Struggle Bug is learned by 38 species by level, 81 by machine and 4 by egg

Effect, in PokéAPI's words

“Has a chance to lower the target’s Special Attack by one stage.”

Who learns it

88 species in all, 8 of 10 games shown

Scarlet and Violet: 51 species

Species	Method	Level
Combee	Level up, Machine	1
Vespiquen	Level up, Machine	1
Volcarona	Level up, Machine	1
Vivillon	Level up, Machine	1
Snom	Level up, Machine	1
Frosmoth	Level up, Machine	1
Nymble	Level up, Machine	4
Lokix	Level up, Machine	4
Tarountula	Level up, Machine	5
Spidops	Level up, Machine	5
Kricketot	Level up, Machine	6
Rellor	Level up, Machine	7
Rabsca	Level up, Machine	7
Iron Moth	Level up, Machine	7
Larvesta	Level up, Machine	12
Volbeat	Level up, Machine	15
Illumise	Level up, Machine	15
Sewaddle	Level up, Machine	22
Swadloon	Level up, Machine	22
Leavanny	Level up, Machine	22
Cutiefly	Level up, Machine	24
Ribombee	Level up, Machine	24
Venonat	Machine
Venomoth	Machine
Scyther	Machine
Mew	Machine
Spinarak	Machine
Ariados	Machine
Yanma	Machine
Pineco	Machine
Forretress	Machine
Gligar	Machine
Scizor	Machine
Heracross	Machine
Surskit	Machine
Masquerain	Machine
Trapinch	Machine
Vibrava	Machine
Flygon	Machine
Staraptor	Machine
Kricketune	Machine
Yanmega	Machine
Gliscor	Machine
Joltik	Machine, Egg
Galvantula	Machine, Egg
Scatterbug	Machine
Spewpa	Machine
Grubbin	Machine
Charjabug	Machine
Vikavolt	Machine
Kleavor	Machine

Legends Arceus: 5 species

Species	Method	Level
Burmy	Level up	1
Wormadam	Level up	1
Mothim	Level up	1
Combee	Level up	6
Vespiquen	Level up	6

Brilliant Diamond and Shining Pearl: 12 species

Species	Method	Level
Venonat	Level up	1
Venomoth	Level up	1
Kricketune	Level up	1
Combee	Level up	1
Vespiquen	Level up	1
Kricketot	Level up	6
Shuckle	Level up	10
Volbeat	Level up	15
Illumise	Level up	15
Ledyba	Level up	22
Ledian	Level up	24
Mothim	Level up	38

Sword and Shield: 19 species

Species	Method	Level
Combee	Level up	1
Vespiquen	Level up	1
Volcarona	Level up	1
Wimpod	Level up	1
Golisopod	Level up	1
Blipbug	Level up	1
Dottler	Level up	1
Orbeetle	Level up	1
Snom	Level up	1
Frosmoth	Level up	1
Shuckle	Level up	10
Larvesta	Level up	12
Shelmet	Level up	16
Accelgor	Level up	16
Cutiefly	Level up	24
Ribombee	Level up	24
Joltik	Egg
Durant	Egg
Sizzlipede	Egg

Ultra Sun and Ultra Moon: 13 species

Species	Method	Level
Vivillon	Level up	1
Ribombee	Level up	1, 10
Wimpod	Level up	1
Golisopod	Level up	1
Kricketot	Level up	6
Cutiefly	Level up	10
Shuckle	Level up	12
Volbeat	Level up	15
Illumise	Level up	15
Shelmet	Level up	16
Accelgor	Level up	16
Sewaddle	Level up	22
Leavanny	Level up	22

Sun and Moon: 13 species

Species	Method	Level
Vivillon	Level up	1
Ribombee	Level up	1, 10
Wimpod	Level up	1
Golisopod	Level up	1
Kricketot	Level up	6
Cutiefly	Level up	10
Shuckle	Level up	12
Volbeat	Level up	15
Illumise	Level up	15
Shelmet	Level up	16
Accelgor	Level up	16
Sewaddle	Level up	22
Leavanny	Level up	22

Omega Ruby and Alpha Sapphire: 61 species

Species	Method	Level
Kricketot	Level up, Machine	6
Shuckle	Level up, Machine	12
Vivillon	Level up, Machine	12
Volbeat	Level up, Machine	15
Illumise	Level up, Machine	15
Shelmet	Level up, Machine	16
Accelgor	Level up, Machine	16
Sewaddle	Level up, Machine	22
Leavanny	Level up, Machine	22
Butterfree	Machine
Beedrill	Machine
Paras	Machine
Parasect	Machine
Venonat	Machine
Venomoth	Machine
Scyther	Machine
Pinsir	Machine
Mew	Machine
Ledyba	Machine
Ledian	Machine
Spinarak	Machine
Ariados	Machine
Pineco	Machine
Forretress	Machine
Gligar	Machine
Scizor	Machine
Heracross	Machine
Beautifly	Machine
Dustox	Machine
Surskit	Machine
Masquerain	Machine
Nincada	Machine
Ninjask	Machine
Shedinja	Machine
Trapinch	Machine
Vibrava	Machine
Flygon	Machine
Anorith	Machine
Armaldo	Machine
Kricketune	Machine
Wormadam	Machine
Mothim	Machine
Vespiquen	Machine
Skorupi	Machine
Drapion	Machine
Yanmega	Machine
Gliscor	Machine
Swadloon	Machine
Venipede	Machine
Whirlipede	Machine
Scolipede	Machine
Dwebble	Machine
Crustle	Machine
Karrablast	Machine
Escavalier	Machine
Joltik	Machine
Galvantula	Machine
Durant	Machine
Larvesta	Machine
Volcarona	Machine
Genesect	Machine

X and Y: 61 species

Species	Method	Level
Kricketot	Level up, Machine	6
Shuckle	Level up, Machine	12
Vivillon	Level up, Machine	12
Shelmet	Level up, Machine	16
Accelgor	Level up, Machine	16
Sewaddle	Level up, Machine	22
Leavanny	Level up, Machine	22
Butterfree	Machine
Beedrill	Machine
Paras	Machine
Parasect	Machine
Venonat	Machine
Venomoth	Machine
Scyther	Machine
Pinsir	Machine
Mew	Machine
Ledyba	Machine
Ledian	Machine
Spinarak	Machine
Ariados	Machine
Pineco	Machine
Forretress	Machine
Gligar	Machine
Scizor	Machine
Heracross	Machine
Beautifly	Machine
Dustox	Machine
Surskit	Machine
Masquerain	Machine
Nincada	Machine
Ninjask	Machine
Shedinja	Machine
Volbeat	Machine
Illumise	Machine
Trapinch	Machine
Vibrava	Machine
Flygon	Machine
Anorith	Machine
Armaldo	Machine
Kricketune	Machine
Wormadam	Machine
Mothim	Machine
Vespiquen	Machine
Skorupi	Machine
Drapion	Machine
Yanmega	Machine
Gliscor	Machine
Swadloon	Machine
Venipede	Machine
Whirlipede	Machine
Scolipede	Machine
Dwebble	Machine
Crustle	Machine
Karrablast	Machine
Escavalier	Machine
Joltik	Machine
Galvantula	Machine
Durant	Machine
Larvesta	Machine
Volcarona	Machine
Genesect	Machine

Changed over time

One change since Generation V

1. Through Gen VPower 3050 from X and Y, Generation VI