BlOTROPlCA 30(2): 179-189 1998 Facultative Ant-Plant Interactions: Nectar Sugar Preferences of Introduced Pest Ant Species in South Florida’ Suzanne Koptur and Ni Truong Department of Biological Sciences, Florida International University, Miami, Florida 331 99, U.S.A.2 Fairchild Tropical Garden, Miami, Florida 33156, U.S.A. ABSTRACT We observed nectar use by native and exotic ant species in nature, garden, and urban situations, and found ants utilizing floral and extrafloral nectar of a variety of flowering plant species. We collected 31 plant nectars (29 floral, 2 extrafloral) and used them in feeding preference tests against standard solutions of sugars (20 percent fructose, glucose, and sucrose, and their mixture), 10 trials for each nectar-ant comparison. We comparcd time-to-discovcry and total ant visits to each droplet using ANOVA, and found that both trial and solution contributed significantly to the variation in most experiments. Seven of the floral ncctars tested were significantly more attractive to certain ant species than the sugar solutions. Not only do ants use floral nectar, but it appears that some floral nectars contain compounds that are especially attractive to ants. RESUMEN Se observ6 que varias especics de hormigas nativas y exoticas utilizaban nectar de florcs y nectar extrafloral dc varias cspecics de plantas en Aoraci6n, localizadas en jardincs, en el casco urbano y en areas naturalcs. Se colectaron 31 nectares dc plantas y su prcfcrencia fut comparada con soluciones de azucar (20 percicnto de fructosa, glucosa, y sucrosa, y sus rnczclas). Esta comparaci6n fuc realizada 10 vcces para cada combinacion de nectar y hormiga. Se utliz6 el Analisis de Varianza para comparar el tiempo quc lc tomaba a las hormigas en descubrir las solucioncs y el n6mero dc visitas a cada gota de nectar o solucibn. El analisis estadistico demonstrb que tanto el nhmero de replicaciones como la clase de soluci6n contribuia significativamcnte a la variacion dc la mayoria de 10s experi- mcntos. Sicte de 10s ncctarcs florales fueron mas significativos a algunas cspccics dc hormigas que las soluciones de azucar. Evidcntamentc, algunos nectarcs florales contcnian algunas substancias especialmente atraycntes a las hor- migas. Key words: mts; feeding; Florida; nectar: preference. ANTS ARE UBIQUITOUS AND PERFORM MANY IMPORTANT tile, ant-repellent substances in petal tissues of FUNc’ric” in the ecology of plants in tropical and plants, and demonstrated their existence in a sim- temperate areas (Huxley & Cutler 1991). Ants are ple experiment. often attracted to plants to collect exudates from The Bakers (1978) were the first to respond to extrafloral nectaries or from insects consuming the Janzen’s hypothesis, pointing out their observations plants. Flowers often contain nectar as the floral of chemicals in nectars such as alkaloids, phenolics, reward, and Janzen (1977) posed the question and nonprotein amino acids that may serve as de- “Why don’t ants visit flowers?”, hypothesizing that terrents to nectar theft. Feinsinger and Swarm floral nectar must contain ant repellents to protect (1978) tested the acceptability of nectar to ants of it for pollinators. Nearly one hundred years earlier, four common plant species in Trinidad, and found A. Kerner von Marilaun (1878) had described that responses of ants varied widely. Schubart and many ways in which plants can exclude nectar- Anderson (1978) offered freshly cut-open flowers thieving ants, including extrafloral nectaries, sticky of three plant species to ants in Brazil, and found stems, dense hairs, water traps, and distasteful sub- all to be readily consumed; though ants may have stances in floral tissue. Van der Pijl (1955) used the been excluded from flowers by structural barriers, term “myrmecophoby” for the deterrence of ants ant-repellent nectars were not indicated. Guerrant from flowers, hypothesizing the existence of vola- and Fiedler (1981) compared the acceptabiliry of floral nectars and floral tissue extracts to sugar so- lutions, and analyzed the plant substances chemi- Received 10 April 1997; revision accepted 29 Scptem- ber 1997. cally to detect deterrent substances; they found that * address for correspondence. floral nectars were attractive to ants, but floral tis- 179 180 Koptur and Truong Adenocalymna comosum Agave decipiens time to discovery time to discovery total ants total ants la 10 15 20 25 30 0 5 Ib 0 10 20 30 40 50 Aloe saponaria Aph elandra jacobinioides time to discovery time to discovery totai ants totai ants Ic 0 10 20 30 40 50 Id 0 5 10 15 20 25 30 Brownea sp. Brun felsia grandiflora time to discovery time to discovery total ants total ants le 0 5 10 15 20 25 If 0 5 10 15 20 FIGURE 1. Results of feeding experiments with crazy ants, Paratrechina longicornis. Bars represent the means of ten trials for each experiment (variation not shown). The upper set of bars represents the time-to-discovery of the solution in minutes; the lower set of bars represents the total number of ants to visit that solution during the one hour that the experiment ran. EFN = extrafloral nectar. Solutions are abbreviated as: N = nectar, F = fructose, G = glucose, S = sucrose, and FGS = fructose, glucose, and sucrose. Significant differences shown by Analysis of Variance and Tukey post hoc tests are indicated by lower case letters, in which case bars with different letters are significantly different within that data set. Nectar Preferences of Pest Ants 181 Cnnum moorei Dipteryx panamensis (efn) time to discovery time to discovery total ants total ants lg 0 5 10 15 20 25 30 35 Ih 0 10 20 30 40 Hamelia patens (squeezed) Hamelia patens (2) time to discovery time to discovery total ants total ants li 0 5 10 15 20 25 30 li 0 10 20 30 40 50 lxora coccinea Justicia fulvicoma time to discovery time to discovery total ants total ants Ik 0 5 10 15 20 25 30 11 0 5 10 15 20 5 ~ ~ FIGURE 1. Continued. sues showed highly variable palatability. Haber et consumed, and many of those enjoyed by ants were al. (1981) had observed ants taking floral nectar known to have phenolics and/or alkaloids. Many from many species of plants in the lowland dry nectars, therefore, are readily consumed by ants forest of Costa Rica, and tested many of these by when they are offered out of their flowers, if they placing nectar droplets on branches of trees and have been collected carefully so as to not contam- noting whether or not they were consumed by ants inate them with any repellents present in the floral within five minutes; all but two tested were readily parts. 182 Koptur and Truong Kalanchoe gastonis-bonnieri Megaskepasma erythrochlamys time to discovery time to discovery total ants total ants 0 10 20 40 Im 0 10 20 30 40 In 30 Odontonema strictata Russelia sarmentusa time to discovery time to discovery total ants total ants 0 5 10 15 20 25 30 10 0 5 10 15 20 25 30 Ip FIGURE I. Continued. In this study we document the use of nectars portant as pollinators since most have antifungal of a variety of cultivated tropical species by native metapleural secretions (.g.,myrmicacin) that kill and exotic ants, and investigate the attractiveness pollen (Beattie 1985). Our study relates primarily of the nectars to noncoevolved pest ant species in to the abilities of ants to obtain food, and to subtropical south Florida. In conducting the ex- whether or not the nectars tested might have non- periments we obtained information on the relative sugar constituents rendering them exceptionally at- preferences of these ants for the various sugar so- tractive to ants. lutions used in our controls as well as their different responses to the nectars tested. The facultative as- MATERIALS AND METHODS sociations observed suggested nectars that might be attractive to ants; we compared the acceptability of Observations were made over two years in the col- these nectars to standard sugar solutions in order lection of Fairchild Tropical Garden and, to a lesser to determine if any of these nectars were excep- extent, the University Park campus of Florida In- tionally attractive. Sugar solutions are nonvolatile, ternational University. We documented all ants us- and ants discover them by touch and taste; floral ing nectars, collecting ant specimens for determi- nectars may have other components that attract nation and later verification. ants from a distance, or affect their taste once they Several milliliters of nectar were collected into are tried. We do not presume that there are nec- a tiny centrifuge tube, and its concentration mea- essarily coevolved positive relationships between sured using a hand-held refractometer (Bellingham ants and floral nectars in natural communities. Un- & Stanley brand, reading percent sugar on a like the other social Hymenoptera, ants are usually weighdweight scale). Nectars were diluted with dis- visiting flowers as nectar robbers, and are unim- tilled water to a concentration of 20 percent, unless Nectar Preferences of Pest Ants 183 xRuttyruspolia sp. Sanchezia speciosa time to discovery time to discovery total ants total ants 5 10 15 20 25 0 10 20 30 40 50 Iq 0 30 35 Ir Sch o tia bra ch yp efa la time to discovery total ants Is 0 10 20 30 40 50 FIGURE 1. Continued. they were less than 20 percent sugar to begin with, with a different colony of ants). We did not replace in which case more dilute sugar solutions were pre- the droplets if they were used up before the end of pared. the trial (except for reruns of Hameliuputens, which For each nectar-ant comparison, we ran ten tri- initially was consumed very quickly; see below). als of the following design. Standard solutions of Trials that were interrupted by external factors (dis- 20 percent fructose (F), 20 percent glucose (G), 20 turbance by urban wildlife, inclement weather) percent sucrose (S), and a mixture of 20 percent were not included.