Opuntia (prickly-pear)
...
1b. Swelling or necrotic area on segment (pad)...2
...
2b. Larva in slight swelling, affected area sometimes yellow...Neolasioptera sp. and "Mayetiola" spp.

Neolasioptera sp. Many red larvae of this species were found in cells 1 cm in diameter. They spun cocoons when full-grown, pupated in the galls, and adults emerged in March. Hosts: O. lindheimeri, Opuntia sp.
...
[only records are from Aguascalientes, Mexico, but the host ranges into the southern USA]

Opuntia (prickly-pear)
...
1b. Swelling or necrotic area on segment (pad)...2
...
2b. Larva in slight swelling, affected area sometimes yellow...Neolasioptera sp. and "Mayetiola" spp.

Neolasioptera sp. Many red larvae of this species were found in cells 1 cm in diameter. They spun cocoons when full-grown, pupated in the galls, and adults emerged in March. Hosts: O. lindheimeri, Opuntia sp.

[only records are from Aguascalientes, Mexico, but the host ranges into the southern USA]