This week Ray wrote to me to ask if he had been seeing an irruption of spiders because he noticed so many webs draped across the vegetation. The webs disappeared after the big rains came.
The word ‘irruption’ means a sudden increase in the relative numbers of a natural population of an organism due to some favourable alteration in the environment.
The number of spider webs naturally tends to increase in the fall. Spiders known as sheet web weavers, especially a species called Collinsia ksenia, can form big webs draped over large areas of vegetation. The spiders themselves are small and hidden underneath the webs.
Big increases in the numbers of webs have been reported to occur after an extended warm sunny stretch of weather. It seems that the female spiders making the webs may be taking advantage of the availability of flying insects. The females may be ‘fattening’ themselves up to lay one last batch of eggs as they near the end of their life, or possibly just eating to help themselves through the winter months.
This might be what Ray observed as we did have a very warm fall and there were lots of insects for food. Then, when the nights started cooling down, condensation occurs on the webs making them more visible than normal. So, it may not necessarily be due to an increase in the actual numbers of spiders present, but rather to an environmental change favouring web building.
The increase in webs could also be a ‘dispersal event.’ Some spiders – usually small ones or spiderlings -are known to go through a dispersal process called ballooning. They climb up to the highest point on the vegetation, stand on tiptoes (‘tippytoeing’) and stick their abdomen up in the air. Using their spinnerets, the spiders shoot out very fine silky strands into the air. The strands join up to form a triangular-shaped ‘balloon’. Even the slightest breeze is enough to pick up one these balloons and lift the attached spider into the air. There are records of some being carried very high into the atmosphere or travelling over very long distances. Most of the time, the dispersal is over shorter distances.
Dispersal events occurred in Australia in 2012 and 2015 involving millions of spiders. When they landed it looked like the whole area was covered in snow. One similar event occurred near Victoria, B.C. in 2021.