Parasitoid case history: an evaluation of methods used to assess host ranges of fire ant decapitating flies.

The first three papers in this section have discussed factors that affect the efficiency and success of laboratory host range tests. This paper presents an evaluation of how well those factors applied to our investigations of host ranges of fire ant decapitating flies in the genus Pseudacteon (Diptera: Phoridae). We initially discuss the nature of the fire ant problem (Hymenoptera: Formicidae: Solenopsis spp.) and the need for effective self-sustaining biological control agents. We briefly review the biology of Pseudacteon decapitating flies, the overall results of our host range tests, and the current status of field releases of these biological control agents. We conclude by discussing how well the recommendations of the three initial papers about 1) statistical procedures, 2) biotypes and cryptic species, and 3) experimental design, plus a recent book on the subject of host range testing, apply to our experiences with fire ant decapitating flies. BACKGROUND OF PARASITOID SYSTEM THE FIRE ANT PROBLEM AND NEED FOR SELF-SUSTAINING BIOLOGICAL CONTROL The major problem with invasive fire ants (Hymenoptera: Formicidae: Solenopsis spp.) is that there are so many of them. In north Florida pastures, fire ant densities average 1,800-3,500 ants per square meter or about 1.5-3.0 metric tons of fire ants per square kilometer (Macom and Porter 1996; converted from dry weight to wet weight). Economic damage to agriculture, electrical equipment, and human health in the United States is estimated at nearly 6 billion dollars per year (Lard et al. 2001; Pereira et al. 2002), not including environmental damage. _____________________________________ Assessing the Host Specificity of Fire Ant Decapitating Flies Second International Symposium on Biological Control of Arthropods 635 Fire ant populations in their South American homeland are about 1/5 as dense as populations normally found in North America (Porter et al. 1997). This intercontinental difference in fire ant densities was not explained by differences in climate, habitat, soil type, land use, plant cover, or sampling protocols (Porter et al. 1997). Escape from numerous natural enemies left behind in South America is the most apparent explanation for the intercontinental population differences. Classical or self-sustaining biological control agents are currently the only potential means for achieving permanent regional control of fire ants. BIOLOGY OF PSEUDACTEON DECAPITATING FLIES Information on the life history, phenology, and biogeography of South American Pseudacteon species, is accumulating (Porter 1998a; Folgarait, et al. 2002; 2003; 2005a; 2005b; Calcaterra et al. 2005). At least 20 species of Pseudacteon flies (Diptera: Phoridae) have been found attacking fire ants in South America (Porter & Pesquero 2001; Brown et al. 2003). Up to nine species of these flies have been found at a single site (Calcaterra et al. 2005). Each species has a distinctively shaped ovipositor that is presumably used in a lock-and-key fashion to lay eggs in a particular part of its host’s body. Female flies usually contain a hundred or more eggs (Zacaro & Porter 2003). During oviposition, one egg is rapidly injected into the ant thorax with a short hypodermic shaped ovipositor (Fig 1A). Shortly after hatching, maggots of Pseudacteon flies move into the heads of their hosts where they develop slowly for two to three weeks (Porter et al. 1995a). Just prior to pupation, the third instar maggot appears to release an enzyme that dissolves the membranes holding the exoskeleton together. The maggot then proceeds to consume the entire contents of the ant’s head, a process that usually results in rapid decapitation of the living host. The headless body is usually left with its legs still twitching (Fig. 1B). Figure 1. A) Female decapitating fly (Pseudacteon) preparing to inject an egg into the thorax of a fire ant worker (Solenopsis). B) Decapitated fire ant worker with a fly maggot consuming the contents of its head. UGA1390062, UGA1390063 The maggot then uses hydraulic extensions to push the ant’s mouth parts aside, after which it pupates within the empty head capsule, positioned so that the anterior three segments harden to form a plate that precisely fills the ant’s oral cavity (Porter 1998a). The rest of the puparium remains unsclerotized and is protected by the ant’s head capsule, which functions as a pupal case. Pupal development requires two to three weeks depending on temperature.