Summary. Because the size of Atta spp. along foragingtrails is partly determined by the characteristics of the plantsharvested, and considering that parasitic phorid flies are at-tracted mostly to large individuals, we hypothesized that planttoughness affects the susceptibility of Atta spp. to these parasitoids. To test this hypothesis, we evaluated parasitismrates of the phorid Neodohrniphora sp. and its effect on Attasexdens (L.) foragers in a laboratory colony. We manipulatedforager size by alternating tough (Anthocephalus chinensis,Rubiaceae) and tender (Rosa chinensis, Rosaceae) plantsgiven to the colony. Ants foraging on tough leaves were larger than ants foraging on soft leaves, and there was a sign-ificant reduction in forager size for both plants when thecolony was exposed to Neodohrniphora sp. However, therewere no relative differences on forager size between the twoplants after the introduction of the parasitoid. The lack of re-sponse of Neodohrniphora sp. to the increase in ant sizewhen the colony was given tough leaves may be attributed tothe unusually large number of suitable hosts in a laboratorycolony. However, large foragers are much less abundant inthe field, in which case shifts in the size of the workforce trig-gered by different substrates could affect the incidence ofparasitism.

Key words: Atta sexdens, leaf-cutting ants, Neodohrniphora,parasitism, Phoridae.

Introduction

Leaf-cutting ants in the genus Atta are subject to parasitismfrom several species of flies in the family Phoridae (Feenerand Moss, 1990), a group of insects whose mere presencemay alter the behavior of Atta workers and reduce their for-aging ability (Feener and Moss, 1990; Orr, 1992; Feener andBrown, 1993). Phorid disturbance may be an important fac-

tor affecting the fitness of leaf-cutting ants and other ant spe-cies (Feener and Brown, 1997).

Neodohrniphora sp. is a phorid parasitoid of one of themost common and economically important leaf-cutting antspecies in Southeast Brazil, Atta sexdens (L.). Parasitismrates of Neodohrniphora sp. are relatively low, but flieshovering over ant trails reduce the number of foraging antsand the average size of foragers and leaf fragments transport-ed (Tonhasca, 1996; Bragança et al., 1998). Neodohrniphorasp., similar to other parasitic phorids (Orr, 1992; Feener andBrown, 1993), has a distinct preference for larger ants (Ton-hasca, 1996). Because the size of foraging workers is strong-ly affected by plant characteristics (Cherrett, 1972; Nichols-Orians and Schultz, 1989), we hypothesized that toughnessof plant species harvested by A. sexdens affects host suscep-tibility to Neodohrniphora sp. We expected that colonies for-aging on tougher leaves would have a higher proportion oflarger ants exposed to parasitism when compared with colo-nies foraging on softer plant tissues. To test this hypothesis,we manipulated forager size by alternating tough and softleaves given to a laboratory colony of A. sexdens and com-pared ant foraging rhythm and parasitism rates of Neodohr-niphora sp. in both substrates.

Materials and methods

The study was conducted with an adult A. sexdens colony (4.5 years old)maintained in a rearing room at the Universidade Federal de Viçosa,Brazil (20°45¢S, 42°51¢W). Temperature, humidity, and photoperiod inthe laboratory were kept at 23 ± 1°C, 85 ± 5%, and L10:D14 respec-tively. An acrylic observation chamber (100 cm long, 45 cm wide, and40 cm high) was placed between the nest and an open foraging arenawhere fresh leaves of Anthocephalus chinensis Walp. (Rubiaceae; toughsubstrate) or Rosa chinensis Jacq. (Rosaceae; tender substrate) wereprovided ad libitum. These species are highly attractive to A. sexdens.The experiment consisted of alternating the plant species in the foragingarena and measuring effects on A. sexdens foragers with and without thepresence of phorids. One of the plants was supplied to the colony for at

Insectes soc. 47 (2000) 220–2220020-1812/00/030220-03 $ 1.50+0.20/0© Birkhäuser Verlag, Basel, 2000

Insectes Sociaux

Research article

Effect of leaf toughness on the susceptibility of the leaf-cutting ant Atta sexdens to attacks of a phorid parasitoid

A. Tonhasca Jr. and M.A.L. Bragança

Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000, Campos dos Goytacazes, RJ 28015-620, Brazil, e-mail: tonhasca@uenf.br

Received 30 August 1999; revised 9 December 1999; accepted 21 January 2000.

Insectes soc. Vol. 47, 2000 Research article 221

least 7 h before each observation. The plant species was changed, and90 min later we sampled foragers returning to the nest and their respec-tive loads. This waiting period allowed workers to adapt to the newresource. We collected five subsamples consisting of five foragers cho-sen at random every 5 min. Ants and their loads were weighed to thenearest 0.1 mg on a precision balance and returned to the nest. We usedmass as a convenient way to express size, which is the actual variablethat affects host selection. To measure foraging intensity, we counted thenumber of loaded and unloaded ants passing a marked point on the plas-tic tube connecting the observation chamber to the nest. Each sampleconsisted of an average of three consecutive two-minute counts. Wethen released one female Neodohrniphora sp. in the observation cham-ber. Flies were previously collected from A. sexdens foraging trails inthe field and kept in the laboratory for up to 6 h until the experiment.Most phorids started to search for their host as soon as they were re-leased; flies that did not pursue ants within 5 min were replaced. Theparasitoid remained in the chamber for 45 min, and during the first 20 min we recorded the number of successful attacks and attacking bouts, when flies only touch the host (Tonhasca, 1996). Despite theirswiftness, Neodohrniphora sp. attacking bouts are as effective as actual attacks in prompting ants to drop their loads and leave the trail(Bragança et al., 1998). Subsequently we sampled foragers as previous-ly described, and the parasitoid was removed from the chamber. Therewere eight observations for each plant species in a two-month period,and ants attacked by Neodohrniphora sp. during five observations foreach plant species were collected, weighed to the nearest 0.1 mg andreturned to the nest. We evaluated differences between tough and softleaves in ant traffic, size of attacked ants, number of attacking bouts andactual attacks of Neodohrniphora sp. with 2-tailed t-tests. Two-wayANOVAs were used to evaluate the effects of phorid presence, plant species and the interaction between phorid presence and plantspecies on the average size of ants and leaf fragments.

Results and discussion

No significant differences were found between plant speciesfor the total number of ants returning to the nest, but the num-ber of loaded ants was significantly greater for the soft lea-ves than for the tough leaves (Table 1). The former resultshould have no effect on ants’ response to the parasitoidbecause while searching for hosts Neodohrniphora sp. doesnot discriminate between loaded and unloaded ants (Tonhas-ca, 1996; Bragança et al., 1998). Differences in the numberof Neodohrniphora sp. attacking bouts and effective attackswere not statistically significant, nor was there a significantdifference for the size of attacked ants (Table 1). Whenresults from both plant species are combined, there were

33.9 ± 4.2 (mean ± 1 standard error) attacking bouts and9.7 ± 1.0 actual attacks/observation period on ants that aver-aged 26.1 ± 1.2 mg. Considering the overall nestbound traffic of 100.6 ± 4.0 ants/2-minute period, we can estimatethe percentages of potentially parasitized and harassed nestbound ants as being about 0.9% and 3.4%, respectively.

The ANOVA results indicated that, as expected, ants for-aging on tough leaves were significantly heavier (thereforelarger) than ants foraging on tender leaves (Table 2, Fig. 1A).When the colony was exposed to Neodohrniphora sp., therewas a significant reduction in forager size (Table 2, Fig. 1A).However, there were no relative differences between the two plants in forager size after the introduction of the parasi-toid, as indicated by the non-significant interaction betweenplant species and presence or absence of phorids (Table 2).Wetterer (1994) observed a gradual reduction in the numberof larger Atta cephalotes (L.) workers when foragers werecutting soft leaves, thus a recruitment effect could cause the reduction of forager size after fresh leaves were given tothe colony. However, in preliminary observations we havedetermined that 90 min is sufficient to stabilize the size of A. sexdens foraging force in laboratory. Moreover, the re-duction of forager size was similar for both plants althoughwe could expected a shift towards larger ants on tough plants (Wetterer, 1994). As larger A. sexdens foragers carrylarger loads (unpubl. data), ants foraging on tough leavestransported heavier leaf fragments than ants foraging on tender leaves, but phorids did not affect load size (Table 2,Fig. 1B).

These results show that A. sexdens foragers are largerwhen foraging on tougher substrates, but Neodohrniphorasp. did not respond to the greater availability of hosts with anincreased number of attacks. It is possible that fully activelaboratory colonies provide a sufficient number of hostsregardless of the plant species utilized by the colony. How-ever, daytime foraging by A. sexdens is scarce, and duringthis time suitable large ants represent a small fraction of theforaging force. In this situation, shifts in forager size trigge-red by different substrates could affect the incidence of para-sitism. Nonetheless, this study supports earlier findings thatthe presence of phorids along foraging trails causes a signifi-cant reduction of forager size even though the risk of parasi-tism is low (Bragança et al. 1998).

Table 1. Means ± standard errors of the number of nestbound A. sex-dens, Neodohrniphora sp. attack attempts, size (expressed as mass inmilligrams) of attacked ants, and results of t-tests for comparing tough(Anthocephalus chinensis) and tender (Rosa chinensis) leaves (n = 8,except for the size data)

Tough leaves Tender leaves t P

All ants 102.1 ± 4.7 99.2 ± 6.8 0.35 0.73Loaded ants 17.5 ± 1.6 34.7 ± 2.7 5.37 0.0002Attacking bouts 38.4 ± 7.2 29.4 ± 4.3 1.07 0.31Successful attacks 10.7 ± 1.5 8.6 ± 1.4 1.03 0.32Size of attacked 26.5 ± 1.5 25.6 ± 2.1 0.36 0.72

ants (n = 45) (n = 33)

Table 2. Two-way ANOVAs to evaluate the effects of phorid presence(yes or no), plant species (A. chinensis or R. chinensis) and their inter-action on A. sexdens and leaf fragment size

Variable Factor df Mean squares F P

Ant size Phorid presence 1 23.29 15.84 < 0.001Plant 1 52.79 35.91 < 0.001Phorid ¥ plant 1 0.05 0.03 0.85Error 28 1.47

Leaf size Phorid presence 1 0.63 0.03 0.86Plant 1 1727.25 91.58 < 0.001Phorid ¥ plant 1 36.77 1.95 0.17Error 28 18.86

222 A. Tonhasca Jr. and M.A.L. Braganca Leaf toughness and phorid parasitism on Atta sexdens

Acknowledgements

T.M.C. Della Lucia provided logistic support, M. Erthal Jr. and D. Moreira were very helpful with field and laboratory work, and two anonymous reviewers made important contributions to the manuscript.A.T.J. was supported by a grant from the International Foundation forScience.

References

Bragança, M.A.L., A. Tonhasca Jr. and T.M.C. Della Lucia, 1998.Reduction in the foraging activity of the leaf-cutting ant Atta sex-dens caused by the phorid Neodohrniphora sp. Entomol. Exp. Appl.89: 305–311.

Cherrett, J.M., 1972. Some factors involved in the selection of vegeta-ble substrate by Atta cephalotes (L.) in tropical rain forest. J. Anim.Ecol. 41: 647–660.

Feener, Jr., D.H. and K.A.G. Moss, 1990. Defense against parasites byhitchhikers in leaf-cutting ants: a quantitative assessment. Behav.Ecol. Sociobiol. 26: 17–29.

Feener, Jr., D.H. and B.V. Brown, 1993. Oviposition behavior of an ant-parasitizing fly, Neodohrniphora curvinervis (Diptera: Phoridae),and defense behavior by its leaf-cutting ant host Atta cephalotes(Hymenoptera: Formicidae). J. Insect Behav. 6: 675–688.

Feener, Jr., D.H. and B.V. Brown, 1997. Diptera as parasitoids. Annu.Rev. Entomol. 42: 73–97.

Nichols-Orians, C.M. and J.C. Schultz, 1989. Leaf toughness affectsleaf harvesting by the leaf cutter ant, Atta cephalotes (L.) (Hymen-optera: Formicidae). Biotropica 21: 80–83.

Orr, M.R., 1992. Parasitic flies (Diptera: Phoridae) influence foragingrhythms and caste division of labor in the leaf-cutter ant, Attacephalotes (Hymenoptera: Formicidae). Behav. Ecol. Sociobiol. 30:395–402.

Tonhasca Jr., A., 1996. Interactions between a parasitic fly, Neodohrni-phora declinata (Diptera: Phoridae), and its host, the leaf-cuttingant Atta sexdens rubropilosa. Ecotropica 2:157–164.

Wetterer, J.K., 1994. Forager polymorphism, size-matching, and loaddelivery in the leaf-cutting ant, Atta cephalotes. Ecol. Entomol. 19:57–64.

Figure 1. Means and standard errors (n = 8) of forager size (A) and leaffragment size (B) for an A. sexdens colony foraging on tough (Anthoce-phalus chinensis) and tender (Rosa chinensis) leaves before and after itwas exposed to Neodohrniphora sp.