It is not often that spiders “make the news”, that is, are covered in non-specialist publications. But in Australia the native and venomous Redback Spider made news in 2011, when for the first time it was observed being preyed upon by a parasitic wasp. The wasp (Agenioides nigricornis), native to Australia, belongs to a family (Pompilidae) of spider predators that is found world-wide. The Redback was well-known from Australia for centuries, but until 2 years ago, the presence of wasps that prey upon the venomous spider, though suspected, had not been verified.
The Redback Spider (Lactrodectus hasselti), is a member of the same genus as the infamous and venomous Black Widow spiders (L. mactans and L. variolus), found in the southern two-thirds of the United States. Other members of the genus occur in all the other continents outside the polar zones. Within the U.S., the state of Florida harbors the most species of widow spiders- four. These include, besides the aforementioned black widows, the specialized red widow (L. bishopi) restricted to the state’s sand hill habitats, and the Brown Widow (L. geometricus), originally from Africa, but now established in peninsular Florida and spreading north.
Like the Brown Widow, the Redback Spider has escaped to foreign lands. It is now established in New Zealand, Japan and perhaps other Asian countries. Redback bites can lead to serious injury or, uncommonly, death, but in other cases, venom is not transmitted in a bite and there may be no symptoms. The apparent preference of the female (the male makes no web) for buildings and human structures for web construction (as in most U.S. species as well) calls for extra caution, especially in damp, hidden areas near the ground.
All members of the Pompilidae wasp family prey on spiders, with most species laying a single egg on the paralyzed host and arranging for a protected location such a prepared cell in the ground or in a crevice or plant stem where the young wasp can develop. The egg hatches and the resultant wasp larva burrows into the spider, consuming it from within. Variations in these steps occur among the wasp species. For example, some species construct a cell first then search for prey while others prepare the cell after paralyzing their prey. A few species attack the spider in its own “home” and leave it there, while other species lay an egg on a spider that’s been stung by another wasp! Perhaps most amazing is that as larvae feed upon their hosts, they delay consuming the hosts’ vital organs, thus assuring the availability of living tissue in later developmental stages!
Apparently there’s enough energy and nutrients available in the single prey to bring the wasp through to adulthood. In fact, depending on host size, the young wasp develops into a larger female or a smaller male wasp. As adults, these rather ferocious little predators, only sip nectar for their nourishment. One might wonder to what extent the wasp helps limit spider numbers, a clear benefit to humans. Surely they do to some degree. However, because both species are Australian natives, they have co-existed for a long time, suggesting that they exist in some sort of population equilibrium. In any case, from an evolutionary viewpoint, it would be counter-productive for a predator to drive down prey numbers to levels too low to support the succeeding generation’s needs.
Wasps belonging to other families, such as the paper wasp, employ a similar approach to provisioning their young although the prey is likely to consist of caterpillars, weevils, grasshoppers or other insects. However the common, blue-black and shiny mud dauber (Chalybion californicum), often seen at pond edges or in wet soil is another spider specialist. It forms a mud nest, often on human structures, and places a paralyzed spider inside.
Often this species “refurbishes” an old nest of another wasp, bringing in water to soften the mud. There may be several compartments to the nest, each with a spider and a wasp egg. Among the blue mud dauber’s chief prey in the U. S. are the black widows.
So, think about that before you elect to knock down that unsightly mud nest (blob-like in some cases) adorning the walls of your porch or entranceway! Inside is or was a wasp larva (or larvae) feeding on widow spiders that otherwise might have been skulking in their webs waiting for their own prey.
References consulted include the following:
Borror, D.J., D. DeLong and C. A. Triplehorn. 1976. An Introduction to the Study of Insects. Holt, Rinehart and Winston, New York.
Edwards, G. B. and S. Marshall 2004. Florida’s Fabulous Spiders. 3rd Edition. World Pubs. Hawaiian Gardens, CA