Host–parasite relationships and life cycles of cuckoo wasps in agro-ecosystems in Argentina (Hymenoptera: Chrysididae: Chrysidini)

Chrysididae is a diverse group of parasitoid/cleptoparasitic wasps; however, host–parasite relationships and life cycles of few species have been studied. Nests of different wasp and bee species were obtained during a trap-nesting programme, in the Pampean region. Some of these nests were parasitised by cuckoo wasps females of Caenochrysis taschenbergi (Mocsáry), Chrysis boutheryi (Brèthes), C. saltana Bohart, C. sp. 1 (ignita-group), C. sp. 2 (ignita-group), Neochrysis lecointei (Ducke), Pleurochrysis ancilla (Buysson) and P. lynchi (Bréthes). This paper reports new data about host–parasite relationships and life cycles for these species. Multiple parasites (from one species or from different families of insects) emerged from single cells of some parasitised nests: from each host cell parasitised by species of Pleurochrysis, two adults emerged successfully, information previously unknown for the genus; and in three cases of cells parasitised by C. boutheryi, two adults successfully emerged from a single cell; in two cases both individuals were chrysidine, and in third one was chrysidine and one Leucospis pulchripes (Leucospidae).


Introduction
The family Chrysididae is a diverse group of parasitic wasps (Kimsey and In the Pampas of Argentina, 90% of the original grasslands have been converted into fields used for crops and/or livestock, threatening native animals through habitat destruction, fragmentation and/or loss of original habitat quality, the introduction of competing animals, and direct human impact (Medan et al. 2011). Trap-nests are a good tool to get biological information on bees and wasps that nest in pre-existing cavities, and provide good evidence about parasites and/or cleptoparasites (Krombein 1967). A number of studies have examined the bees and wasps found in trap-nests in agro-ecosystems in the Pampean region, including Torretta and Durante (2011), Torretta et al. (2012) and Torretta et al. (2014), and their host-parasite interactions (Torretta 2014). Adults of different species of cuckoo wasps emerged from nests of different species of bees and wasps during distinct sample periods; therefore, the objectives of this study were: (1) to describe host-parasite relationships and (2) to study life cycles and emergence patterns of the different species. Additionally, sex ratio, associated organisms and parasitism percentage are provided for some cuckoo-wasp species.

Study sites
The trap-nests were located in four sites of the Pampean region. The sites represent different subdivisions of the Pampean grassland ( Figure 1a) and have distinct main economic activities (Table 1). The Toay site is located in a xerophytic open forest system (Caldenal) which is a transitional ecosystem between the Pampas grasslands, to the east, and the dry Monte shrublands, to the west. It is dominated by the caldén tree (Prosopis caldenia Burkart), with an understory of predominantly perennial grasses (Cabrera 1994). The Hortensia site is a mosaic of crop fields and semi-natural grasslands used for livestock grazing. The Pila site is a seasonally flooded grassland dedicated to cattle breeding, and the Colonia Elía site is a mosaic of crop fields and grazing land in similar proportions.

Nest collection
Trap-nests were placed between November and March during three consecutive periods (Table 1), and were visited monthly. In Toay, trap-nests were located in two areas of forest in two (2009)(2010) and in four (2010-2011) transects perpendicular to the field margin at heights of 1-2 m on trees. In Hortensia and Pila, trapnests were placed on fence posts in field margins associated with diverse crops, seminatural grassland and/or sown pasture. In Colonia Elía, they were located on trees and on fence rows (Figure 1b, c). Trap-nests (numbered in each sampled period from #1) were arranged in bundles (numbered with Roman numerals, from I) of 14 canes, and the bundles were placed at intervals of 50-100 m, covering areas from 3-10 ha. The number of trap-nests varied among sites and sampled periods (Table 1).
Trap-nests used in this study consisted of hollow bamboo canes (5-11 mm internal diameter), which were cut so that a nodal septum closed one end of the cane (Aguiar and Garófalo 2004). At each monthly visit, the traps with nests were removed and taken to the laboratory. There, the cells were separated in plastic vials with cotton plugs and numbered from 1 to n (starting from the innermost) and were kept in the laboratory at room temperature (ca. 15-25°C) until adult emergence. Since trap-nests were collected at monthly intervals, development time can only be estimated with an error of ± 15 days (Thiele 2005).
For each chrysidine species, I calculated the percentage of parasitism as [number of parasitised host(s) cells]/[total number of host(s) cells].
The material studied is deposited in the Entomological Collection of the General Botany Unit (FAUBA), Facultad de Agronomía, Universidad de Buenos Aires, Argentina, and at the Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina (MACN).
During the 2010-2011 period, I reared 20 individuals of Chrysis boutheryi (Brèthes) from 13 nests of vespid wasps, and a megachilid bee. Only one nest was parasitised in each bundle of canes, except one bundle with three parasitised nests. Ten nests were found and removed during early summer (January), and the remaining three during late summer (March). Adult emergence showed a clear bimodal pattern (Table 2; Figure 2a). The sex ratio was strongly female-biased (19/1). In 15 host cells, a single adult chrysidine was developed. However, in three cases, two adults successfully emerged from a single cell (Table 2) Seven nests were removed during January and one during February, and the remainder in March. Estimated emergence time showed a unimodal pattern, although one female took nearly 22 months to emerge (Figure 2b). In both sampled periods, the sex ratio was female-biased (1.2/1 and 2/1, respectively; Table 2).
Two specimens of chrysidine successfully emerged for each cell of the two species of Pleurochrysis. During 2010-2011, I reared two individuals of Pleurochrysis ancilla (Buysson) from a nest of Hypodynerus sp. from Toay, and six individuals of Pleurochrysis lynchi (Brèthes) from three nests of Stenodynerus sp. from Pila (Table 2).

Discussion
This paper is the first report about host-parasite relationships and life-cycle data for these eight species of chrysidine in the highly modified Pampean region.
Four species in the genus Chrysis were studied in this work. In two of them, the numbers of emerged adults and attacked nests provide good information about their biological traits. The females of Chrysis boutheryi seem to be generalist in the hosts  which they parasitise: eumenine vespid and megachilid bees. However, most parasitised nests were of mud cell-building species of the family Vespidae. Also, Megachile catamarcensis used mud (with flower petals and/or leaf pieces) to build their nests ). These are the first records about host-parasite relationships for a species of the nisseri species group. One cell was attacked by C. boutheryi and L. pulchripes, and both adults successfully emerged. This is an interesting finding given that there are few reported cases where two parasites emerge from a single cell. Rosenheim (1987) reported cells of the ground-nesting Ammophila dysmica Menke (Sphecidae) successfully parasitised by the cuckoo wasp Argochrysis armilla Bohart and the miltogrammine fly Hilarella hilarella (Zetterstedt) (Sarcophagidae). Also, from another cell of this nest of Hypodynerus sp., two female of C. boutheryi emerged.
These facts indicate that in the preimaginal stages of Hypodynerus sp., food was sufficient for both individuals to develop and emerge successfully in a single cell. The bimodal emergence pattern of Chrysis boutheryi trait is reflected in the activity pattern of oviposition of females (note dates of nest collection in Table 2). Sex ratio was strongly female-biased. The significance of the female-biased sex ratio of emergence of this cuckoo wasp is unclear.
The females of Chrysis saltana seem to be host specific, only parasitising cells of the carder bee Anthidium vigintipunctatum. The only other record for a species of the comparata-gibba group is for Chrysis tripartita Aaron, closely related to saltana (Bohart 1985), which also attacks bees in the genus Anthidium Fabricius (Grigarick and Stange 1968). Chrysis saltana shows a univoltine life cycle; however, one female took about two years to emerge, suggesting parsivoltinism (Torchio and Tepedino 1982). In many species of Apoidea, the duration of the life cycle varies among individuals of the same generation (Torchio and Tepedino 1982;Wcislo and Cane 1996) and may be a bet-hedging strategy related to unpredictable resource availability (Neff and Simpson 1992) or to the abundance of natural enemies (Wcislo and Cane 1996). Parsivoltinism can favour the persistence of some populations by staggering the emergence of sequential generations (Torchio and Tepedino 1982). Chrysis saltana has been recorded from Salta, Santiago del Estero, Catamarca and La Rioja (Bohart 1985). The population studied herein is the southernmost record for this species, and temperature accumulation required for diapause termination possibly takes longer than for other populations. Therefore, at lower latitudes, this species could exhibit a bivoltine life cycle.
Two species of Pleurochrysis were reared in this study, both nest parasites of mudpot nesting wasps. The host wasps of P. ancilla and P. lynchi were Hypodynerus sp. and Stenodynerus sp., respectively, species that supply their nests with numerous Lepidoptera larvae. There are few host records for other species of this genus. Pleurochrysis bruchi (Brèthes) was reported from Pachodynerus gayi (Spinola) (Janvier 1933); P. postica (Brullé) from Trypoxylon rogenhoferi Kohl (Garcia and Adis 1995), Trypoxylon xanthandrum Richards (Coville and Griswold 1983) and one species of Sceliphron Klug (Kimsey and Bohart 1991); and P. morosa (Buysson) from Trypoxylon rogenhoferi Kohl (Garcia and Adis 1995). All female wasps of these species of Vespidae: Eumeninae, Sphecidae and Crabronidae, used mud as cell partitions and captured a large number of prey for their offspring.
Possibly, the species of Pleurochrysis are cleptoparasitic and their larvae consume host provisions, or they are parasitoids and consume pre-imaginal instars host. The cocoons of chrysidines were formed inside the host cell, and therefore I could not see if these wasps attacked host larvae, their provisions or host prepupae/pupae. For both species of Pleurochrysis, two adults emerged successfully from each host cell parasitised. This information is previously unknown for the genus. While the number of obtained parasitised nests was low (n = 4), I suggest that it might be possible that females of this genus lay two eggs in each host cell due to a sufficient provision of caterpillars (Hypodynerus, Pachodynerus de Saussure and Stenodynerus de Saussure) or spiders (Sceliphron and Trypoxylon Latreille). Another explanation is that more than one female will lay eggs in the same host cell. Rosenheim (1987) found up to six Argochrysis armilla Bohart produced from a single cell of Ammophila dysmica Menke (Sphecidae).
The other chrysidine species of this study were reared from diverse species of Trypoxylon, which build mud nests and capture spiders for larval provision. These results agree with known records for other species of these genera. The Chrysis ignita group has numerous species, and a wide range of hosts (Crabronidae and Vespidae: Eumeninae) has been reported (Kimsey and Bohart 1991). At the same time, the few available records about hosts of Caenochrysis taschenbergi and Neochrysis lecointei indicate a marked specialisation on Trypoxylon (Coville 1981;Kimsey and Bohart 1991;Garcia and Adis 1995).
Very little is known about sex ratio in chrysidine species (Yokoi et al. 2012). Highly female-biased sex ratios (as has happened in Chrysis boutheryi) for single batch rearing of cuckoo wasps are not uncommon (Medler 1964), and sex ratio may vary between sites and years (Krombein 1967). Therefore, more data should be obtained to clarify sex-ratio dynamics in these species.