Labial Gland Disease

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"Pseudogynes" (Greek meaning "false queens") have long been known in various European Formica species, especially the wood ants. They were initially related to queens based on their similarity to true queens and because their origin was unclear. It was once thought that the guest beetle Lomechusa strumosa was responsible for their development, but this is not the case. It is now known that these individuals have enormously swollen labial glands (salivary glands), which are located in the mesosoma and exit to the mouth. This swelling is thought to be caused by a virus, however details are unknown. The method of spread and infection are also unknown. Infection causes a disease known as labial gland disease.

Labial gland disease causes swelling of the labial glands in the pupal stage of numerous Formica species. The resulting adults have enlarged thoraces and are called secretergates (Wasmann's pseudogynes). These adults infect younger larvae, probably during feeding. It soon becomes non-infectious outside the labial glands. Diseased queens were not seen to feed larvae, yet their offspring included secretergates. This discrepancy remains unexplained. (Elton, 1991)

It is unknown whether only deformed individuals are infected, or whether "normal" workers can also harbour the pathogen but without displaying morphological modifications. As infection normally (as far as known) takes place in the larval stage the conspicuously hunched thorax can develop during the pupal stage. If adult workers were to be infected, the shape of the thorax would already be fixed and would unable to change. Thus morphology alone may not be a reliable indicator of infection.

It has been proposed to differentiate between "Secretergate" (infected workers), "Secretogyne" (queens) and "Secretaner" (males). "Secretogyne", queens who are even more hunchbacked than normal thanks to the disease, can mate and lay fertile eggs; however, some infected queens do not lay eggs despite mating.

  • Labial gland disease in Formica polyctena. Healthy queen (top left; colour bleached by storage in ethanol), healthy worker (top right), and two “pseudogyne” or “secretergate” specimens with labial gland disease (bottom, note humped mesosoma) (from Laciny (2021), Fig. 7, originally from Accessed 15 Aug 2020).

Known Ant Hosts

Secretoforms have been found in six of the eight European species of the Formica rufa group, and may also occur in Formica uralensis and Formica pratensis (as Formica nigricans). They do occur in other species of the genus viz. in Formica sanguinea (Wasmann, 1915), Formica fusca (Wasmann, 1915), Formica lemani (Collingwood, 1956), Formica rufibarbis (Wasmann, 1915), and in some North American Formica species. They are a major mortality factor in F. sanguinea (Wasmann, 1915: 272-281) and could be so in other species. (Elton, 1991)

Ant Pathogen Species Locality Source Notes
Formica fusca unknown agent Spain Espadaler & Riasol, 1981; Elton, 1991
Formica japonica unknown agent Japan Sonobe, 1974; Elton, 1991
Formica lemani unknown agent Spain Collingwood, 1956; Espadaler & Riasol, 1981; Elton, 1991
Formica lugubris unknown agent Spain Espadaler & Riasol, 1981; Elton, 1991
Formica polyctena unknown agent Elton, 1991
Formica pratensis unknown agent Japan Elton, 1991 as ''Formica nigricans'', needs confirmation
Formica rufa unknown agent Elton, 1975; Elton, 1991
Formica rufibarbis unknown agent Elton, 1991
Formica sanguinea unknown agent Elton, 1991
Formica uralensis unknown agent Japan Elton, 1991 needs confirmation