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Parental care might help invasive brown widow spiders spread. Valeria Arabesky and colleagues at Ben-Gurion University of the Negev found that invasive brown widow spiders effectively defend their egg sacs against wasp parasites. The spiders use multiple defense strategies, including guarding behaviors and constructing their egg sacs with denser spikes when wasps are around. These strategies may benefit the invasive species and protect the spiders against high levels of predation and parasitism.

The study was just in Proceedings of the Royal Society B.

A major question as species increasingly spread around the world is what enables these species to survive and thrive in new locations. One important factor could be parental care, or how parents protect and help their offspring survive.

"For creatures that start their life as eggs, such as spiders, parental protection can be crucial. The eggs themselves are defenseless and very attractive to parasites and predators, thus their parents are their only hope for survival," explained Valeria Arabesky, the first author of the study.

The brown widow spider, Latrodectus geometricus, is an urban invasive species that generally lives under garbage cans, in fences, and around buildings. This study compared behaviors of the invasive brown widow spider and a native, the white widow spider, common in desert and semi-arid habitats in the Middle East.

Both spider species became agitated when tiny parasitic wasps, Philolema latrodecti, approached the spider webs that contained the spiders' egg sacs, eggs wrapped in silk cocoons. The brown widow spider tapped her egg sacs and circled it protectively with her legs, while the white widow spider shook her whole body and the web to dislodge the wasp.

Moreover, the brown widow captured and killed wasps, something that the white widow spider never did. Brown widows even captured wasps and threw them out of the web without feeding on them. Overall, the invasive brown widow spider's behaviors were more effective in preventing attacks by wasps.

Brown widow spiders also cover their silken egg sacs with silk spikes, something that the native species doesn't do. When researchers removed spikes from half of the brown widow's egg sac and introduced the parasitic wasps, they found that the wasps preferred the smoothed side for laying eggs.

That suggests that spikes really do work as a defense. Brown widow mothers even increased the density of spikes on egg sacs after being around wasps, which suggests that spiders increase defenses when a threat is detected. These extra defenses, guarding and protecting the egg sacs, may help brown widows avoid having their offspring attacked by wasps.

Future research could continue to find out whether parental care may increase invasion success in other species.