Fire ant decapitating flies turn fire ants into zombies

    Posted on Tuesday, Nov 10, 2020
    Zombies are real!
    Did you know that some insects can manipulate the behavior of other insects? There are many parasites known as parasitoids that are able to hijack other arthropods (e.g., spiders, cockroaches, etc.). Parasitoids are specialized parasites that feed on or within their host, which ultimately leads to the hosts death. The fire ant decapitating fly (Pseudacteon spp.) is such a bug. These parasitoid flies complete their life cycle by using fire ants as a host. Since the early 2000’s, a few species of Pseudacteon decapitating flies have been introduced into North America from South America (Argentina and Brazil) to help combat the red imported fire ant (Solenopsis invicta). This is a concept known as biological control (or simply, biocontrol): searching the native location of your invasive pest for potential natural enemies–the enemy of your enemy is your friend.
    Fire ant decapitating flies are host specific on fire ants. They only lay their eggs in fire ants. When a female decapitating fly successfully lays an egg in a fire ant worker’s thorax, the journey of parasitism begins. The fire ant will return back to its nest. It will no longer forage. This is thought to aid in the decapitating fly’s survival because conditions are ideal in the nest and the risky behavior of foraging is avoided. It takes a couple of days for the egg to hatch. Once the egg hatches, the first instar larvae (or small maggot) quickly molts into a second instar larvae and moves to the ant’s head. At this point, little tissue other than hemolymph (analogous to blood) is consumed. Once the maggot molts into a third instar, it gets bigger and bigger and will thus need to consume the contents of the fire ant’s head--the brain last. It wouldn’t be wise to do this inside the nest. Can you imagine emerging inside a fire ant nest? To avoid this, zombie ant behavior has evolved. Several hours prior to decapitation and consumption of head tissues and the brain, the fire ant worker will wander far away from its colony alone, controlled by the Pseudacteon. Once a suitable location is found, it will stop and (presumably) secrete enzymes that loosen the connective tissues of the ant. The head typically falls off--thus the common name, decapitating flies. After a couple of weeks, the fly pupates and emerges as an adult fly. The adults are short lived. They mate and the females search for fire ant hosts to continue the cycle. 
    Researchers in the Rollins Urban & Structural Entomology Facility of the Entomology Department at Texas A&M are investigating the molecular basis of this manipulation of fire ants by decapitating flies. Through RNA-Seq and differential gene expression analysis, it will be possible to get a glimpse at how these parasitoid flies are manipulating their hosts. For example, genes involved in immunity and foraging may be lowly expressed in fire ants after they are successfully parasitized by the decapitating fly. Unraveling the molecular basis of host manipulation may help lead to novel pest management of the red imported fire ant.

    -- Joanie King

    Joanie is a doctoral student in the Department of Entomology

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