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PEST ARCHAEOLOGY: A HOLE NEW FIELD OF STUDY?

Research scientist and industry expert Warwick Madden shares an interesting case of investigative pest management at an archaeological site in western Sydney.

Sometimes in this industry we think we have all the pests we are likely to encounter well covered. We have the main range of pests that appear on every general pest spray label and then there are the more marginal pests, that are seasonal or only occur under certain conditions. Then occasionally there’s a pest and a situation completely out of left field which requires going right back to first principles to even identify what is causing the problem: identify the pest; work out a monitoring system; develop a control method; and then determine whether the control method is actually working.

 

An unusual case

In May 2020 I was contacted by James Buckland of Systems Pest Management with just such a left-field scenario. James told me about an archaeological heritage site in Parramatta, NSW, which had everyone scratching their heads. During the construction of a new hotel in the CBD of Parramatta, Australia’s second oldest European settlement, the foundations and lower walls of convict-era buildings were found. The new development had to preserve this site after an archaeological dig was carried out and the result was an open courtyard within the hotel showcasing the remains along with some educational information and displays of artefacts collected on site.

 

The open courtyard within the hotel showcasing archaeological remains

 

International Conservation Services (ICS) was managing the heritage site and had discovered some problems, one of which involved damage that appeared to be occurring to the convict-era mortar of the foundations. Small 5 mm diameter holes were found in the soft mortar and at each inspection, there appeared to be more. If the damage kept occurring, it could lead to the collapse of the foundations. This is when I was called in. In coordination with James from Systems, and John Dedes of Civil Pest Management, we arranged a site inspection to see if we could solve the problem.

 

Unusual holes kept appearing in the soft mortar

 

Visiting the site

In October 2020 we were able to visit the site at last – given all the restrictions due to the pandemic it wasn’t easy to arrange. It is a fascinating site. To stand among the foundations of buildings from the early 19th century and walk where convicts and British soldiers walked with our special access was quite the experience.

We examined the walls and the tiny holes in the mortar, which were quite common. We inserted probes into the holes to see if we could find anything inside, but could find nothing. It was assumed it was some kind of mortar bee or wasp that had made the holes to lay eggs in and that the larvae had probably already emerged leaving the holes behind. But we were lucky enough to have a moment of serendipity. While we were looking around wondering how to work out what the insect was, something flew in and investigated the mortar near the holes. Suddenly the chase was on! From our combined efforts and armed with collection tools, a specimen was captured.

It appeared to be a small dark reddish-black wasp with a yellow band on the abdomen. We now had a suspect. We took plenty of photos of the damage and the wasp and arranged to send the specimen to be identified. We discussed plans for monitoring and control and agreed to meet up again when we had a plan to put in place.

The wasp specimen was sent to Derek Smith at the Australian Museum for identification. It was narrowed down to being a Vespid wasp in the subfamily Eumeninae (the mud, potter or mason wasps), but with 350 species in 35 genera in Australia, and no key available, that was as far as he could classify it.

 

One of the Vespid wasps (subfamily Eumeninae)

 

A plan of action

Given the fact we now knew the time of year they were most likely to appear and infest the workings, we could work with a window. A combination of UV light traps, sticky traps and various wasp monitors containing a variety of lures was decided upon for our plan of attack. However, things move very slowly in the conservation world, and it was nearly a year before we were back on site to install our monitors.

Two Vectothor Peregrine UV light traps with sticky boards were installed at two corners of the site where power was available. Three types of wasp trap were also placed around the foundations, with Vespex stations and liquid bait provided by Sundew Solutions, and the other two types obtained from a hardware chain. Some traps were suspended from the light fitting frames around the perimeter and others were placed on plastic-covered milk crates closer to the foundations and the wasp workings.

Regular visits over the following weeks picked up a lot of flying insects in the various traps. The UV light traps captured numerous flies and gnats. The Vespex traps captured plenty of fermentation flies that may have been coming from the cafe nearby. The other traps varied in their catch but there were no wasps seen in any traps for a few weeks. Finally, early in October 2021, John Dedes found a couple of the wasp culprits in a UV trap and another stuck in a spider web near the workings. Then, two weeks later, 11 wasps were picked up in the UV traps and a wasp was seen visiting the workings. With nothing in the liquid wasp traps, it was determined that they should be removed and replaced with sticky traps on the milk crates. As the liquids in the traps were largely sugar based it was assumed the wasps weren’t attracted to such substances.

 

A total of six UV light traps were installed to monitor wasp activity

 

By November, it was determined that the window for wasp activity was from late September through to November, meaning the wasp traps were only effective during this period. Whether enough were caught to reduce the damage was not yet known. ICS carried out some work to fill the holes with matching mortar to restore the structural integrity of the walls. Our original thoughts were to cover the mortar with a type of hard resin to prevent attack by wasps. However, the problem was that this would make a seal, trapping moisture inside the wall, which could then cause expansion and collapse.

The other idea was to replace the mortar with heritage lime mortar but with the addition of a hardening agent to prevent wasp excavations. However, ICS had warned that the ruins were too fragile for this level of disturbance. This left us with monitoring, trapping and possible application of a very light spray of a repellent insecticide.

It was decided upon to maintain the sticky traps for another two weeks, until the end of February 2022, but they were unsuccessful. Further wasps were caught in the UV light traps. More flying wasps were sighted and captured on the next visit undertaken by Mr Dedes. It was decided that the best approach would be to install more Ensystex Peregrine 3 UV light traps, which were proving to be the most successful. It was further proposed that a ‘sacrificial’ wall of soft mortar be created and placed in the ruins to distract the wasps. A protective repellent spray was applied to the mortar at a very low volume, to minimise potential water damage.

 

The wasps remain elusive

As it stands, the situation at the Parramatta site is still a work in progress. The sacrificial wall has not been constructed, however it is a desirable option and is strongly being considered by ICS. As at January 2022, only one target wasp has been caught, despite six UV light units having been installed. The plan is to continue monitoring the situation until zero wasps are caught in the traps, which we believe could be sometime in early March. Then, we will re-commence the monitoring mid-September 2022.

We are currently waiting for further analysis and identification from CSIRO Canberra to help gather more data on this particular species. There are many unanswered questions about its life cycle, which will ultimately help determine the control measures to be applied going forwards. Keeping in mind that our ultimate aim is to preserve the site, protect it from constant attack by wasps, using environmental control strategies, without having to resort to chemical application… this is the real challenge!

It has been a very interesting exercise for all of us and showed that pest control can always throw up new and exciting challenges to be reckoned with. Many thanks to all who were involved for their assistance, cooperation and teamwork.

 

Warwick Madden, Principal Research Scientist, Further Research and Consulting