Colony - level impacts of parasitoid ies on re ants

The red imported Ž re ant is becoming a global ecological problem, having invaded the United States, Puerto Rico, New Zealand and, most recently, Australia. In its established areas, this pest is devastating natural biodiversity. Early attempts to halt Ž re ant expansion with pesticides actually enhanced its spread. Phorid  y parasitoids from South America have now been introduced into the United States as potential biological control agents of the red imported Ž re ant, but the impact of these  ies on Ž re ant populations is currently unknown. In the laboratory, we show that an average phorid density of as little as one attacking  y per 200 foraging ants decreased colony protein consumption nearly twofold and signiŽ cantly reduced numbers of large-sized workers 50 days later. The high impact of a single phorid occurred mainly because ants decreased foraging rates in the presence of the  ies. Our experiments, the Ž rst (to our knowledge) to link indirect and direct effects of phorids on Ž re ants, demonstrate that colonies can be stressed with surprisingly low parasitoid densities. We interpret our Ž ndings with regard to the more complex Ž re ant– phorid interactions in the Ž eld.