This is a male long-horned bee that is largish, with very long antennae, a white clypeus, all-pale hairs on its vertex and thorax and a prominent pale-yellow spot near the base of each jaw.  It has unusual front-leg tibial spurs.  It was flitting between shrubby-blue salvia and sweet-stem when I found it.  I saw similar males of its kind on palafoxia.  

Eucera (Pectinapis ) fasciatella
Size:  10.5 mm (male) 

Associated plant:
Shrubby blue salvia
(Salivia ballotiflora)

Plant family:Laminacaea

When seen: May 30, 2021
Falcon State Park
Starr County

Male bee

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Front view

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Dorsal view: All of the hairs on the bee's vertex, thorax and legs are pale.

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Lateral view of head & thorax

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male bee

Megachile sidalceae

Size:  14 mm (male) 

Associated plant:

Sweet stem / shrubby blue salvia

(Aloysia macrostachya / Salvia ballotiflora)

Family:   Verbenaceae / Laminaceae 

When seen: May 30, 2021
Falcon State Park
Starr County

Male bee

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Male bee

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Dorsal view

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Dorsal view of thorax

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male bee

Megachile sidalceae

Size: 14 mm (female) 

Associated plant:

Texas palafoxia

(Palafoxia texana)

Family:  Asteraceae  

When seen: May 30, 2021
Falcon State Park
Starr County

Female bee

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Face and jaws: the female bee's mandibles have four teeth each, with a complete cutting edge between the second and third teeth. Above this is what looks like the suggestion of an incomplete cutting edge. The apical margin of the bee's clypeus is straight.

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Vertex & dorsal view of thorax

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Lateral view of abdomen

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Dorsal view of abdomen

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female bee