Plant-Insect Interactions: Acacia falcata and native ant visitors

Abstract

<p>The recruitment of arthropod defenders to host plants is a means by which host plants restrict damage from insect herbivores. Extrafloral nectaries (EFN) are a common plant feature associated with the recruitment of ant defenders. Three native ants <i>Iridomyrmex purpureus, Polyrhachis ammon</i> and <i>Camponotus aeneopilosus</i> (Formicidae, Hymenoptera) forage upon <i>Acacia falcata</i> EFNs for the nectar reward (NSW, Australia). These ants and host plant were observed to determine, a) which plant features are attractive to the foraging ants, b) if there are antennal structures responsible for the detection of those attractive cues, and c) if an electrophysiological response can be observed in the generalist forager, <i>I. purpureus</i>. Exudate from EFN and volatiles of <i>A. falcata</i> were examined using Gas Chromatography Mass Spectrometry (GCMS). Exudate was found to be comprised of sugar, while host plant volatiles from phyllodes were a collection of common volatile organic compounds. Several attractive compounds were found in the volatile suite of undamaged phyllodes, acting as potential attractants for the native ants. The antennae of the native ant foragers were examined using electron microscopy (EM) to determine the location of chemoreceptive sensilla, responsible for the detection of the aforementioned volatiles. Two primary chemoreceptive structures, a robust sharp-tipped and robust blunt tipped trichodea, were identified on the flagellum segments of the antennae of all three species. When antennae of I. purpureus was exposed to <i>A. falcata</i> volatiles, using an electroantennogram (EAG), electrophysiological response was minimal. However, response to food based cues such as sugar were observed. Foraging ants are likely attracted to <i>A. falcata</i> EFN for the sugary reward and, as response to general plant volatiles was minor, are recruited to the host plant for that reward alone.</p>