Fatal diseases and parasitoids: from competition to facilitation in a shared host

Abstract

Diverse parasite taxa share hosts both at the population level and within individual hosts, and their interactions, ranging from competitive exclusion to facilitation, can drive community structure and dynamics. Emergent pathogens have the potential to greatly alter community interactions. We found that an emergent fungal entomopathogen dominated pre-existing lethal parasites in populations of the forest defoliating gypsy moth, Lymantria dispar. The parasite community was composed of the fungus and four parasitoid species that only develop successfully after they kill the host, and a virus that produces viable propagules before the host has died. A low-density site was sampled over 17 years and compared with 66 sites across a range of host densities, including outbreaks. The emergent fungal pathogen and competing parasitoids rarely co-infected host individuals because each taxa must kill its host. The virus was not present at low host densities, but successfully co-infected with all other parasite species. In fact, there was facilitation between the virus and one parasitoid species hosting a polydnavirus. This newly formed parasite community, altered by an emergent pathogen, is shaped both by parasite response to host density and relative abilities of parasites to co-inhabit the same host individuals.

Reduced Compsilura concinnata parasitism of New England saturniid larvae

“…Starting in the late‐1980s, the establishment of E. maimaiga in the northeastern U.S.A. reduced both the frequency and magnitude of L. dispar outbreaks (Hajek, ; Hajek et al , ). This pathogen also kills L. dispar larvae so quickly that co‐infecting parasitoids such as Compsilura are unable to complete their development (Hajek & van Nouhuys, ). Compsilura also tends to attack L. dispar during early instars, whereas Entomophaga kills later‐instar larvae.…”

Ecological Factors Mediate Immunity and Parasitic Co-Infection in Sea Fan Octocorals

“…There are several systems where competing forces have created such unexpected co-occurrence patterns ( 27 , 30 , 33 , 34 ). Another possibility is that host mortality drives patterns of co-occurrence ( 30 , 32 ). Whole-colony mortality is infrequent over the scale of a few years in these sea fans (Tracy et al., unpublished data), but colony mortality may not be the correct metric to evaluate the impact of mortality in sea fans.…”

“…One overarching finding is that patterns of parasite co-occurrence cannot alone reveal the nature of the interaction between parasites. Factors including host mortality, parasite resource use, and host immune responses together shape co-occurrence patterns, meaning that parasites facilitating each other immunologically may not positively co-occur ( 30 – 32 ). Similarly, the negative pattern expected from immune-mediated antagonism between parasites combined with the positive force of common resource use could equilibrate as random co-occurrence, as suggested for an amphibian co-infected with two trematodes ( 33 ).…”

Gypsy moth larval necropsy guide

“…On its own, LdMNPV only increases in abundance in high density populations (Doane and McManus 1981). LdMNPV is now found infecting insects in high density gypsy moth populations, but its prevalence is often low, and E. maimaiga infections have become much more common, at times leading to the collapse of gypsy moth populations at a variety of densities (Hajek et al 2015, Hajek and van Nouhuys 2016, Liebhold et al 2013.…”

“…Since the early 1900s, intensive efforts have been made to introduce biological control agents against the gypsy moth, including 34 parasitoids, 1 predator, and 5 pathogens (Fuester et al 2014). Successful establishment of parasitoid introductions has been limited, as has the effects of parasitoids on the regulation of gypsy moth populations (Hajek and van Nouhuys 2016). While parasitoids contribute to the overall mortality of gypsy moth populations, pathogens often play a more vital role.…”