There are at last nine species of Bumble Bees in Texas. There are two good places to go on the Interent to help you identify bumble Bees in Texas and learn a bit about when they are flying and what plants they may be attracted to.
The first site, Bumble Bees of the Eastern United States, covers all but one Texas Bumble Bee, Bombus sonorus. This site is nice because it gives information on lifestyle and plant preferences as well as provides identification keys. It also has very nice photographs to help with identification.
A related site, Bumble Bees of the Western United States, is similar to the site listed above. It includes information for Bombus sonorus (also sometimes called Bombus pennsylvanicus sonorus) which is missing from the Eastern Guide above.
Another site, Texas Bumble Bees, has very nice drawings for identification of all nine species of Texas Bumble Bees.
Bug Guide has a nice group of photographs of Bumble Bees which can help you identify a Texas Bumble Bee and Discover Life contains keys and photographs to help identify Bombus sp. . . .although their is a bit of a learning curve to learn the terms which make the keys easier to understand.
By using a combination of these resources, I think you should be able to learn to identify our Texas Bumble Bees and learn a bit about their lifestyles, biology, and plant preferences. Enjoy.