The mealworm-and-sand mistake that kills leopard geckos
What impaction is
Impaction is when something obstructs the gecko's digestive tract. The animal can't pass it. Food backs up behind it. The gecko stops eating, becomes lethargic, and — without intervention — declines and dies.
In leopard geckos specifically, impaction is one of the leading causes of preventable juvenile mortality. The animals are small. Their digestive tract is small. It takes very little material to obstruct it.
Mistake 1: Loose particulate substrate
The pet trade has been selling loose substrates as "natural" leopard gecko substrate for decades:
- Calci-sand — marketed as calcium-fortified and "safe to ingest." It is not safe to ingest. It clumps in the digestive tract and impacts geckos at an alarming rate.
- Loose sand (silica or play sand) — better than calci-sand but still risky, especially for juveniles and during feeding when geckos snap at prey on the substrate.
- Walnut shell or crushed corncob — sharp particulate that causes both impaction and internal abrasion. Avoid.
- Reptile bark / chips — too large to be passed safely if swallowed.
The mechanism is straightforward: the gecko eats a cricket sitting on the substrate, accidentally swallowing some substrate with the prey. Over weeks or months, particulate accumulates in the digestive tract. Eventually the load is enough to obstruct.
Younger geckos impact faster because their digestive tracts are smaller. A juvenile on calci-sand can impact within months. An adult on the same substrate may go a year or more before symptoms appear. Either way, the outcome is bad.
The fix
Use a solid or compacted substrate that can't be swallowed:
- Tile (ceramic or slate) — simple, easy to clean, holds heat well, looks good in a naturalistic setup
- Reptile carpet — fine for short-term but harbors bacteria and tears claws over time; replace regularly
- Paper towel — best for hatchlings or quarantine; not a long-term aesthetic choice
- Excavator clay packed flat — naturalistic, supports digging behavior, doesn't get loose particles in the gecko's mouth
- Topsoil-and-sand mix, compressed — works for adults if compacted tightly; risky for juveniles
Mistake 2: Mealworms as a staple diet
Mealworms are cheap, easy to store, and last forever in the fridge. Pet stores push them. Owners use them as the entire diet. This is the second half of the impaction problem.
The issue: mealworms have a very high ratio of chitin (the hard exoskeleton material) to soft body content. Chitin is indigestible and passes through the digestive tract slowly. On a staple diet of mealworms, a leopard gecko's gut is constantly processing a high-chitin load. Combine this with any substrate ingestion or any feeding inefficiency, and impaction follows.
Mealworms are also nutritionally inferior to better prey options. The shell takes up most of the prey item; the actual nutrition density is low.
The fix
Build the staple diet around better prey:
- Dubia roaches (best option) — high nutrition density, soft body, easy to gut-load, low chitin ratio. The clear staple choice.
- Crickets — solid staple. Slightly higher chitin than dubia but well within safe range. Most widely available.
- Black soldier fly larvae (BSFL) — naturally calcium-rich, excellent variety prey.
- Hornworms, silkworms — soft body, hydrating, great variety prey for occasional feeding.
Mealworms can be part of variety — once a week or so for adults — but should not be the staple. Waxworms should be rare treats only (high fat).
Other impaction causes worth knowing
- Prey too large. A prey item wider than the space between the gecko's eyes is too big. Adult geckos can handle larger prey, but oversized prey is a common impaction cause.
- Chronic dehydration. Geckos need a humid hide and access to water. Chronically dehydrated geckos can't pass material efficiently.
- Cold temperatures. A gecko with a too-cool warm side (under 88°F surface) digests poorly. Slow digestion increases impaction risk.
- UVB deficiency-driven MBD. Geckos with metabolic bone disease lose digestive motility. MBD prevention is its own topic but it touches impaction risk.
Signs of impaction
- Not passing waste for more than a week
- Loss of appetite
- Lethargy, hiding more than usual
- Visibly distended abdomen
- Straining without producing waste
- Sudden weight loss combined with the above
Any combination of these warrants a vet visit. Mild impaction can sometimes be resolved with warm soaks and gentle abdominal massage, but anything advanced typically requires veterinary intervention. Find an exotic vet through ARAV's directory.
Prevention summary
- Solid or compacted substrate. No calci-sand, ever. No loose sand for juveniles.
- Dubia roaches, crickets, or BSFL as staple — not mealworms.
- Prey items no wider than the gap between the gecko's eyes.
- Warm-side surface temperature 88-92°F. Measure with a digital probe.
- Humid hide always available for shed and microclimate.
- Fresh water daily.
Six cheap, easy changes. The animals that get them right rarely impact. The animals that don't, do.