Competitive mechanisms underlying the displacement of native ants by the invasive Argentine ant

Ecology, Jan, 1999 by David A. Holway

Defensive-compound repellency experiment

Although Argentine ants employ chemical defensive compounds when fighting (Brown 1973, Lieberburg et al. 1975, Tremper 1976), no information exists on whether these compounds are more or less repellent than those produced by other ants. Using an approach similar to that used by Adams and Traniello (1981) and Anderson et al. (1991); I compared the repellency of the Argentine ant's defensive compounds with those produced by native ants. One colony fragment (i.e., workers and brood) of each of the eight species was collected near Davis, California, in early June 1996, transported to Salt Lake City, Utah, housed in the laboratory, and fed fresh insects, apple jelly, and a standard laboratory diet (Bhatkar and Whitcomb 1970) ad libitum. Defensive compounds of each species were isolated as follows. For the four species in the subfamily Dolichoderinae (D. insanus, T. sessile, L. occidentale, and L. humile), which store defensive compounds in pygidial glands in the gaster (Holldobler and Wilson 1990), I dissected out pygidial glands from freshly freeze-killed workers and crushed each gland in 0.2 mL of a 95%-ethanol solution. For the four species in the subfamilies Formicinae (F. aerata and F. moki) and Myrmicinae (A. occidentalis and M. ergatogyna), which store defensive compounds in poison glands in the gaster (Holldobler and Wilson 1990), dissection of the glands proved difficult. For the Formicines I collected defensive compounds of freshly freeze-killed workers by milking the posterior tip of the gaster against the lip of a glass vial and rinsing the gaster tip and vial lip with 0.2 mL of 95% ethanol. For the Myrmicines, I removed the gasters of freshly freeze-killed workers and crushed each gaster in 0.2 mL of 95% ethanol. Following preparation, all treatment solutions were stored at -25 [degrees] C and used within a few hours of preparation.

I estimated the repellency of each compound using the recruitment behavior of the pavement ant (Tetramorium caespitum) in the field as a bioassy. I presented each T. caespitum colony with a pair of baits (each bait [approximately] 1-g tuna) placed at opposite ends of a microscope slide, and then I waited until at least 20 workers recruited. Using a 1.0-mL syringe, I surrounded the treatment bait with 0.2 mL of a 95% ethanolic solution containing one worker equivalent of the defensive compounds of one of the eight ant species and surrounded the control bait with 0.2 mL of 95% ethanol. I waited until at least 20 workers had returned (usually within 15 min) and counted the number of workers at each bait. I tested each T. caespitum colony (n = 13) with each of the eight defensive compounds on one occasion and on separate days during the last two weeks of June 1996. All experiments were conducted over narrow intervals of time (1800-2000) and temperature (22 [degrees] 26 [degrees] C). For each experiment, I calculated the recruitment differential (i.e., number of workers recruiting to the treatment bait - number of workers recruiting to the control bait); these differentials are a measure of the absolute repellency of worker-equivalent extracts. Using paired t tests, I compared the recruitment differentials from the Argentine ant treatment to the recruitment differentials from each of the seven native-ant treatments, correcting for seven comparisons using the Bonferroni method suggested by Rice (1989). The pavement ant was used in this bioassay because it exhibits strong recruitment behavior and shares no evolutionary history with any of the eight species of ants used in this study (thus reducing the chance that it has developed a tolerance towards any of the defensive compounds). Because T. caespitum might nonetheless exhibit idiosyncratic aversions to certain defensive compounds, I also tested the repellencies of the eight compounds using colony fragments of L. humile, L. occidentale, and M. ergatogyna in the laboratory. Methods used were similar to those above except that each colony fragment was provided with each of seven defensive compounds (no conspecific comparisons) on one occasion only.