POPULATIONECOLOGY Microgeographic and Vertical Distribution of Acromynnex landolti (Hymenoptera: Formicidae) Nests in a Neotropical Savanna STEPHEN L. LAPOINTE,l MIGUEL S. SERRANO,2 ANDPETER G. JONES Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713, Cali, Colombia Environ.Entomo!.27(3): 636-641 (1998) ABSTRACT Nests of the grass-cutting attine ant Acromyrmex Landolti (Forel) were excavated in a lowland tropical savanna in eastern Colombia over a period of 27 mo. Of 135 colonies, nests excavated during the dry season were deeper, had more chambers, and were'" 2.5 times larger than those excavated in the same area of native savanna during the wet season. Chambers containing fungus gardens were found to a depth exceeding 2 m during the dry season. During the rainy season, all chambers containing fungus gardens were located within 30 cm of the soil surface. Individual chamber size did not vary seasonally. During the rainy season, colonies relocated to superficial chambers and were smallest in terms of total colony volume and number of chambers per colony. During the dry season, colonies excavated new chambers to depths where soil moisture asymptot- ically approached a maximum level. Maps of colony location and microtopography of the savanna revealed an aggregated distribution associated with microrelief. Colonies were aggregated on slightly raised ridges that were most likely caused by surface water runoff. Implications for sampling and control are discussed. KEY WORDS Attini, leafcutter ants, nest phenology, distribution, Colombia LEAFCUTIERANTSOF the myrmicine genera Atta and Acromynnex (tribe Attini) are common throughout the lowland neotropical savannas where they build complex colonies. Attine colonies consist of a series of connected underground chambers in which a symbi- otic fungus is cultured on plant material as food for larvae and adults. Colonies of the grass-cutting attine Acromynnex landolti (Forel) are present in high den- sities in neotropical savannas, and they damage pas- tures and crops (Robinson and Fowler 1982, Lapointe et al. 1990). Because A. landolti cuts only grasses, the term grass-cutter is sometimes used to distinguish it from its more catholic attine relatives, the leafcutter ants of the genus Atta. The habits of A. landolti are not as well known as those ofleafcutters. Unlike species of Atta, A. landolti has no soldier caste, does not maintain obvious foraging trails, and has relatively small colo- nies with a single entrance (Lapointe et al. 1993). A. landolti is capable of periodic colony relocation and rapid colonization of susceptible plantings (Lapointe et al. 1993, 1996). Colonies of A. landolti typically reach densities of 600-1,000 colonies per hectare in native savanna and >5,000 colonies per hectare in swards of susceptible forage grasses (Lapointe et al. 1990). In these extreme cases, total collapse of the pasture is inevitable. I Currentaddress:U.S.HorticulturalResearchLaboratory,USDA- ARS, 3020 PlymouthOaksRoad,Apopka, FL 32712. 2 Current address:DepartmentofEntomology,Universityof Mis- souri,Columbia, MO 65211. The relatively small size and high density of colonies of A. landolti make these attines more difficult to control by direct poisoning than other attine species, but also more amenable to studies of nest architecture and colony phenology. An understanding of colony distribution and phenology is needed to develop sam- pling methods, optimize control by direct poisoning, and determine the best methods for land preparation. The disruption of colonies that occurs with seedbed preparation can reduce colony density during the crit- ical establishment phase of improved pastures and susceptible crops, such as rice and maize (Lapointe et al. 1990). We repOlt here the results of mapping stud- ies that reveal the distribution of colonies of A. landolti in savanna grasslands of eastern Colombia, and of excavations that show seasonal variation in the vertical location of colony fungus gardens in the soil profile. Materials and Methods Excavations. A lightly-grazed area (Yopare) of semi-natural savanna, known to be densely colonized by A. landolti, was selected to study seasonal variation in the vertical position of colonies. The area, located at the Carimagua Experiment Station in eastern Meta department of Colombia, was typical of native savanna used for grazing under minimal management, and con- sisted of native species of grasses and forbs dominated by grasses of the genera Andropogon, Axonopus, Pas- palum, and Trachypogon. Annual burns were common during the dry season to improve the forage quality of these native grasses. 0046-225X/98/0636-0641$02.00/0 © 1998 EntomologicalSocietyofAmerica Downloaded from https://academic.oup.com/ee/article/27/3/636/2464536 by guest on 20 December 2022