rapidly appeared following contact of parental species. Initial distributions and dispersal of Diorhabda spp. and hybrids are mapped for Texas, New Mexico, Oklahoma, and Kansas, where they produced large-scale tamarisk defoliation and localized dieback for 3–4 yr. However, populations subsequently severely declined, now producing only isolated defoliation and allowing tamarisk to recover. Diorhabda sublineata and D. elongata temporarily produced nontarget spillover defoliation of ornamental athel, Tamarix aphylla (L.) Karst, along the Rio Grande. Hybrid phenotypes were generally bimodally distributed, indicating some degree of reproductive isolation. Additional diagnostic phenotypic characters in males allowed more precise hybrid scoring. Character mixing in some hybrid populations approached or reached that of a hybrid swarm. The significance of hybridization for tamarisk biocontrol is discussed.

Hybridization and range expansion in tamarisk beetles (Diorhabda spp.) introduced to North America for classical biological control

“…However, we also found some of these hybrids in Texas, in close proximity to the locations sampled by Knutson et al, (2019). These results highlight the value of population genomics to monitor hybridization in cryptic species.…”

“…The large extent of the tamarisk invasion, which is estimated to cover at least 360,000 hectares (Nagler, Glenn, Jarnevich, & Shafroth, 2011), coupled with the high value of ecologicallysensitive riparian areas and the cost of conventional control which runs in the millions of dollars (US) per project (Knutson et al, 2019), provided impetus for development and implementation of a biocontrol program.…”

“…carinulata, which are sympatric in the native range (Tracy & Robbins, 2009), did not break down upon secondary contact. Although we were not surprised to find hybrids between D. carinata, D. elongata, and D. sublineata (Bean et al, 2013;Bitume et al, 2017;Knutson et al, 2019) of the other species (Bean et al, 2013). Sex-biased asymmetry of hybridization could be examined by employing existing mt-CO1 efforts with genome-wide ancestry analyses like these (Ozsoy et al, 2019;Ozsoy et al, 2018Ozsoy et al, , 2021.…”

“…Stands of tamarisk can form dense monotypic thickets that cause substantial economic and environmental damage including increased fire intensity and frequency (Drus, Dudley, Brooks, & Matchett, 2013), increased evapotranspiration (Nagler et al, 2014), diminished soil mycorrhizae critical for native plant species (Meinhardt & Gehring, 2012) as well as a number of other negative impacts on native flora, wildlife habitat and recreation (Di Tomaso, 1998; Gaskin & Schaal, 2002; Shafroth et al, 2005; Zavaleta, 2000). The large extent of the tamarisk invasion, which is estimated to cover at least 360,000 hectares (Nagler, Glenn, Jarnevich, & Shafroth, 2011), coupled with the high value of ecologically-sensitive riparian areas and the cost of conventional control which runs in the millions of dollars (US) per project (Knutson et al, 2019), provided impetus for development and implementation of a biocontrol program.…”

“…1) (DeLoach et al, 2003) and the species established at northern locations but failed to establish below the 38 th parallel in California and Texas (Lewis, DeLoach, Knutson, Tracy, & Robbins et al, 2003) leaving many heavily invaded river systems without a biocontrol option. To address this problem and improve ecological matching across the diverse invaded range (Sands & Harley, 1980), four additional tamarisk-feeding Diorhabda populations were collected from ecologically distinct locations in North Africa and Eurasia and introduced primarily in tamarisk-infested areas of western Texas (Knutson et al, 2019; Michels et al, 2013; Tracy & Robbins, 2009). Three ecotypes were elevated to species status in a taxonomic revision based in part on morphology of the genital sclerites (Tracy & Robbins, 2009).…”