Seasonal Variations of Metapleural Secretion in the Leaf-Cutting ant Atta sexdens piriventris Santschi (Myrmicinae: Attini), and Lack of Fungicide Effect on Beauveria bassiana (Bals.) Vuillemin

Variações Sazonais da Secreção Metapleural da Formiga Cortadeira Atta sexdens piriventris Santschi (Myrmicinae: Attini), e Ausência de Efeito Fungicida Sobre o Entomopatógeno Beauveria bassiana (Bals.) Vuillemin

a02v30n4

ECOLOGY, BEHAVIOR AND BIONOMICS

Seasonal Variations of Metapleural Secretion in the Leaf-Cutting ant Atta sexdens piriventris Santschi (Myrmicinae: Attini), and Lack of Fungicide Effect on Beauveria bassiana (Bals.) Vuillemin

ELENA DIEHL¹ AND LUCIANE K. JUNQUEIRA²

¹ Lab. Genética, Setor de Insetos Sociais, Universidade do Vale do Rio dos Sinos

C. Postal 275 - 93001-970, São Leopoldo, RS, [e-mail]

2 Lab. Entomologia Florestal, ESALQ/USP, 13418-900, C. postal 9,

Piracicaba, SP, [e-mail]

Variações Sazonais da Secreção Metapleural da Formiga Cortadeira Atta sexdens piriventris Santschi (Myrmicinae: Attini), e Ausência de Efeito Fungicida Sobre o Entomopatógeno Beauveria bassiana (Bals.) Vuillemin

RESUMO – Os mecanismos envolvidos na manutenção das colônias de insetos sociais livres de patógenos são aspectos importantes relacionados à biologia evolutiva do grupo. No caso das formigas, tem sido sugerido que a secreção das glândulas metapleurais teria importante papel na assepsia da colônia. Neste trabalho, as diversas subcastas de operárias da formiga cortadeira Atta sexdens piriventris Santschi foram comparadas quanto à presença de secreção metapleural ao longo de um ano. Simultaneamente, a atividade tópica da secreção foi testada em operárias tratadas com o fungo entomopatogênico Beauveria bassiana (Bals.) Vuillemin. De acordo com a largura média da cápsula cefálica, foram definidas seis subcastas de operárias: jardineira, mínima, média, grande, máxima e soldado. Foi encontrada uma correlação positiva entre subcasta e comprimento do reservatório metapleural. As subcastas apresentaram diferenças significativas na presença de secreção metapleural ao longo do ano exceto nos meses de abril, maio, julho e novembro. Foi observada uma correlação positiva entre as temperaturas médias sazonais e presença de secreção metapleural. Foram registradas altas freqüências de operárias de todas subcastas infectadas por B. bassiana durante os bioensaios, indicando que a secreção metapleural não tem atividade fungicida contra esse patógeno. Os resultados sugerem que a secreção metapleural não está entre os principais mecanismos de assepsia das colônias de A. sexdens piriventris.

PALAVRAS-CHAVE: Insecta, formigas cultivadoras de fungo, glândula metapleural, fungo entomopatogênico, controle biológico.

ABSTRACT – Mechanisms involved in maintaining colonies of social insects free of pathogens are among the main aspects of interest in the evolutionary biology of this group. For ants, it has been suggested that secretion from the metapleural glands play an important role in colony asepsis. In this study, different worker subcastes of the fungus-growing ant Atta sexdens piriventris Santschi were compared in relation to presence of metapleural secretion during a year. At the same time, the topical activity of the secretion was tested on workers treated with the entomopathogenic fungus Beauveria bassiana (Bals.) Vuillemin. Based on the mean width of the cephalic capsule, six worker subcastes were defined: gardener, minor, media, large, major and soldier. A positive correlation between worker subcaste and metapleural reservoir length was observed. Significant differences for presence of metapleural secretion were found throughout the year among subcastes, except in April, May, July and November. A positive correlation between seasonal mean temperature and presence of the metapleural secretion was observed. High frequencies of infected workers of all subcastes were observed during bioassays with inoculation of B. bassiana, indicating that metapleural secretion has no fungicidal activity against this pathogen. The results suggest that metapleural secretion of workers is not the main mechanism of A. sexdens piriventris colony asepsis.

KEY WORDS: Insecta, fungus-growing ants, metapleural gland, entomopathogenic fungus, biological control.

In most terrestrial habitats, social insects have higher biomass and energy consumption than vertebrates. Among the eusocial insects, ants are dominant, widely distributed throughout the world, excepting the polar regions. According to Hölldobler & Wilson (1990), the eusocial behavior is an ecological strategy of high efficiency. The ants' evolutionary success may be attributed to the appearance of small and wingless workers, capable of exploiting hardly accessible sites, gathering a variety of energetic resources and nesting in the soil. The occurrence of metapleural glands whose acid secretion inhibits microrganism growing (Maschwitz et al. 1970) was an adaptation which allowed the ants to explore other habitats (Hölldobler & Wilson 1990).

In some ant species, the metapleural secretion is active against bacteria and fungi (Maschwitz 1974, Beattie et al. 1986, Veal et al. 1992, Mackintosh et al. 1995, Nascimento et al. 1996) besides having insecticide properties (Attygalle et al. 1989). For some authors, it may reduce spore germination and colony growth of some soil fungi, and the pollen quality of certain plants is also reduced (Beattie et al. 1985). However, Ramsey (1995) found a species of Iridomyrmex pollinating Blandfordia grandiflora (Liliaceae), suggesting that metapleural secretion has no such great effects on pollen activity. Other roles of the metapleural gland have been reported, such as intraspecific recognition (Brown 1968) and territorial marker, increasing workers aggressiveness in the areas where present (Jaffé & Puche 1984, Cammaerts & Cammaerts 1998).

Atta sexdens piriventris Santschi is a fungus-growing ant widely distributed in Rio Grande do Sul state, Brazil, and responsible for serious damage to crops. Microbial control has been suggested as an alternative control method for this pest (Diehl-Fleig et al. 1993, Specht et al. 1994). However, the metapleural secretion and its possible antifungal effect should be an obstacle to the use of entomopathogenic fungi as biological control agents (Kermarrec et al. 1986). Wilson (1980) has suggested that the development of glands should be higher on ant castes where they would be more needed. Then, if the metapleural glands are responsible by colony asepsis one should expect to find larger and more active glands in minor workers and gardener ants, which take care and protected the symbiontic fungus, the immatures and the queen. Thus this study was carried out aiming: a) to relate the size of metapleural reservoir according to the subcastes (defined from the cephalic capsule mean width) of A. sexdens piriventris workers; b) to evaluate the frequencies of workers by subcaste with metapleural secretion in the gland reservoirs throughout a year; and c) to test the topical activity of metapleural secretion of workers against the entomopathogenic fungus Beauveria bassiana (Bals.) Vuillemin.

Material and Methods

The ant workers were collected from an Atta sexdens piriventris colony located situada in the municipality of São Leopoldo (29º45'S; 51º08'W), Rio Grande do Sul state, Brazil. The climate is subtropical with a short dry season in the summer and an annual rainfall of 1347.4 mm. The annual mean temperature is 19.5ºC, with the mean maximum of 30.2ºC in January and the mean minimum of 10.7ºC in June and July (MARA/SNI/DNM 1992, Pereira et al. 2001).

Morphometric Measurements of Worker Subcastes. The width of the cephalic capsule was determined as the distance between eyes, and the length of metapleural gland reservoir were measured in A. sexdens piriventris workers (N=1000) collected, separated and arbitrarily grouped into subcastes according to their size (see Wilson, 1980). To set the subcastas, the workers were arbitrarily distributed in six classes of cephalic capsule width: gardener (0.85 mm - 1.15 mm), minor (1.16 mm - 1.45 mm), media (1.46 mm - 1.75 mm), major (2.36 mm - 2.65 mm), large (2.96 mm - 3.25 mm) and soldier (3.26 mm - 3.65 mm). Thirty individuals from each subcaste, randomly chosen, were measured using reticulated micrometer attached to an optical microscope.

Mean size of the cephalic capsule and mean length of the metapleural gland reservoir and their standard deviations were calculated for each subcaste. Pearson correlation coefficient (r) was used when analyzing the relation between these two measures.

Evaluation of Metapleural Reservoir. From January to December 1994, 50 workers/subcaste in the same colony, were collected monthly and had their reservoir perforated in the median-superior portion with micro-needle under stereomicroscope (40X). Presence or absence of secretion in the reservoirs was individually registered for both left and right glands. Considering that production of metapleural secretion, such as occurs in others glands of several organisms may be related to climatic factors, possible relations between frequencies of workers with secretion in the metapleural reservoirs, seasonal mean temperature and relative humidity were analyzed through Pearson correlation coefficient (r).

Bioassays with Workers. To determine whether the metapleural secretion is active against the entomopathogenic fungus Beauveria bassiana, and to evaluate this action throughout a year, bioassays were conducted during middle months (January, April, July and October) of each season with all six subcastes of fresh collected A. sexdens piriventris workers.

For the bioassays it was used the B_SA strain of B. bassiana that is a microbial agent for leaf-cutting ants control in some experimental areas of Rio Grande do Sul. A suspension of 1.6 x 10⁶ conidia/ml of this strain was applied, using a microcapillary, either to the metapleural gland atrium or to the antero-superior portion of the metathorax (sites where the ant is unable to spread the metapleural secretion). Fifty ants/subcaste were used for each application. As a control, sterile saline solution was applied in equal number of individuals of each subcaste.

Ants were individually distributed in sterile vials containing filter paper with 0.05 ml of sugar solution and kept in humid chamber at 27±1ºC, 80% RH and 12h photoperiod. The frequencies of B. bassiana infected ants by treatment (site of pathogen application), the season on the year and subcaste were compared by the chi-square test.

Results

Morphometric Measurements of Worker Subcastes. Based on cephalic capsule mean width (Table 1), six subcastes of A. sexdens piriventris workers (gardener, minor, media, large, major and soldier) were defined. Significant positive correlation between cephalic capsule size and metapleural reservoir length was found for gardener, large and soldier subcastes (Table1).

Evaluation of Workers Metapleural Reservoirs. Monthly comparisons among subcastes showed that they significantly differ for metapleural secretion production along the year, except for April, May, July and November (Table 2). In general, a larger number of soldiers, compared to other subcastes, had secretion in their metapleural reservoirs. Throughout the year the frequencies of workers with secretion in the metapleural reservoir varied from 13.0% in July to 61.7% in November (Fig. 1).

Considering the number of workers of all subcastes, for each month, there was a significant positive correlation between the presence of secretion in the metapleural reservoir and the seasonal mean temperature (r=0.6967; P<0.01), However, there was no correlation between presence of secretion and seasonal mean relative humidity during the sampled period (r=-0.1559, P>0.05).

Bioassays with Workers. During winter, spring and summer, independent of the inoculation site (metapleural gland or metathorax antero-superior portion), there were no significant differences for infection frequencies among subcastes. Only during autumn, a lower frequency of infected gardeners was found after inoculation in the metathorax antero-superior portion (Table 3). However, when infection frequencies of each inoculated subcaste were compared to their respective controls (inoculated by sterile saline solution), differences were significant except for gardeners, minors and media ants that had no difference during winter (Table 3). Despite carefull handing during manipulations of all four seasons, pathogen contamination occurred in some control groups.

In relation to seasonal data, during spring and summer, we found the highest frequencies of workers infected with B. bassiana, all subcastes together (Table 3). A lower occurrence of saprophytes was observed in the bioassays during these seasons if compared with autumn and winter.

Discussion

In the study of social insects, descriptive and numeric data are necessary for recognizing species-specific characteristics (Tschinkel 1991). Thus, for this study, tests were designed considering the mean size of A. sexdens piriventris subcastes of workers. Differential production of metapleural secretion was observed among the six worker subcastes, and often, the highest frequencies of reservoirs with secretion were found in the larger subcastes. In relation to the season of the year, there was a positive correlation between secretion presence and seasonal mean temperature.

Metapleural secretion has been studied in several ant species, with demonstration of its activity against bacteria and fungi (Maschwitz et al. 1970, Maschwitz 1974, Beattie et al. 1986, Veal et al. 1992, Nascimento et al. 1996), insects (Attygalle et al. 1989) and pollen (Beattie et al. 1985). However, none of this authors reported seasonal differences or differences among production from subcastes, possibly because their studies were conducted with ant colonies kept under laboratory conditions, tests performed during only one period of the year, with a small ant number, and perhaps, with the whole ants belonging to the same subcaste. Recently, Bot & Boomsma (1996) evaluating workers from colonies of two Acromyrmex species of Panama, found that the metapleural gland relative size gradually decreases as body size increases. According to them, their data follow what has been proposed by Wilson (1980) that organs achieve maximum development in those ant castes where they would be more important. In other words, if metapleural gland is associated with colony asepsis, it would be expected to be larger and to produce more secretion in the gardeners and minor workers, based on their colony role. However, for A. sexdens piriventris, a correlation between cephalic capsule size and metapleural reservoir length was found. A higher frequency of secretion in the reservoirs was found in the soldiers which, according to Wilson (1980), are more involved in colony defense than in hygiene.

Maschwitz et al. (1970) found that workers of A. sexdens poorly fed have an air bubble in their metapleural reservoirs. Variation in metapleural secretion presence in A. sexdens piriventris should be related to environmental factors, such as temperature, not excluding genetic factors as proposed by Bot & Boomsma (1996).

High frequencies of A. sexdens workers infected during the bioassays were observed, indicating that metapleural secretion has no fungicidal and/or fungistatic activity against the strain of B. bassiana. All subcastes were equally sensible to the pathogen, thus, it seems to have no relationship between the role of each subcaste (which could be more or less exposed to entomopathogens) and the antimicrobial effect of metapleural secretion, as suggested by Wilson (1980). However Alves & Sosa-Gómes (1983) observed greater resistance of A. sexdens rubropilosa (Forel) soldiers to the fungi B. bassiana and Metarhizium anisopliae in relation to the other workers.

During bioassays with A. sexdens piriventris, high incidence of saprophytic fungi occurred through fall and winter in dead workers, which are commonly present during wet periods specially in soil dwelling insects. Considering that the metapleural gland, which is found exclusively in ants, has been suggested as responsible for colony asepsis, growth of saprophytes suggests that A. sexdens piriventris secretion is not an efficient fungicide. The hypothesis of metapleural secretion not presenting fungicidal activity is also supported by the occurrence of high frequencies of infected workers during the periods with the greatest number of workers presenting secretion in their reservoirs.

Beattie et al. (1985) found that the secretion of a species of Myrmecia has fungicidal activity against B. bassiana and other entomopathogens. More recently, Nascimento et al. (1996) confirmed the presence of substances with bactericidal and fungicidal activities in the metapleural secretion from A. sexdens rubropilosa, Atta cephalotes (Linn.) and Acromyrmex octospinosus (Reich). However, our results with A.sexdens piriventris workers does not show any fungicidal activity, corroborated by Billen (1990) citing Vander Meer (unpubl. data) who found that Solenopsis invicta Buren metapleural secretion has no antibiotic activity.

It seems that the metapleural secretion of A. sexdens piriventris workers, by itself, cannot affect biological control programs using B. bassiana as agent. Other chemical mechanisms should also be considered, such as, for example, mandibular glands secretion (Brough 1983) or pathogen recognition and/ or hygiene behavior (Kermarrec et al. 1986, Machado et al. 1988, Diehl-Fleig & Lucchese 1991) as well as the occurrence of a bacterium which produces antibiotic substances on the ant body (Currie et al. 1999, Schultz 1999). These mechanisms could be more effective than metapleural secretion in protecting the ant against pathogens.

Acknowledgments

We thank Prof. Marcia Eloisa da Silva for technical assistance, to Eduardo Diehl-Fleig for the English version of the manuscript and two anonymous referees. This work was supported by Conselho Nacional de Pesquisa - CNPq and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul – FAPERGS (grants to E. Diehl).

Literature Cited

Received 02/02/01. Accepted 30/09/01.

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Publication Dates

• Publication in this collection
  17 June 2002
• Date of issue
  Dec 2001

History

• Accepted
  01 Sept 2001
• Received
  02 Feb 2001