Inverse density-dependent and density-independent parasitism in a ...

Tropical Zoology 16: 83-92, 2003

Inverse density-dependent and density-independent parasitism in a solitary ground-nesting bee in Southeast Brazil Y. ANTONINI

1,3,

R.P. MARTINS

1

and C.A. ROSA

2

1

Laboratório de Ecologia e Comportamento de Insetos, Departamento de Biologia Geral, Universidade Federal de Minas Gerais, Cx. Postal 486, 30.270-901 Belo Horizonte, Brazil 2 Departamento de Microbiologia, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil Received 5 August 2002, accepted 14 January 2003

Aspects of the population dynamics of a solitary ground-nesting oligolectic bee, Diadasina distincta (Holmberg 1903) (Hymenoptera Anthophoridae), were studied in relation to the number of parasitized nests in aggregations of different sizes. Inversely density-dependent parasitism by bombyliid flies and densityindependent mortality by fungi together explained most of the variation in population size. There was a significant negative correlation between the number of nests in an aggregation and parasitism by bombyliid flies. Bombyliids accounted for 19.4% of mortality and two fungal species for an additional 19%. It is hypothesized that male patrolling in larger aggregations accounted for lower parasitism rates by bombyliid flies. KEY WORDS:

solitary bee, Diadasina distincta, Anthrax sp., nest aggregation, parasitism, mortality, bombyliid flies, fungi.

Introduction . . . . . . . . . . . . . . Material and methods . . . . . . . . . . . Results . . . . . . . . . . . . . . . General mortality . . . . . . . . . . . . Parasitism by insects . . . . . . . . . . . Mortality due to fungi . . . . . . . . . . Discussion . . . . . . . . . . . . . . General mortality . . . . . . . . . . . . The effect of aggregation size on the number of parasitized nests Fungi and mortality rate . . . . . . . . . . Conclusion . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . References . . . . . . . . . . . . . . 3

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Corresponding author (Fax: +55 31 4992567, E-mail: [email protected]).

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Y. Antonini, R.P. Martins and C.A. Rosa INTRODUCTION

Numerous species of bees, parasitic wasps and bombyliid flies are the principal insect taxa that parasitize the immature stages or stored food of solitary ground-nesting bees, often taking advantage of the provisioning female’s absence to steal or otherwise appropriate unattended resources (BATRA 1965, ROUBIK 1989). There is great interest in the impact of parasites on the dynamics of hosts, including bee and wasp host populations (EVANS 1977; ANDERSON & MAY 1978, 1979; LASALLE & GAULD 1991; WCISLO et al. 1994). Parasite pressure and predators may be extrinsic factors that promote social evolution in bees and wasps (MICHENER 1974, EVANS 1977). Bombyliid flies are particularly common parasites of bees. Of 428 cases of parasitism of bees by bombyliid flies, 304 were by 70 species of the genus Anthrax Scopoli 1763 (YEATES & GREATHEAD 1997). This suggests that flies of this genus may have a profound impact upon populations of a wide variety of bee species. Aculeate Hymenoptera that nest in aggregations may demonstrate patterns of parasitism related to the distribution or density of nests. Some studies support the notion that a high nest density represents a refuge from natural enemies, i.e., inversely density-dependent mortality (ALCOCK 1974, ROSENHEIM 1990), whilst others support both the reverse pattern of directly density-dependent mortality and density-independent mortality (MICHENER 1974, BROCKMAN 1979, ROSENHEIM 1990). Variation in these patterns seems to be created by interactions of different strategies of parasite searching and host defenses (ROSENHEIM 1990). Diadasina distincta (Holmberg 1903), is a multivoltine solitary, neotropical, ground-nesting bee (MARTINS & ANTONINI 1994, MARTINS et al. 1999). The adult bees emerge at the middle of the rainy season during February and nesting activity occurs until October, through the drier and colder months of the year. Females nest in aggregations, the size of which varies seasonally (MARTINS & FIGUEIRA 1992). Each nest is a burrow ending in a single cell in the ground approximately 4 cm deep and 1 cm in diameter. A single female constructs each nest (on average five nests per female) and old nests are not reused (ANTONINI 1995). Adult females sleep in the nests, and males sleep in the surrounding vegetation on which they also rest between patrolling periods. Males strongly compete for females and pursue both newly emerged females and those females already provisioning nests. Females mate repeatedly throughout their lives that last, on average, 12 days (ANTONINI 1995). Here we documented mortality rates, especially due to bombyliid parasites (Diptera Bombyliidae) and to fungi, as recorded in nest aggregations of different sizes.

MATERIAL AND METHODS

Study site The study was carried out at the Ecological Station of the Campus Pampulha of the Federal University of Minas Gerais (UFMG), Belo Horizonte, MG, Brazil (19o52’S, 43o58’W) from February to October of 1994, totalling about 600 hr of observations. Aggregations of D. distincta were located along dirt trails, almost free from vegetation. The mean distance between the aggregations in the study area was 2 ± 1.5 m (n = 9).

Parasitism in a ground-nesting solitary bee

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Nests of this bee are always in areas with bare soil, and nest entrances are concealed by small mud turrets whose openings are directed laterally, and thus not visible from directly above (see illustration in MARTINS & ANTONINI 1994).

Number of parasites The number of parasites was recorded by analyzing nine aggregations of different sizes. To quantify parasitism rates and other possible mortality causes, we used emergence traps with metallic frames measuring 40 × 40 × 40 cm, covered with netting, and fastened to the soil. These emergence traps were placed after all nests had been definitively closed by the nesting females. In three aggregations, the traps enclosed all the nests (about 50 nests in each aggregation). In three other aggregations, which were of larger (each about 2000 nests) than the area covered by the traps, they covered only part of the aggregation, enclosing 150, 170 and 180 nests. In the others three aggregations (30 nests each) parasitism was recorded by counting the pupal skins of Anthrax sp. (Diptera Bombyliidae) that emerged from the bee nests. It was very easy to count pupal skins because the pupa crawls to the nest entrance, opens it and stops. Then, the adult fly emerges and the pupal skin is left at the nest entrance. After provisioning a cell and ovipositing, the female bee closes the nest entrance, so that each parasitized nest will have just one bee larva to be consumed by a bombyliid larva. Data were collected monthly, from March to September, comprising the beginning, middle, and end of the reproductive season. Emergence traps were inspected daily to record the number of newly emerged adults of D. distincta, the parasite Leucospis genalis Boucek 1974 (Hymenoptera Leucospidae), as well as the number of pupal skins of the Anthrax sp. Afterwards the bees and the parasites were released. The emergence traps were above the aggregation for about 1 month. If no further individuals had emerged 15 days after the emergence of the last individual, the soil where the six established aggregations were located was excavated to collect and examine cell contents from which bees and parasites had not emerged. After the cells were removed, the emergence traps were moved to another part of the same aggregation. A similar procedure was adopted for the three small aggregations where the fly pupal skins were counted. We analyzed in all 650 cells using emergence traps plus 90 cells from the aggregations where fly pupal skins were counted, totaling 740 cells. Living adult bees and parasites found in the cells were considered as having emerged, but immature stages were included in mortality (as “unknown factors”) as their survival to adulthood was not guaranteed. A Spearmann correlation (ZAR 1998) was used to analyze the relationship between the number of nests in each aggregation and the number of parasitized nests.

Identification of fungi To isolate and characterize the fungi, cells from which bees had not emerged were removed from nests and transported in sterile Petri dishes to the laboratory where they were opened aseptically with sterile forceps. Larval provisions, larvae and pupae were cultured for a few hours after sampling. Approximately 0.1 g of larval provisions from each sample was diluted in 1 ml of sterile distilled water and stirred for 1 min. One loopful was streaked out or 0.1 ml of successive decimal dilutions was spread on acidified YM agar (1.0% glucose, 0.5% peptone, 0.3% malt extract, 0.3% yeast extract, 2.0% agar, 100 mg/l chloramphenicol, acidified with HCl to pH 3.7). Larvae and pupae were homogenized in 1 ml sterile distilled water using a hand-held glass-teflon tissue homogenizer and one loopful was spread on YM agar plates. The plates were incubated for 3-10 days at 25 ± 3 ºC. The fungal colonies were counted and representatives of each morphological type were purified and maintained in YM slants. Fungi were identified by the keys of BOOTH (1971), GAMS & HOLUBOVÁ-JECHOVÁ (1976), EAPER

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Y. Antonini, R.P. Martins and C.A. Rosa

& FENNELL (1977), DOMSCH et al. (1980), STOLK & SAMSON (1983), PITT (1985) and HANLIN (1990). Voucher specimens of bees, parasites and fungi were deposited, respectively, in the Laboratório de Ecologia e Comportamento de Insetos, Departamento de Biologia Geral, and Departamento de Microbiologia, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil.

Determination of bee population size Estimates of population size were made every 15 days, using the method of Petersen, modified by BAILEY (1952), by passing an insect net only once above the aggregation. Captured individuals were marked on the thorax with acrylic paint. In each marking period, different colors were used from those used previously to avoid any interference among successive estimates. One day after marking, recaptures in the same aggregation were made, giving proportions of marked and unmarked individuals (MARTINS et al. 1999). This was performed to evaluate any relation between parasitism and population size using the population size estimates (see MARTINS et al. 1999).

RESULTS

General mortality The most important parasitic insect was a species of Anthrax, followed by L. genalis (Table 1). Fungi were another important agent of mortality and we found 12 species in all (Table 1). The mortality rate was high; of the 650 examined cells (only those from emergence traps), 317 (49.8%) were destroyed by parasites, fungi (Table 2), or unknown factors. The most important causes of mortality were parasitism by Anthrax sp. and L. genalis, responsible for only 2% of parasitized cells, together responsible for 21.4% mortality, followed by fungi (19.0%). Unknown factors result-

Table 1. Spectrum of insect brood parasites that emerged from nests of D. distincta and of fungi affecting bee brood pollen provisions in nine aggregations at the Ecological Station of the UFMG, Belo Horizonte, MG, Brazil. Parasites

Fungi

Anthrax sp. (Diptera) Leucospis genalis (Hymenoptera)

Aspergillus flavus Link: Fries Aspergillus janus Raper & Thorn Aspergillus flavipes (Bainer & Santory) Thorn & Church Aspergillus terreus Thorn Aspergillus puniceus (Bain) Thorn & Church Cladosporium spharospermun Penzig Cladosporium cladosporioides (Fresenius) de Vries Fusarium sp. Booth Mucor sp. Schipper Penicillium implicatum Biourge Rhizopus oryzae Went & Prinsen Geerligs Rhizopus sp. Went


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Table 2. Mortality of Diadasina distincta due to parasitism by Anthrax sp. and fungi on larvae in six aggregations (totaling 576 cells)* at the Ecological Station of UFMG, Belo Horizonte, MG, Brazil from March to September of 1994. Mortality due to Months

March April May June July August September Total

Anthrax sp. no. (%)

(%)

Bee cells no.

Total

no.

13 69 20 10 — 8 6 126

42 35 24 11 9 2 — 123

56.0 23.5 22.0 15.5 14.3 3.3 — —

20 45 66 51 54 50 41 327

75 149 110 72 63 60 47 576

17.4 46.3 18.0 14.0 — 13.3 12.7 —

Fungi

* Cells in which mortality was due to unknown factors were not analyzed here.

ed in 9.4% of mortality (61 cells). Data collected from the aggregations with 30 nests each were not analyzed here. Parasitism by insects The parasitic behavior of L. genalis differs from that of Anthrax sp. Female L. genalis enter nests when the nesting female is away. Anthrax sp. females fly around the nest entrances and flick their eggs inside the nest regardless of the presence of the female. Smaller aggregations, with about 30 nests, had higher rates of parasitism (Fig. 1). Thus, there was a significant negative correlation between the size of aggregation (number of nests) and the number of parasitized nests (rs= – 0.93, P < 0.001, n = 9); i.e., the number of parasitized nests was lower when the population size of the bee was higher (Fig. 2). The three aggregations with 30 nests each (colonies in low density) were also parasitized heavily by Anthrax sp. (89%); adult bees emerged from only 20 cells. An increase in parasitism was noted in aggregations early in the season with a peak of Anthrax parasitism in April 1994; it then decreased from the middle of May until the end of the activity period (Fig. 3). Mortality due to fungi We found a large number of fungus species contaminating cells of D. distincta. An elevated rate of mortality due probably to fungi, like fly parasitism, was noted in aggregations early in the season (during March, April and May) (Table 2). The two most abundant species of fungi that appeared in the latter part of the season (present in 90% of the samples) were Aspergillus flavus Link: Fries and Rhizopus oryzae Went & Prinsen Geerligs. There was no correlation between the number of nests in each aggregation and the number of cells contaminated by fungi. Therefore, parasitism by fungi is not density-dependent.

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Fig. 1. — Percentage of parasitism and the number of nests in the aggregations at the Ecological Station of the UFMG, Belo Horizonte, MG, Brazil.

Fig. 2. — Percentage of parasitism by Anthrax sp. and population size of the host Diadasina distincta at the Ecological Station of the UFMG, Belo Horizonte, MG, Brazil.


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Fig. 3. — Seasonal variation in the emerging parasites Anthrax sp. and their host bee Diadasina distincta at the Ecological Station of the UFMG, Belo Horizonte, MG, Brazil.

DISCUSSION

General mortality Bees, which dig or construct nests, are potential hosts to a wide variety of organisms. The resources stock-piled in the nest (stored food or brood) may be stolen or used in situ by parasites, taking advantage of the mother’s absence. Such cumulative losses are hypothesized to influence the evolution of various traits of the parasitized species, in particular those relating to social behavior. Diadasina distincta is no exception to this pattern, as it was parasitized heavily by Anthrax sp., the most important brood parasite found in this study. The tendency to nest in large aggregations could be evolutionary advantageous. Dense aggregations of bees provide large quantities of resources for potential predators and parasites. Therefore, it is not surprising that aggregations often support a diverse assemblage of natural enemies which may include microbial pathogens, fungi, nematodes, mites, bombyliid, conopid, and miltogrammine flies, Strepsiptera, and meloid and rhipiphorid beetles in addition to cleptoparasitic bee species (PACKER 1988).

The effect of aggregation size on the number of parasitized nests ROSENHEIM (1990) showed that a larger number of nests in an aggregation can lead to a decrease in the rates of parasitism. This was evident with D. distincta since, in spite of fluctuating numbers of parasitized nests during the reproductive period, there were significant differences in the number of parasitized nests according to the number of nests in the aggregations.

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Variations in host life-history may be viewed as alternative strategies that confound parasites, like construction of accessory burrows or early emergence (MINCHELLA 1985). Diadasina distincta possesses some behaviors that supposedly can help to reduce rates of parasitism, such as (i) constant patrolling of males in the aggregations, which, while searching for females, chase away parasites, (ii) females sleeping inside their nests during construction. BOHART et al. (1960) noted that some bees in an aggregation can emerge later than their parasites, nesting only after the death of the adult parasites. In D. distincta, however, there was a synchrony of the emergence of adult bees and parasites. Nonetheless, the rates of parasitism decreased from May, when the population of bees was still rapidly growing, and this trend continued until the end of the season (Fig. 3). The months of lower rates of parasitism also coincided with the months in which there were a larger number of males and, as the patrolling in aggregations became more intense, this may also have decreased the success of the parasites.

Fungi and mortality rate The fungi, whose spores probably came with the pollen or through the soil, also appeared with greater frequency at the beginning of the reproductive period, mainly in those cells where prepupae were in dormancy from the end of the previous season, throughout the whole rainy period. Although the D. distincta cell wall is impermeable to water (MARTINS & ANTONINI 1994), the soil, in turn, is quite moist during the rainy season, which can facilitate the proliferation of fungi. PACKER (1988) found an association between the incidence of infection of brood by fungi and rainfall. LINSLEY & MAC SWAIN (1952) found that 34% of the cells of Diadasia bituberculata Cresson 1878 were contaminated, mainly by the fungi Aspergillus flavus and Rhizopus spp., coincidentally the most frequent genera found in D. distincta cells. We do not know if the pathogenic organisms we detected differentially affect particular host developmental stages. However, PACKER (1988) found that in Halictus ligatus Say 1837 the pupal stage lasts longer than any other but seems to suffer the least mortality, indicating that pupae may be protected against infection by fungi. The mortality of brood attributed to “unknown factors” could be partially due to the action of ants and termites, abundant in the aggregations of D. distincta (MARTINS & ANTONINI 1994).

CONCLUSION

Despite the high number of parasitized nests and other mortality factors, aggregations of some bee species can persist in the same places for many years (MALYSHEV 1935). This was true of D. distincta, located in the Ecological Station of UFMG, since 1988 (MARTINS & ANTONINI 1994). Probably there is a lag between increasing parasite populations and bee populations so that the bee population grows faster than the parasite population. On several occasions, during studies of the biology of solitary bees, we have found abandoned nesting sites of gregarious ground-nesting species. In some cases,


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the disappearance of the bees could be attributed to an extended period of drought or other physical factors in the environment. However, a few cases did not seem explicable on any such basis and, since abundant evidence was present that there had been a high incidence of parasites and predators in the population, we suspect that biotic factors may have played an important role in the disappearance of the bees. However, while the population of D. distincta at UFMG remained stable for many seasons, in 1997 and 1998 it virtually disappeared from the area for unknown reasons. It should be noted that since D. distincta is an oligolectic specialist bee (MARTINS & BORGES 1999), and could be locally rare, care must be taken in the management of areas where it occurs so as not to run a great risk of extinction. Studies to determine requirements for nesting are also clearly needed.

ACKNOWLEDGMENTS Doug Yanega for the comments and help with the English in the first draft; D. Greathead and an anonymous referee for the useful comments that improved the manuscript; A. Roig-Alsina for the identification of Diadasina distincta; N. Evenhuis for the identification of Anthrax sp., Z. Boucek for the identification of L. genalis; E.M. Viana for fungi identification. We would like also to thank Coodenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the author’s fellowships. This is a contribution of the Graduate Program in Ecology, Conservation and Wildlife Management (ECMVS), Federal University of Minas Gerais (UFMG), Belo Horizonte, MG, Brazil.

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