Our Fight Against Worms and Larvae

Significant damage is caused to Royston Cave by worms ingesting the chalk. In some areas, whole sections of the cave’s surface have been eaten away and its detail lost. In other areas, the worms colonise cracks in the wall, causing the cracks to enlarge. Eventually these sections of chalk collapse.

An investigation found two main species involved in the cave’s deterioration; large, purple worms and small, translucent larvae. Both appeared to damage the chalk in the same way, although the scale of damage caused by the larger worms was greater. They are thought to feed on nutrients in the chalk which enter the cave as a result of leaks and flooding, especially from sewage pipes.

Brandling Worms © Tobit Curteis Associates LLP. 2020.

The large, purple worms, typically about 20 mm long, were identified as the brandling worm (Eisenia foetida) and are generally found in the lower part of the cave. The small, translucent larvae, usually less than 8 mm long, were identified as that of the fungus gnat (Speolepta leptogaster) and are generally located higher up the walls. Their adults, which look similar to mosquitoes, were also present in the cave.

Fungus gnats are almost always associated with caves and cave-like habitats and, as the name suggests, usually feed on fungal matter. It's thought they may also consume other organic material including microscopic protozoa, mites and other small insects which they trap on silk threads. Their life cycle is short and it is thought that they are active during the day and night.

Fungas Gnat Larvae © Peter Askew, IMSL. 2012.

Although the damage caused by the worms and larvae is widespread it generally appears to occur in discrete areas, either around bedding plains or along existing irregularities in the chalk. These seem to be areas where the chalk is softer, but may also correlate to sections with a greater concentration of nutrients from sewage leaks. Damage has left areas with a honeycomb appearance.

The worms and larvae also excrete fine casts across the surface of the cave. These vary considerably in size but are usually 1 mm thick. Colours vary from white to brown to black, depending on the location and material consumed. The casts tend to accumulate in indents and gaps or form small, individual piles. While unsightly and disfiguring, the casts do not pose a risk to the structure of the cave.

Possible methods to reduce the worm and larvae populations were considered. No biocide or saline solution was viable due to the possible negative effect on the chalk. There is also a health and safety risk if biocides were to leak into the water table. A possible alternative was to use carbon dioxide, or other gasses, but again this posed a risk to public health and safety.

Instead, it was decided to control the brandling worm population by reducing their habitat. To do this, soil and dirt was removed from the base of the cave and from horizontal surfaces in the walls. The material, believed to be a mixture of soil left behind from the original 1742 excavation and debris which had fallen through the grille at the top of the cave, was heavily compacted and well bonded to the chalk surface beneath. It was removed manually with scalpels, small dental tools and soft brushes. Residue in crevices were removed with soft brushes and vacuum cleaners. While a small number of worms remain present, their population have greatly decreased.

An unexpected benefit of this treatment was that it revealed additional floor carvings of figures, similar to those recorded by William Stukeley in the 18th century. A number of simple geometric designs were also observed as well as numerous small holes and irregularities in the chalk. Some of these are likely to be naturally occurring but some are clearly man made.

Adult Fungas Gnat © Peter Askew, IMSL. 2012.

For the fungus gnats, a UV insect lamp was installed to attract and trap them in their flying adult stage. This interrupts and limits their breeding cycle, reducing the reproduction of new larvae. Subsequent tests showed that the UV lamp was successful in decreasing the gnat population.

The initial introduction of a UV light saw an increase in microbiological growth so the lamp’s operational hours are now restricted by a timer to minimise further growth. The sticky board traps and UV bulbs are checked and replaced regularly as part of ongoing conservation management.

In addition, repairs were made to nearby drains to prevent sewage leaks and reduce nutrient sources.

Also identified in the cave were springtails, various spiders, common earthworms and green slugs.

We would like to thank everyone involved in our efforts to preserve Royston Cave including; Tobit Curteis Associates for their conservation work; Industrial Microbiological Services LTD. for their identification of the larvae and microbiological growth; and Historic England for their ongoing support.

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References

Tobit Curteis Associates LLP. (2014). Research and Conservation Treatment of Royston Cave, Hertfordshire.

Industrial Microbiological Services LTD. (2012). Microbiological Survey of Selected Wall Surfaces at Royston Cave.

Tobit Curteis Associates LLP. (2011). Preliminary Report on the Remedial Conservation Treatment at Royston Cave, Hertfordshire.

Tobit Curteis Associates LLP. (2009). Condition Survey and Development of a Conservation Programme for Royston Cave, Hertfordshire.