Carnivorous Garden Spiders Are Actually Omnivores, Researchers Find

A common variety of carnivorous garden spider may be in need of a new classification after researchers discovered that as much as a quarter of its diet is made up of pollen.

The study on orb-weaver spiders in the open-access journal PLOS One suggests that the arachnids choose to eat pollen even when insects are readily available. The orb spiders' webs not only trap insect prey, but also quite a lot of aerial plankton such as pollen and fungal spores.

Dirk Sanders, an entomologist at University of Exeter, along with colleagues from the University of Bern, conducted a series of feeding experiments on orb-weaver spiders, as well as a stable isotope analysis on juvenile spiders to see whether the arachnids incorporate plant matter into their diet, learning that 25 percent of the spider's food intake was made of pollen, while the remaining three-quarters consisted of flying insects.

"Most people and researchers think of spiders as pure carnivores, but in this family of orb web spiders that is not the case. We have demonstrated that the spiders feed on pollen caught in their webs, even if they have additional food, and that it forms an important part of their nourishment," Sanders said.

"The proportion of pollen in the spiders' diet in the wild was high, so we need to classify them as omnivores rather than carnivores," he added.

Prior research on orb-weaver spiders suggested that they consumed pollen as a side effect of eating their own webs to recycle the the silk proteins. However, Sanders and his research team found that this cannot be the case because of the size of the pollen grains ingested.

If the pollen grains were all small, it is possible they the could have more or less accidentally been ingested. But many of the larger grains of pollen found ingested by the spiders were also covered in a digestive enzyme.

"Feeding on larger pollen grains can only be achieved by active consumption, which would then need to be regarded as herbivory, and these spiders as omnivores as they feed on insect prey and pollen," the team wrote in their research paper.

"The importance of pollen in nutrition may decrease over time, when insect prey becomes more abundant in summer and the flowering season of the most abundant plants is over. However, feeding on pollen as juveniles is sufficient to classify these spiders as omnivores, rather than being pure predators, as they are carnivores and herbivores in an important life stage," they concluded.

Further studies may investigate whether spiders choose their web-building sites based on the pollen availability in the environment.