Chemical Defense by the Native Winter Ant (Prenolepis imparis) against the Invasive Argentine Ant (Linepithema humile)

The invasive Argentine ant (Linepithema humile) is established worldwide and displaces native ant species. In northern California, however, the native winter ant (Prenolepis imparis) persists in invaded areas. We found that in aggressive interactions between the two species, P. imparis employs a potent defensive secretion. Field observations were conducted at P. imparis nest sites both in the presence and absence of L. humile. These observations suggested and laboratory assays confirmed that P. imparis workers are more likely to secrete when outnumbered by L. humile. Workers of P. imparis were also more likely to secrete near their nest entrances than when foraging on trees. One-on-one laboratory trials showed that the P. imparis secretion is highly lethal to L. humile, causing 79% mortality. The nonpolar fraction of the secretion was chemically analyzed with gas chromatography/mass spectrometry, and found to be composed of long-chain and cyclic hydrocarbons. Chemical analysis of dissected P. imparis workers showed that the nonpolar fraction is derived from the Dufour's gland. Based on these conclusions, we hypothesize that this chemical defense may help P. imparis to resist displacement by L. humile

Effect of chemical secretion on <i>L. humile</i>.

<p>Proportion of <i>L. humile</i> workers that demonstrated specific behaviors after contact with the <i>P. imparis</i> secretion. There were <i>N</i> = 14 trials in which the <i>P. imparis</i> ant secreted on the <i>L. humile</i> ant.</p

Effect of the relative numbers of <i>L. humile</i> and <i>P. imparis</i> on aggressive behavior.

<p>Shown are the mean number of observations per trial of secretion (filled bars), gaster-flagging (hatched bars), and fighting (open bars) in relation to the proportion of <i>P. imparis</i> workers. Each assay was performed with 20 total <i>P. imparis</i> and <i>L. humile</i> workers and <i>N</i> = 15 trials for each proportion. Error bars show standard error of the mean.</p

The <i>P. imparis</i> secretion.

<p>A single <i>P. imparis</i> worker is shown with a liquid droplet containing bubbles at the tip of its raised abdomen. The secretion is then applied to the body of the <i>L. humile</i> ant.</p