Diorhabda sublineata is a leaf beetle known as the subtropical tamarisk beetle (STB). The species was first described by Hippolyte Lucas in 1849. It feeds on tamarisk trees from Portugal, Spain and France to Morocco, Senegal, Algeria, Tunisia, Egypt, Yemen, and Iraq. It is used in North America as a biological pest control agent against saltcedar or tamarisk (Tamarix spp.), an invasive species in arid and semi-arid ecosystems (where the STB and its closely related sibling species also may be less accurately referred to as the 'saltcedar beetle', 'saltcedar leaf beetle', 'salt cedar leaf beetle', or 'tamarisk leaf beetle') (Tracy and Robbins 2009).
Taxonomy
The STB was first described from Annaba, Algeria as Galeruca sublineata H. Lucas (1849). Reiche and Saulcy (1858) erroneously placed G. sublineata as a junior synonym to the sibling species G. elongata Brullé (the Mediterranean tamarisk beetle, Diorhabda elongata). Weise (1893) created the genus Diorhabda and proposed the variety Diorhaba elongata var. sublineata (H. Lucas). Gressitt and Kimoto (1963) proposed the subspecies D. e. sublineata. Tracy and Robbins (2009) restored D. sublineata (H. Lucas) as a valid species based on comparisons of the male and female genitalia, and provided illustrated taxonomic keys separating the STB from the four other sibling species of the D. elongata (Brullé) species group: Diorhabda elongata, Diorhabda carinata (Faldermann), Diorhabda carinulata (Desbrochers), and Diorhabda meridionalis Berti and Rapilly. In literature prior to 2009, D. sublineata was usually also referred to as D. elongata, or as a subspecies or color variant of D. elongata.
Host plants
Field collections in Spain and north Africa reveal that the STB feeds on at least five species of tamarisks, including Tamarix gallica which hybridizes with the widely invasive T. ramosisima in western North America. The STB can become numerous in Egypt and Senegal, but reports on defoliation of tamarisk are lacking (Tracy and Robbins 2009). Extensive laboratory host range studies verified that STB is a specialist feeder on tamarisks, feeding only on plants of the tamarisk family, Tamaricaceae. In laboratory and field cage studies, the STB will also feed and complete development on Frankenia shrubs, distant relatives of tamarisks in the same plant order Caryophyllales, but STB greatly prefer to lay eggs upon tamarisk (Milbrath and DeLoach 2006).
Life cycle
The STB overwinters as adults on the ground. Adults become active and begin feeding and mating in the early spring when tamarisk leaves are budding. Eggs are laid on tamarisk leaves and bark and hatch in about a week in warm weather. Three larval stages feed on tamarisk leaves for about two and a half weeks when they crawl to the ground and spend about 5 days as a "C"-shaped inactive prepupa before pupating about one week. Adults emerge from pupae to complete the life cycle in about 4–5 weeks in the summer. (For images of various life stages of a related species, see Diorhabda carinulata at Commons.) Five generations of STB occur through spring and fall in central Texas (Milbrath et al. 2007). Similar to the northern tamarisk beetle, adults begin to enter diapause in the late summer and early fall, ceasing reproduction and feeding to build fat bodies before seeking a protected place to overwinter (Lewis et al. 2003). Larvae and adults are sensitive to shorter daylengths as the summer progresses that signal the coming of winter and induce diapause (Bean et al. in prep.). Robert Bartelt and Allard Cossé (USDA-ARS, Peoria, Illinois) found that male STB emit a putative aggregation pheromone, similar to that found in Diorhabda carinulata (Cossé et al. 2005), that could serve to attract both males and females to certain tamarisk trees.