Tuesday, March 13, 2012

Hymenoepimecis argyraphaga: the controller/destroyer of spiders


Introduction
     The Costa Rican wasp Hymenoepimecis argyraphaga is found in the forests of, you guessed it, Costa Rica! It is a parasitoid that uses the spider Plesiometa argyra as a host. H. argyraphaga  is able to use the spider for food as well as control it to build it a "cocoon web" when necessary. 

Symbiont description
     There is an entire family of wasps who use other species to raise their young [1] and H. argyraphaga is a prime example. This Costa Rican wasp has found a way to keep their young protected before they are able to cocoon. Unlike many parasitic wasps which lay hundreds or thousands of eggs into their hosts, this meticulous mom only lays one egg on the host spider's abdomen [1]. Since the larva stage of the wasp is defenseless, the egg is planted on the P. argyra spider where it will hatch. The larva will reside on the spider, virtually free of predation, until it becomes old enough to cocoon into the adult stage. 

Host description

     The spider P. argyra spends most of its day spinning a carefully detailed web to catch the required insects it needs to live. It is often described as an orb--a platform suspended by strong "cables" of spider silk, which resists wind and rain [2]--weaving spider.

Life cycle
     Once the adult female wasp has mated, she will find a suitable P. argyra spider to parasitize. The H. argyraphaga female wasp stings the orchard spider, temporarily paralyzing it so it can lay an egg on the spider's abdomen [3]. The egg will hatch into a larva and it will feast on the spider's hemolymph (the circulatory fluid of invertabrates [4]) through small holes. The larva will continue to grow for about 2-3 weeks while the spider carries on with its normal activities, apparently not noticing it has a larva on its back!


     When the larva is ready to pupate, it injects a chemical into the spider [5]. This chemical drastically changes the web-spinning ability of the P. argyra spider. Instead of beautifully patterned webs that it had been spinning, the spider, under the zombie poison of the larva, starts a completely different, sloppy-looking web that is reinforced and will hold a much heavier object--the larva's cocoon [3]. 


     With the platform ready and the spider patiently awaiting his ultimate demise, the larva then freely molts, kills the spider with a toxic poison, and sucks the remains of the juicy insides out before discarding the carcass and building its cocoon [1]. The web allows the larva to cocoon in midair away from other insects/predators that can be found on the ground. The adult wasp will emerge from the cocoon and the cycle will begin again. 

Ecology:
     Spiders are among the most abundant insectivorous predators of terrestrial ecosystems [6]. Thus they can have an ecological impact on the insect population in their habitats. Medical research using spider venom has yielded several chemicals that may be useful to control or treat diseases in humans [7]. Thus P. argyra serves to keep insect populations down and research on its venom could lead to the discovery of treatment for human diseases. The effect of the Costa Rican wasp could potentially alter the population of the P. argyra but keep in mind that H. argyraphaga needs the spider in order for their offspring to survive. Thus, the Costa Rican wasp must find the balance necessary to allow the spider to procreate while also making its own offspring.

An example of manipulation
     Manipulation of a host can take many forms but as Combes states "In many other cases, however, the manipulation is done partially or totally at the molecular scale" [8]. H. argyraphaga is an example where the manipulation is done solely by injecting the host with a chemical that alters its behavior. More research needs to be done to fully understand how this chemical is making the spider change its behavior and make a web it has never woven before. H. argyraphaga is also an example of a durable interaction that results in the death of the host--parasitoidism. 

References
[8] Combes, Claude. Parasitism. Chicago: The University of Chicago Press. 2001. Print. 


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