From Coffee Culture, Jerry Baldwin’s blog at The Atlantic:

A few specialty roasters in the US have begun to experiment with putting robusta into their espresso blends. The typical reasons are to make it more like Italian blends or make a thicker crema. I do understand the preference for the texture of a good crema, but I don’t understand sacrificing flavor to achieve it.

whoa, I thought I was a coffee snob but I never would have thought to ask the robusta question.  Wikipedia says this about robusta:

[Robusta] is easier to care for than the other major species of coffee, Coffea arabica, and, because of this, is cheaper to produce. Since arabica beans are considered superior, robusta is usually limited to lower grade coffee blends as a filler. It is however often included in instant coffee, and in espresso blends to promote the formation of “crema“. Robusta has about twice as much caffeine as arabica.

back to the Coffee Culture article:

Why try to make espresso “more Italian” when specialty roasters in Italy are either abandoning robusta completely or have developed 100 percent arabica blends at the top of their product range? Torrefazione Mexico in Milan, Illycaffe in Trieste, and Caffe Kimbo in Naples are just a few of countless roasters who are 100 percent arabica or, “prefer the excellence of a 100 % arabica blend,” as Kimbo says on its website. Even Lavazza, the Folgers of Italy, has 100 percent arabica blends that are at the high end of their offerings.

Lavazza:  the Folgers of Italy. (cap tap:  The Browser.)