For the last two episodes we have been fascinated by the Asian giant hornet and what it’s potential introduction and establishment in North America might mean to bee keepers and average citizens. This week we bring it home to the front flower bed to learn about a member of the brachonid wasp clan, a tiny American wasp called Aphidius. Each spring I am treated to hordes of prolific aphids colonizing my bee-balm and other perennials. Their tiny beaks probe meristematic buds, sucking sap and curling and twisting expanding leaves. These rascals excrete gobs of sticky honeydew on leaves and branches below; altogether they are quite disagreeable. Rather than panic and pull out an insecticide, I wait a few days or a week, and usually papery brown aphids will appear amongst the other plump, juicy members of the aphid colony. These papery brown aphids go by the colorful name of “aphid mummies”. Aphid mummies are the exoskeletons of once living aphids, aphids that have had their internal organs consumed by an Aphidius wasp larva which developed within.
As aphids feed on plants, plants release volatile chemicals that can attract Aphidius wasps to an individual plant under attack. In addition to plant attractants, the very honeydew excreted by aphids may also serve as attractants to their lethal Aphidius enemy. Once a colony of aphids is located, the tiny female wasp begins her attack. By curling a marvelously jointed abdomen beneath her body and between her legs, the stinger at the tip of the abdomen faces forward just like the lance of a charging medieval knight on horseback. She deftly jabs with her stinger, a.k.a. ovipositor in entomological jargon, until she finds just the right spot to pierce the exoskeleton and deposit an egg inside the hapless aphid. Upon hatching from the egg, the wasp larva begins to consume tissues within the aphid, but its choice of meals is selective. It does not consume the vital organs immediately and does not kill its aphid host outright. It nibbles around the edges so to speak, allowing the aphid to survive and feed while the wasp larva develops within. And you thought James Cameron was the first to come up with that idea for Aliens.
Cool wet weather has spawned an outbreak of aphids on my perennial plants. Among the plump juicy aphids are brown, papery aphid mummies, the product of attacks by parasitic Aphidius wasps. These tiny braconid wasps play an important role in reducing aphid populations on many woody and herbaceous landscape plants. Watch as a wasp tangles with a hapless aphid. The aphid does its best to kick and push the female wasp away. Her articulated and highly mobile abdomen curls beneath her body, providing a frontal assault on the aphid. At the tip of her abdomen is an ovipositor and as she stings the aphid her eggs are inserted into the aphid’s body.
Parasitic (a.k.a. parasitoid) wasps developing within their host are called endoparasites and those that develop while their host continues to feed and move about are called koinobionts. Other species of parasitic wasps, including the steel blue cricket hunter, potter wasp, and cicada killer we met in previous episodes, first paralyze their host, arresting its development before depositing an egg. These undead but paralyzed victims soon become food for the wasp larva. Bad way to go. Wasps with this modus operando are known as idiobionts. As they develop, some species of Aphidius exert a zombie-like mind control over their aphid host, causing it to wander away from the rest of the aphid colony. By creating distance from the rest of the herd, these parasitized zombie aphids may be less likely to be consumed by predators that would also kill the wasp larva developing within. As if this was not enough abuse, the parasitized aphid has one final chapter in its doomed existence. As the larval wasp nears the end of its development, it finally consumes the vital internal organs and tissues of the aphid, leaving behind only the papery exoskeleton and thereby creating a mummy. Within the mummy, the larva pupates and with time a tiny adult wasp emerges from the pupa, still within the aphid’s exoskeleton. Ready for what’s up when aphids have a hole in their rear end? To free itself from the mummy, Aphidius uses sharp jaws to chew a near perfectly round hole in the aphid’s abdomen as an escape hatch. Once out, it’s off to find some food, maybe a mate, and hunt more unfortunate aphids.
Aphidius wasps are not only important agents of biological control in my garden, but they also wage war on aphid pests found in many agricultural crops around the world. Some species can be purchased commercially and are released in greenhouses and conservatories to reduce aphid populations as an alternative to insecticides. While the larvae require aphid hosts in which to develop, adult wasps use floral resources like nectar as a carbohydrate source. So, be sure to have a season long diversity of flowering plants in your landscape to help sustain these tiny warriors and if you spot a papery brown aphid with a hole its rear end, you will have found your mummy.
Bug of the Week thanks Dr. Shrewsbury for providing inspiration for this episode. The fascinating articles “Host behaviour modification by the endoparasitoid Aphidius nigripes: A strategy to reduce hyperparasitism” by Jacques Brodeur and Jeremy McNeil, “Identification of Semiochemicals Released During Aphid Feeding That Attract Parasitoid Aphidius ervi” by Yongjun Du, Guy M. Poppy, Wilf Powell, John A. Pickett, and Lester J. Wadhams, and “Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops” by Sara G. Prado, Sarah E. Jandricic, and Steven D. Frank, were used in preparation of this episode.