Riccardoella limacum or the slug mite is a member of the Acari (mite) family which is parasitic on slugs and snails. Slug mites are very small (less than 0.5 mm in length), white, and can be seen to move very rapidly over the surface of their host, particularly under the shell rim and near the pulmonary aperture. While once thought to be benign mucophages, more recent studies have shown that they actually subsist on the host's blood, and may bore into the host's body to feed.
Mite infection among gastropod populations varies greatly. Dense gastropod population favors infection; isolated populations may remain uninfected. Older and larger gastropods are more likely to show infection. Mites have been observed to move from host to host when hosts mate, and when gastropods congregate in moist soil and under rocks during the day. It has been shown that mites move preferentially towards fresh mucus when they travel along mucus, enabling them to follow mucus trails to new hosts. Once infected, individual gastropods take longer to mature and show reduced mating, activity, and feeding. Infected slugs and snails lay fewer eggs than uninfected individuals. Infected gastropods also show decreased winter survival rates.
At least 31 species of mollusks are exploited. Common hosts include the following:
- Agriolimax agrestis
- Arianta arbustrum
- Arion ater
- Arion hortensis
- Helix aspersa
- Helix pomatia
- Limax maximus
- Milax budapestensis
- Milax gagates
- Milax sowerbyi
Slug mites are a one-host mite. It is possible for a mite to be born, live, and die on a single host.
Mites have two sexes. Their five-stage life cycle is as follows: Females lay eggs in the host lung, and then the eggs hatch in 8–12 days as six-legged larva in the lungs of hosts and undergo three nymph stages. The whole life cycle can take place in 20 days under ideal conditions. Eggs do not hatch while the host is hibernating.
Economic impact and treatment
Slug mites are a concern for commercial and hobbyist breeders of snails; as little as six mites can sicken an individual and make it susceptible to infection by threadworms and bacteria such as Pseudomonas aeruginosa. Countermeasures include regular washing and introduction of Hypoaspis miles, a predatory species of mite which feeds upon slug mites.