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Strange zig-zag marking on glass puzzles Aussies: 'What is this?'

The strange shape appeared on a glass door unexpectedly and has left people online stumped.

A strange mark which emerged on a homeowner's glass door has left Aussies stumped — but a pest expert has revealed what has most likely caused it to appear.

Posting on Reddit, the Sydney resident said the zig-zag build-up — which at first glance appears to be on his living room ceiling, but was actually attached to a glass door leading to an outdoor area — turned to dust when wiped off.

“What is this?” the Sydneysider wrote next to a photo of the strange black trail.

The strange zigzag marking on a glass door.
This strange marking on an Aussie's glass has puzzled people online. Source: Reddit

Pest controller Christopher Moschella told Yahoo News Australia that while it was “very hard to determine” what the markings were for sure, he believed it was left by a “swarm of gnats” resulting from recent heavy moisture in the garden.

He said the alfresco area could be home to a lot of plants or exposed soil which attracted the insects.

“Gnats fly around like that and flying ants do swarm up like that as well,” Moschella told Yahoo. “I don’t think it’s ants though. Ant colonies can be found in down lights and roofs.”

Fixing the problem could be as simple as spraying plants with pesticides and replacing soil in any indoor plant pots. Moschella said moisture from a garden could also be a contributing factor.

“It always comes down to keeping up with general pest control,” Moschella said. “Plants outside and indoor plants should be sprayed as well.”

He said insect swarms could develop along window sills where moisture accumulated but added, “formations like this, you don’t see that often”.

The Redditor also shared a second photo in the comments of his original post which showed a brown cocoon-like growth on a heat strip.

Moschella said this photo was clearly of a mud wasp nest.

White growths on a large grey heat strip.
These growths on a heat strip are mud wasp nests, a pest expert says. Source: Reddit

Mud wasps can 'get creative' with their nests

Mud wasps, also known as “dirt dauber”, are from either the family Sphecidae or Crabronidae and – as the name suggests – build their nests from mud.

Moschella said they were not harmful and did a lot of work in pest control, preying on insects and spiders which they fed to their lavae, and like other wasp species, return to the same spot to build their nests.

“Just spray them off with a bug spray such as Mortene and get a general pest control yearly or every six months,” he told Yahoo.

Moschella, who shares TikToks of his more unusual pest finds under the handle RoachSniper, said nests looked like cocoons, while others were oval-shaped and he had seen one that was a “wormy shape”.

“They get creative and sometimes get a bit funny,” he added. “I saw one shaped like a spider – they built a mud nest over the top of a spider. It was crazy.”

People online puzzled by markings

Some people in the comments of the Reddit post debated whether the marks were caused by mould or bugs, with most agreeing that they were somehow insect related.

Most commenters however, were stumped by the image and made jokes about a "parasite invasion" or said the grown was reminiscent of the creatures from the Upside Down in the popular Stranger Things series on Netflix.

Others helpfully suggested "burning the place to the ground" and said that because the growth had appeared in Australia it would "most likely kill you". Another had the simple advice: "Get out", while someone else said: "It's going to be a no from me".

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