Behavioral Modification of Leucauge mariana Induced by an Ichneumonid Spider-Parasitoid, Hymenoepimecis castilloi, in the Colombian Andes

Hymenoepimecis is a genus of Darwin wasps in the Polysphincta group of genera (Hymenoptera: Ichneumonidae: Pimplinae) known as ectoparasitoids of a broad spectrum of spiders. The parasitoid induces production of a web known as cocoon web, which provides shelter and support for the wasp pupa. In this study, we describe for the first time the interaction between Hymenoepimecis castilloi Pádua & Sääksjärvi (Hymenoptera: Ichneumonidae) and its host spider Leucauge mariana (Taczanowski) (Araneae: Tetragnathidae) in the Colombian Andes, provide new records of wasp genus distribution, and described the behavioral modifications induced in the spider. Web modifications occurred in the webs of both solitary and aggregated individuals. Adhesive spirals were lacking, and webs were connected to vegetation by multiple threads in all cocoon webs, which was not seen attached to webs of non-parasitized spiders. All parasitoid cocoons were observed hanging on a vertical line in the hub of the cocoon web. As previously described for other species, we believe that this modified web design results in increased web strength and favors parasitoid development during the pupal stage.

A New Darwin Wasp (Hymenoptera: Ichneumonidae) and New Records of Behavioral Manipulation of the Host Spider Leucauge volupis (Araneae: Tetragnathidae)

Some ichneumonid wasps of the Polysphincta group of genera (Hymenoptera: Ichneumonidae: Pimplinae) induce behavioral modifications in their host spiders during a specific moment of their development, resulting in the construction of webs that differ in several aspects from those constructed by unparasitized individuals. In this study, we describe the parasitoid wasp Hymenoepimecis pinheirensis sp. n. (Ichneumonidae: Pimplinae) and present information on behavioral modifications in the orb-web structure of its host, the spider Leucauge volupis (Keyserling 1893). Previously, reported observation on this host/parasitoid interaction was restricted to one locality, and the wasp species was misidentified as Hymenoepimecis jordanensis Loffredo and Penteado-Dias 2009. Modified webs built by parasitized spiders lack adhesive spirals and have several radii that converge to the web hub. The cocoon built by the wasp larvae is attached to the web hub, suspended by horizontal radial lines, and surrounded by a tridimensional tangle positioned below the hub. This modified web structure is similar to the most frequent architecture of webs constructed by individuals of Leucauge mariana (Taczanowski 1881) parasitized by Hymenoepimecis tedfordi Gauld 1991. However, cocoon webs built by L. volupis parasitized by H. pinheirensis sp. n. differ from the cocoon webs described for the other Leucauge species parasitized by Hymenoepimecis wasps. This evidence suggests that the modified web pattern in Leucauge species is determined by specific responses of each spider species to the behavioral manipulation mechanism displayed by the wasps.

Two lines of defense in the pupas of ichneumonid wasp parasitoids associated with spider hosts

Insects in the pupal stage are vulnerable to various predators because the pupa is immobile. The pupas of parasitoid ichneumonid wasps (Ichneumonidae) associated with spider hosts have evolved two lines of defense against predators, namely a cocoon spun by the parasitoid larva and a web provided by the spider host. The web is derived from a normal or modified spider web built by the spider under manipulation by the penultimate instar of the parasitoid wasp. In laboratory experiments, we tested the efficacy of these two defensive lines using six potential predators with two different types of mouthparts coming from three foraging guilds. The presence of the cocoon significantly reduced predation. Scavengers with chewing mouthparts, e.g., cockroaches and crickets, attacked and consumed pupas within both sparse and strong cocoon walls. Scavengers with piercing mouthparts were able to attack pupas in cocoons with a sparse wall, but not with a strong wall. Collectors and true predators showed no interest in cocoons. The presence of a web increased pupa protection by up to 80% when the web was on the ground and by up to 95% when the web was in the air. Only scavengers with chewing mouthparts were able to reach and consume pupas sheltered by the web. We provide the first evidence of how the two lines of defense contribute to parasitoid defense during the pupal stage.

Evolutionary aspects of the parasitoid life strategy, with a particular emphasis on fly–spider interactions

Parasitoids are significant ecological elements of terrestrial food webs and have evolved within seven insect orders. Interestingly, however, associations with spiders as hosts have evolved only in two insect orders, Diptera and Hymenoptera. Here, we summarize various aspects of host utilization by dipteran flies with an emphasis on associations with spiders. Our synthesis reveals that spider flies (family Acroceridae) have evolved a unique life strategy among all the parasitoid taxa associated with spiders, in which koinobiont small-headed flies utilize an indirect oviposition strategy. This indirect oviposition in spider flies is inherited from Nemestrinimorpha ancestors which appeared in the Late Triassic and is characterized by the evolution of planidial larvae. Further, we discuss the advantages and disadvantages of indirect oviposition in spider flies. On the one hand, indirect oviposition allows the fly to avoid contact/wrestling with spider hosts. On the other hand, larval survival is low because the planidium must actively seek out and infect a suitable host individually. The risk of failure to find a suitable spider host is offset by the fly’s extremely high fecundity.