By MARTY FINK
This summer, I was amazed by the vast number of white snails clinging to desiccated vegetation along the trails of Mission Trails Regional Park. I wondered whether the snails are native to our area. Why had they attached themselves to dead plants? Were they themselves now dead?
They are invasive Italian white snails (Theba pisana) and are found in coastal habitats with warm-to-hot arid climates, thriving on alkaline soils rich in loose calcium carbonate (limestone). The shell is almost always creamy white and may have a solid, dashed, or dotted dark-brown spiral, or no markings at all.
Originating in the Mediterranean region, the first North American sighting was in La Jolla in 1914. The snails were thought to have been brought in as a food delicacy. Then, the great escape. They were either intentionally thrown away because they were “dead” or crawled off unnoticed.
By 1918, an infestation covered four La Jolla city blocks. By 1922, they were found in 22 different La Jolla locations and at the Scripps Institute for Biological Research (now Scripps Institute of Oceanography), about two miles north of downtown La Jolla, on virtually everything — houses, fences, bridges, curbs, telephone poles, trees, shrubs, and weeds, and averaged 21 snails per square foot. They spread north to Orange and Los Angeles counties, probably on hay used as citrus grove mulch, but have reportedly been eradicated in those two counties, although isolated clusters have been sporadically found. They remain firmly entrenched in San Diego County.
The snails feed ravenously on green plants and ground-level organic material. Their lifespan is one to two years. The right combination of coolish temperatures and moisture (as little as 1-2 mm) induces activity such as mating and reproducing but they only reproduce over a single breeding season.
The Italian white snail is a hermaphrodite so any snail can lay eggs after mating. Up to 4,500 eggs are laid in a 1-3-centimeter-deep cavity dug by the snail. The eggs hatch no sooner than 20 days later depending on surrounding moisture. Hatching may be delayed in dry conditions, but the eggs cannot survive long dry spells.
Snail activity decreases as the weather warms and ceases completely during summer heat. The climbing behavior is characteristic of this species as an adaptation to avoid ground-level high temperatures, which can exceed their lethal limit, 115 degrees Fahrenheit.
Upon climbing they go into estivation, a prolonged torpor or dormancy during the hot, dry period. They also form a wall of thick, dried mucus across their shell opening to reduce water loss, thus being able to survive extended periods of low moisture and high temperature conditions.
The snails may appear lifeless and dead, but under cooler temperatures and the right water levels, they awake, revive, descend, and slither away.
— Marty Fink is a trail guide at Mission Trails Regional Park.