I don't know what it is, but it's obviously a sphinx moth closely related to our white-lined sphinx moths and banded sphinx moths. The caterpillar would have had a 'horn' on its rear end. Here's a picture I made some years ago of a banded sphinx moth -

{{gwi:452464}}

Sherry

Possibly White Lined Sphinx Moth.

Here is a link that might be useful: White Lined Sphnix Moth (Bug Guide)

The Oehlke silkmoth website mentions the White-lined Sphinx being recorded in Argentina and also lists a few placenames within that country.

http://www.silkmoths.bizland.com/hlinelin.htm

and this bugguide page mentions SA distribution and shows that the caterpillar's appearance varies considerably.

http://bugguide.net/node/view/3071

White Lined Sphinx, Hyles lineata, is one of my favorite moths.
Larvae feed on a wide variety of hosts; moths are found mostly at late afternoon and in the early AM.
Mine love to nectar on Hostas, of all things.
Range is from Canada through Central America.

John