What a mealworm actually offers a gecko
A mealworm is a beetle larva wrapped in a relatively hard shell of chitin, and that shell is the heart of the matter. A leopard gecko can digest mealworms, but it works harder to do it, and undigested shell fragments in droppings are a common sight in geckos fed little else. Nutritionally the worm itself is middling: reasonable protein, modest fat, very little moisture compared with a cricket or a roach, and like all feeder insects it brings almost no calcium of its own. None of that makes mealworms dangerous. It makes them a supporting act that has somehow ended up headlining in many tanks, mostly because the tubs are cheap, quiet and keep for weeks in a fridge door.
Why a mealworm-only gecko runs into trouble
The problems arrive through monotony rather than poison. A gecko fed one insect eats one nutritional profile at every meal, so any gap in that profile becomes a gap in the gecko. Mealworms are also less stimulating to hunt: they barely move in a dish, and leopard geckos are ambush hunters whose feeding response is triggered by movement. Some individuals become fussy, striking only at worms and refusing better feeders later, which locks the monotony in. Low moisture intake adds a quieter risk, since leopard geckos take much of their water from their food and from licking droplets, and dry feeders push more work onto the water bowl they may rarely use.
The feeders that should carry the staple role
Build the diet around insects that beat mealworms on nutrition and digestibility. Crickets are the traditional staple: soft-bodied by comparison, active enough to trigger hunting, and easy to gut-load. Small dubia roaches are meatier, slower to escape and keep well. Black soldier fly larvae are naturally richer in calcium than most feeders and arrive soft and wriggling. Silkworms and hornworms, in sizes the gecko can manage, bring moisture and variety as occasional additions. Whatever the mix, every feeder should be gut-loaded on nutritious food for a day before serving and dusted with calcium powder at feeding time, with a vitamin dust on a regular rotation.
Where mealworms still earn their place
Keep the tub. Mealworms are useful as a change of pace, as a taming aid offered from tongs while a new gecko learns your hand is safe, and as emergency backup when the cricket delivery fails. Freshly moulted, pale, soft mealworms are easier on digestion than the dark hardened ones, so pick those out for feeding where you can. Serve them in a dish the gecko knows, in modest numbers alongside or between staple feeds rather than as a heaped bowl that replaces dinner. If your gecko only ever rushes the dish for mealworms and ignores everything else, withdraw them for a few weeks until the better feeders are accepted again.
Watch the gecko, not the tub
The verdict on any diet shows up in the animal: a leopard gecko on a sound rotation is thick-tailed, bright-eyed, sheds cleanly and hunts with enthusiasm, while one sliding on a poor diet loses tail condition and interest together. Note which feeders were offered and eaten at each feed, because memory blurs a varied month into a vague impression. Putting the feeding rotation on the pet's Care Teams schedule in KinsLe, with each feed marked off as it happens, turns the plan you made on paper into the diet the gecko actually gets.