Parasitoids inside spider egg sacs
The brown widow, Latrodectus geometricus, is the only one of the 35 known widow-spider species to have spread across most of the world. One possible advantage is that its egg sacs may be unusually difficult or unrewarding targets for natural enemies.
The parasitoid wasp Philolema latrodecti lays eggs inside widow-spider egg sacs. Its developing larvae consume spider eggs before emerging as adult wasps. To compare how often this happens, researchers collected egg sacs from 20 sites across Israel and incubated them until either spiderlings or wasps emerged.
The difference was enormous
Only one of 1,895 brown widow egg sacs was parasitized: approximately 0.05%. Parasitism was much more frequent in all three native widows sampled—7.4% in Latrodectus pallidus, 18.3% in Latrodectus tredecimguttatus and 23% in Latrodectus revivensis.
The difference remained statistically significant when the comparison was restricted to sites in the Negev Desert where the parasitoid was present. That matters because brown widows were commonly collected around human structures, while the native species were generally found in less disturbed habitats where wasps may be easier to encounter.
Why brown widow egg sacs may be poor targets
Brown widow egg sacs were smaller than those of the native species, and the wasps emerging from them were also smaller. The authors suggest that these sacs may offer fewer resources and therefore be lower-quality nurseries for developing parasitoid larvae.
The field study did not isolate every defence. Earlier laboratory work found that the brown widow’s distinctive spike-covered silk can hinder a wasp trying to lay eggs through the sac wall. Females can also actively guard their sacs and drive approaching parasitoids away. The near absence of parasitism may therefore reflect a combination of habitat, egg-sac architecture, maternal defence and host quality.
An advantage during invasion
Invasive species sometimes succeed partly because they leave important enemies behind or are affected by them less severely than native relatives. The brown widow’s exceptionally low parasitism rate could give more of its offspring the chance to hatch, potentially contributing to its continued spread.
This does not establish parasitoid escape as the sole—or even the dominant—cause of the brown widow’s global success. Its tolerance of urban habitats, transport by people, aerial dispersal by spiderlings and interactions with native widows may all contribute.
Why biological control could backfire
Egg-sac parasitoids might look like appealing biological-control agents against an invasive spider. These results point in the opposite direction: releasing more wasps could have little effect on brown widows while placing much greater pressure on native widow species.
That is a warning rather than the result of a management trial. The study did not release parasitoids or measure population-level control. It shows that the proposed agent attacks the native species far more readily under the sampled conditions—exactly the imbalance that a responsible control programme would need to avoid.
Important caveats
The species were not distributed evenly among identical habitats, and the parasitoid was detected only in the central Negev. Earlier controlled experiments support the idea that the brown widow’s advantage is not explained by habitat alone, but the new field survey cannot assign a precise share of the difference to each defence.
The researchers also did not directly test whether wasps prefer larger egg sacs. Smaller sacs and smaller emerging wasps are consistent with lower host quality, but that proposed mechanism still needs a direct choice experiment.