Eucharitid Wasps

All but one species of Eucharitid wasps attack ant larvae.

Peeters et al (2015) - Among 9 families of wasps parasitic on ants, only Eucharitidae (56 genera) specialize in attacking the immature stages of ants. Unlike most parasitoid wasps, eucharitid females deposit their eggs away from the host, in or on plant tissue (leaves and flower buds). The very active, minute, strongly sclerotized first-instar larvae (‘planidia’) are transported by foraging ants to their nest where they attach to ant larvae and development proceeds once the latter have spun a cocoon (Clausen, 1940; Heraty and Murray, 2013). The eucharitid parasitoids attacking Ponerinae, Ectatomminae, Myrmeciinae and Formicinae belong to a monophyletic derived lineage of Eucharitidae (Murray et al., 2013).

Heraty et al. (2015) - All known Eucharitidae are parasitoids of the immature stages of ants. Larval stages were first described by Wheeler (1907) for Orasema Cameron. The late instar larvae and pupae of Eucharitinae and Oraseminae were then described in a series of early papers summarized by Clausen (1940). The first-instar larvae are highly conservative in their morphology, but late instars can be strikingly ornate with numerous odd protuberances and swellings over the body.

Another oddity of Eucharitidae is the inverse evolutionary association with their ant hosts. Phylogenetic results based on molecular data suggest that the more ancestral eucharitid lineages are associated with more derived groups of ants (e.g., Formicinae and Myrmicinae), and the more derived eucharitid lineages attack the more primitive groups (e.g., Ponerinae) (Murray et al. 2013).

Schwitzke et al. (2015) found an unspecialized Eucharitid laid its eggs on stipules of the plant Leea manillensis (Leeaceae). The widespread forest plant, occurring throughout the Philippines and parts of Southeast Asia, is a large erect shrub up to 6 m tall. The plant also produces extrafloral nectaries (EFN) that attract a broad array of ant species.

In Leea manillensis, the stipule that encloses the next generation of shoot is the spatial focus of investment into defensive mechanisms via EFN and FB. Obviously not general herbivores are the most damaging threats for Leea manillensis in the study area, but the specialized ant-parasitoid.

The hatching planidia had a massive impact on the whole ant–plant–herbivore system. In our study, about 53 % of the plants’ shoots were observed to be infested with planidia. All of the common ant visitors at the extrafloral nectaries strictly avoided the shoots after the eclosion of the wasps’ larvae. That might explain why we noticed planidia to leave the shoot and disperse. The avoidance of shoots and the observed aggressive behaviour of ants towards adult wasps indicate that the ants were able to recognize this parasitoid as a threat. On 81 % of the planidia-infested shoots, ants were never observed to visit the extrafloral nectaries. Our observations suggest that this avoidance finally led to a cessation of EFN secretion in most of the shoots. Hence, the avoidance of planidia directly resulted in a lasting lack of ants on the shoots, which then caused an almost 10-fold increase in complete shoot losses compared to the unaffected shoots.

We found no evidence that the parasitoid wasp on Leea manillensis had its focus on a specific ant species, genus, or subfamily. In the contrary, we observed a broad range of ant species to get attacked and killed by the planidia, but have no information about the effect of the parasitoid on the ant colony level. As far as it is known, hatching planidia are carried into the colony via foraging ants (Pe´rez-Lachaud et al. 2006; Lachaud and Perez-Lachaud 2012). On Leea, the number of larvae on the shoots declined over time, and we observed planidia to spread over the whole plant along strands. We assume but do not know whether they were carried away by ant foragers. So far, the attack of adult ants has not been reported for the family Eucharitidae. In our experiments with planidia and different ant species, all of the ants died quite fast. On Leea, we found foraging ants killed and attached to leaves and stems. Ants tend to keep foraging trails free of dead nestmates to avoid disease transmission (e.g. Wilson et al. 1958). Besides lurking on the shoot and other plant parts, lurking on corpses might be another way to ensure a contact with foraging ants. Access to ant colonies might also occur via alternate hosts (Heraty et al. 2004). As we observed the planidia to attack all kinds of insects in the vial experiments, they might indeed use a number of different taxa of insect prey as a transport vehicle. However, the killing of potential host ants foraging on the plant does not fit to what is known so far about phoretic behaviour of the Eucharitidae. At this point, we could only speculate what strategies planidia might use to gain access to the ant host colony.