The Nature of Ant Attendance and the Survival of Larval Icaricia acmon (Lycaenidae)

I examined ant attendance and its importance to larval survivorship in a facultatively myrmecophilous butterfly, Icaricia acmon (Westwood and Hewitson) (Lycaenidae), in a population that uses two host plant species, Eriogonum compositum Doug!. and E. strictum Benth. (Polygonaceae). Third and fourth instar larvae of I. acmon were tended by three ant species: Tapinoma sessile (Say), Formica neogagates Emery, and an unidentified Formica species. Third instar larvae were tended less frequently than fourth instar larvae on both plant species, and T. sessile was the attendant ant species for a higher proportion of third instar than fourth instar larvae developing on E. compositum. Over the duration of the study, all switches of attendant ant species on individual plants were from early T. sessile attendance to later F . neogagates attendance. An exclosure experiment revealed that ant attendance had no significant effect on larval mortality. Additional key words: Tapinoma, Formica, facultative myrmecophily, Washington, Eriogonum. The association of lycaenid larvae with ants (Formicidae) has provided researchers with model systems for studying the costs and benefits of mutualisms. Recent work has focused on both ecological and evolutionary aspects of these mutualisms, including host-parasitoid interactions (Pierce & Mead 1981), oviposition behavior (Atsatt 1981a, Pierce & Elgar 1985), and the evolution of host choice (Atsatt 1981b, Pierce 1985). To understand the evolution of these mutualisms, it is critical to examine the costs and benefits to both partners. Several field studies have documented the importance of ants to the survival of larval lycaenids (Pierce & Mead 1981, Pierce & Easteal1986, Pierce et al. 1987, Fiedler & Maschwitz 1989b), but few have experimentally addressed this issue in facultative ant-Iycaenid associations (but see Pierce & Mead 1981, Pierce & Easteal 1986). This is particularly surprising when one considers that most lycaenid-ant associations are facultative (see Fiedler 1989a for a recent review). Although the mutualisms studied so far provide excellent ecological and evolutionary case studies, it is important to recognize how variability in the intensity of ant-Iycaenid associations might affect generalizations about these systems. To broaden our understanding of facultative ant-Iycaenid mutualisms, I conducted an ant exclosure experiment to determine the importance of ant attendance and host plant choice to larval demography in a population of Icaricia acmon (Westwood and Hewitson) (Polyommatinae: Polyommatini) in central Washington. I Present address: Section of Ecology and Systematics, Cornell University, Ithaca, New York 14853. VOLUME 47, NUMBER 1 9 STUDY SITE AND ORGANISM The population of Icaricia acmon lutzi (dos Passos) that I studied occupies a bench of the Yakima River, 11.2 miles north of Yakima, Washington at 46°47'N, 120027'W and 375 m elevation. The habitat represents the Artemisia/Festuca zone described by Franklin and Dyrness (1973), but has fewer Artemisia and more Eriogonum (Polygonaceae) species than typical Artemisia/Festuca communities. The four commonest Eriogonum species were E. compositum Doug!., E. strictum Benth., E. microthecum Nutt., and E. elatum Doug!. The climate in this region is arid to semiarid with relatively warm summers and cold winters. The mean annual precipitation in nearby Yakima is 20 cm with only a small amount (2.9 cm) of this falling during June through August. Average temperatures in the Yakima area in July and January are 21.7°C and -2.5°C, respectively (Franklin & Dyrness 1973). Although populations of Icaricia acmon in this area of Washington have been called hybrids with I. lupini (Boisduval) (Goodpasture 1973), regional systematists refer to all populations in the state as I. acmon (J. P. Pelham pers. comm.). Icaricia acmon ranges throughout much of western North America, and has been recorded feeding on Polygonum and at least seventeen species of Eriogonum (both Polygonaceae) as well as Lotus, Astragalus, Lupinus, and Melilotus (all Fabaceae) (Scott 1986). Subspecies lutzi ranges from British Columbia south to central Oregon and east to central Colorado (Goodpasture 1973). In Washington, I. acmon lutzi specializes on Eriogonum, having been recorded from E. compositum, E. pyrolifolium Hook., E. sphaerocephalum Doug!., and E. strictum, and has one or two broods, depending on the site (Peterson, un pub!. data). Larvae diapause in the third instar through winter and resume feeding when their host plant leafs out in the spring. Larvae feed on leaves or flowers by chewing holes in the surface of the structures and inserting their heads to mine out the internal tissues. Several ant species tend the third and fourth instar larvae of I. acmon, but earlier instars are not tended (Peterson pers. obs.). California populations of I. acmon have four instars and larvae have both a honey gland and eversible tentacles, structures associated with myrmecophily (Ballmer & Pratt 1988). Washington populations are similar in these regards (Peterson pers. obs.). It is not known when these structures first appear in development or if pupae of I. acmon are tended by ants.