Arizona Bark Scorpions: Why Block Walls and Citrus Yards Make Them Worse

Block walls and citrus yards make Arizona bark scorpions worse because they hand the scorpion the two things it needs on a desert lot: a cool, protected place to spend the day and a steady supply of crickets and other insects to eat at night. A house with both is a scorpion property whether the owner knows it yet or not.

What the Arizona Bark Scorpion Actually Is

The Arizona bark scorpion (Centruroides sculpturatus) is a small, slender, tan scorpion, usually 2 to 3 inches long, and it is the only scorpion in the United States with a sting that is medically significant. Two things separate it from the larger, harmless desert scorpions people also find in the yard. First, it climbs. It walks straight up stucco, block, rough wood, tree bark and the underside of a patio table, which is how it ends up on a bedroom wall or in a bathtub. Second, it likes company. Bark scorpions gather in groups in the same protected cavity, so where there is one in a block wall there are usually a dozen more.

They are nocturnal. During a summer day when the wall surface is 130 degrees they are wedged into a cool crevice, and after dark they come out onto the wall face and the ground to hunt. Their prey is almost entirely insects, and on an Arizona lot that means field crickets first, then roaches, earwigs, moths and spiders. They need very little water and get most of it from what they eat.

Why a Block Wall Is Perfect for Them

Nearly every subdivision in the low desert has a concrete block wall around the yard, and the wall is built from hollow block. Each block has two cavities, the mortar joints crack with heat and settling, and the wall cap is often a row of open cells. The result is a 6 foot tall apartment building with hundreds of shaded, thermally stable rooms and a hunting surface on both faces. Scorpions rest inside the cells by day and hunt along the wall at night, and because the wall is continuous, they move between yards without ever touching open ground.

The house itself adds more of the same. Stucco over foam is finished with weep holes, small openings along the base of the wall every few feet that let moisture drain out. To a scorpion a weep hole is a doorway into the wall cavity behind the stucco. Pool equipment pads, irrigation valve boxes, stacked pavers, the gap between the AC condenser and the wall, and the corners of the garage are all the same kind of space. The more block, stucco and concrete a yard has, the more daytime shelter it offers.

Why Citrus Makes It Worse

A citrus tree does three things. It drops fruit, which feeds crickets, roaches and fruit flies. It is watered, usually on a basin or a drip line, which keeps a patch of soil moist all summer and holds even more insects. And it has rough bark and a dense canopy that scorpions climb and rest in during the day. A mature orange tree planted a few feet from a block wall is a feeding station with a shelter attached, and the leaf litter under it is where the crickets breed.

Older neighborhoods in Mesa, Phoenix, Tempe and Glendale on flood irrigation with 40 year old citrus have this in the extreme. The flood irrigation soaks the whole yard every 2 weeks, the grass and the citrus hold crickets in numbers a granite yard never sees, and the scorpions follow. Newer subdivisions with a granite yard and one small citrus in a raised bed have less of this, but the tree still matters.

How to Tell If Your Wall Has Them

The reliable test takes 5 minutes after dark. Bark scorpions fluoresce under ultraviolet light, so a UV flashlight with a 365 to 395 nanometer bulb, the kind sold for a few dollars, lights them up as bright green shapes on a wall from 20 feet away. Walk the block wall on both faces, the base of the house, the pool pad, the citrus trunk and the underside of the patio furniture. Look at the wall cap and inside any open cells. Look at the garage door seal.

What you find tells you the scale. One or two scorpions on the back wall in July is typical desert edge background. A dozen on one wall run, several on the house, and a few at the door thresholds means the property is holding a population and treatment needs to cover the wall, the house and the food supply together.

What Actually Reduces Them

A single spray around the foundation does not, because the scorpions are inside the wall cells and behind the weep holes, not on the ground where the spray went. What works is a program built around where they live and what they eat.

• Treat the block wall itself, both faces, the cap and the open cells, with a residual labeled for scorpions, and repeat as the heat breaks it down.

• Treat the weep holes and the foundation, and screen the weep holes so scorpions cannot use them as entries.

• Cut off the crickets with granular bait and a residual along the foundation, the grass edge and under the citrus, because a scorpion with nothing to eat leaves.

• Clean up the citrus: pick up fallen fruit weekly, thin the lower canopy so it does not touch the wall or the house, and keep the drip basin from flooding the block wall footing.

• Seal the house after treatment: door sweeps, the garage door seal, copper mesh at plumbing and AC line penetrations.

• Check at night with the UV light every few weeks through the summer to see whether the count is dropping.

A sting from a bark scorpion causes sharp pain, numbness and tingling that spreads from the site, and in a small child or an elderly person it can cause muscle twitching, trouble breathing and roving eye movements. Wash the site, apply a cold pack, and call Poison Control at 1-800-222-1222. Anyone with breathing trouble or a stung child under 5 should go to an emergency room. Antivenom is stocked in Arizona hospitals.

FAQ

Will removing the block wall or the citrus fix the problem? No, and it is not necessary. Treating the wall, screening the weep holes, controlling crickets and keeping the citrus cleaned up and trimmed off the wall brings a scorpion lot down to occasional sightings without changing the landscaping.

Why did I see more scorpions right after a treatment? Because the product reached them in the wall cells and flushed them out. Seeing dead or sluggish scorpions on the patio for a few nights after treatment is the treatment working.

Do scorpions come from the neighbor's yard? Along a shared block wall, yes, constantly. That is why the wall gets treated on both faces and why corner lots often need the neighbor side of the wall treated too.

If your wall lights up green at night, a scorpion control program that treats the wall, the weep holes and the cricket supply together is the fix, and Green Ninja Pest Control is happy to walk the property with a black light and show you what is there.