Russian Dolls of the Bug World

A parasitic Aphidius wasp descends on its victim: a pea aphid. The wasp thrusts its abdomen forward between the legs to inject an egg into the tiny insect.
A parasitic Aphidius wasp descends on its victim: a pea aphid. The wasp thrusts its abdomen forward between the legs to inject an egg into the tiny insect. "It all happens in an instant," said Molly Hunter of the UA's department of entomology. (Photo: Alex Wild)

A research team including Martha (Molly) Hunter from the department of entomology in the University of Arizona's College of Agriculture of Life Sciences has disentangled relationships in an assembly of players that resemble Russian dolls: a bacterium that lives inside a tiny insect, a virus that infects those bacteria, and a parasitic wasp that lays its eggs in the insect.


In a war between parasite and host, the parasitic wasp, Aphidius ervi, and the pea aphid, Acyrthosiphon pisum, are locked in a battle for survival.


Hailing from Europe, the pea aphid hitchhiked to North America before the turn of the 20th century, where it spread as a pest of peas and related crops in the legume family. Later, the parasitic wasp was introduced as a biological pest control agent, in an effort to keep the pest in check.
 
The wasp, A. ervi, lays an egg inside the pea aphid, where the egg hatches and converts the aphid's insides into a wasp nursery. The wasp larva uses the still-living aphid as a food source, eventually pupating inside the aphid and emerging as a fully formed mature wasp.


However, the pea aphid is not defenseless. It is protected by the bacterium Hamiltonella defense, and its associated bacteriophage called APSE. Bacteriophages are viruses that exclusively infect bacteria.


The bacteria with their incorporated phage float around in the aphid's hemolymph, "the insect's equivalent of blood" said Hunter, and also make their home in specialized cells provided by the aphid for the purpose of hosting their symbionts. In addition to conferring greater resistance to heat stress on the aphids, the main job of the bacteria and their phages appears to be protection against the parasitic wasps.


Although the wasps still lay their eggs inside aphids hosting the bacterial symbiont, the larvae that hatch from those eggs are unable to develop normally. The research group, which included Judith Becerra, a chemical ecologist in the UA's department of ecology and evolutionary biology also affiliated with Biosphere 2, set out to investigate what tactics the wasps might use to overcome the aphids' symbiont defense strategy.


Read the rest of this February 28 UANews article at http://uanews.org/node/45176

Date released: 
Mar 2 2012
Contact: 
Molly Hunter