A successful biocontrol agent in the USA, Diorhabda carinulata (Coleoptera: Chrysomelidae) on Tamarix spp. (Tamaricaceae), rejected in South Africa due to insufficient host specificity

Abstract

Several countries globally, including South Africa, have been invaded by at least one of five species of Tamarix. South Africa therefore considered using one or more species of leaf-feeding beetles in the genus Diorhabda (Coleoptera: Chrysomelidae), including Diorhabda carinulata, against invasive T. ramosissima and T. chinensis, since the beetles are highly damaging in the USA. The situation in South Africa is possibly more complicated than that in the USA because there is an indigenous species, Tamarix usneoides, which could potentially serve as a host for the beetles. To investigate this possibility, a series of field and laboratory host specificity tests were conducted using D. carinulata against invasive target Tamarix species and the indigenous non-target T. usneoides. Field tests showed that D. carinulata had a preference for invasive Tamarix species, but readily settled and laid eggs on T. usneoides. Laboratory paired-choice tests showed that adult beetles preferred T. usneoides over T. chinensis and preferred T. ramosissima over T. usneoides, for both feeding and oviposition. Laboratory no-choice tests showed the egg-to-adult survival rate to be higher for individuals reared on T. usneoides than on T. ramosissima. Furthermore, the fecundity of females reared on T. usneoides was higher than that of females reared on T. ramosissima. Diorhabda carinulata is thus not a suitable biocontrol agent against invasive Tamarix in South Africa. An alternative biocontrol agent is currently being sought, and a short-list of candidate agents has already been compiled.

Increased soil salinity has no effect on the host preference and suitability of the Tamarix biological control agent, Diorhabda carinulata

“…Similar to this study, Marlin et al (2019) observed that D. carinulata readily selects T. usneoides during host specificity testing. The selection and utilisation of T. usneoides by D. carinulata represents a new association, since the native range of D. carinulata and T. usneoides are geographically isolated (Tracy & Robbins, 2009).…”

“…This study provides two fundamental implications for the release of D. carinulata as a biocontrol agent against invasive alien Tamarix taxa in South Africa. Firstly, D. carinulata selects T. usneoides in the presence of its target host and is also able to reproduce and survive on the indigenous T. usneoides, which is in agreement with Marlin et al (2019). These findings emphasise the likelihood of the beetles having non-target effects on T. usneoides.…”

“…These findings suggest that the salt concentration used in this study did not alter the suitability of the Tamarix taxa as host plants. The findings also suggest that increased soil salinity may not reduce the suitability of T. usneoides as a host plant, on which D. carinluata has previously been found to successfully complete its lifecycle (Marlin et al, 2019). Schile and Mopper (2006) found that salinity accelerated leaf mortality of the herbaceous freshwater plant, Iris hexagona (Iridaceae), and was also detrimental to the survival and development of the iris leaf mining fly, Cerodontha iridophora (Diptera: Agromyzidae).…”

“…Because the T. chinensis x T. ramosissima hybrid dominates the South African Tamarix invasion (Mayonde et al, 2016), the effect of salt on its suitability as a host of D. carinulata was investigated. The effect of salt on the non-target indigenous T. usneoides, as a host for D. carinulata, was included because the beetle is known to also attack this species (Marlin et al, 2019), causing its rejection as a biocontrol agent in South Africa. However, this relationship might be altered by the influence of salt on the host plant.…”