The lethal and sub-lethal consequences of entomopathogenic nematode infestation and exposure for adult pine weevils, Hylobius abietis (Coleoptera: Curculionidae)

Abstract

a b s t r a c tEntomopathogenic nematodes (EPN) frequently kill their host within 1-2 days, and interest in EPN focuses mainly on their lethality. However, insects may take longer to die, or may fail to die despite being infected, but little is known about the effects of EPN infection on insects, other than death. Here we investigate both lethal and sub-lethal effects of infection by two EPN species, Steinernema carpocapsae and Heterorhabditis downesi, on adults of the large pine weevil, Hylobius abietis. Following 12 h nematode-weevil contact in peat, S. carpocapsae killed a significantly higher proportion of weevils (87-93%) than H. downesi (43-57%) at all concentrations tested. Less than 10% of weevils were dead within 2 days, and weevils continued to die for up to 10 days after exposure (LT 50 of 3 days or more). In a separate experiment, live weevils dissected 6 days after a 24 h exposure to nematodes on filter paper harbored encapsulated and dead nematodes, showing that weevils could defend themselves against infection. Some live weevils also harbored live nematodes 6 days after they had been removed from the nematode infested medium. Feeding by weevils was not affected by infection with, or exposure to, either species of EPN. We discuss these results in relation to the use of EPN in biological control against H. abietis.

Pine weevils modulate defensive behaviour in response to parasites of differing virulence

“…More weevils groomed in response to both species but grooming insects groomed for longer than controls only when the infective juveniles were S. carpocapsae. Since S. carpocapsae killed twice as many adult pine weevils as H. downesi at an equivalent concentration under conditions in which behavioural avoidance was precluded (Girling et al 2010), these findings support the hypothesis that hosts invest more in defence against more virulent parasites, but raise the question, how does the host recognize that a parasite is more virulent?…”

“…Natural populations have probably exerted strong selection pressure on soil-associated insects to evolve multiple defences (Villani et al 1999). Hylobius abietis adults display each of the three levels of defence identified for insects by Gross (1993): (1) reduced probability of being found and contacted, as shown here by evasion of a contaminated surface; (2) behavioural and morphological defences, including typical thick cuticle of weevils and grooming as also shown here; and (3) physiological mechanisms to kill the parasites, including encapsulation (Girling et al 2010). It is unclear which level is most important in protecting the weevils against entomopathogenic nematodes, but avoiding infection has advantages over strategies initiated after infection (Hart 1994), being relatively lower in cost (Poulin et al 1999).…”

“…In this study, we tested the hypothesis that adult pine weevils, Hylobius abietis, respond to the presence of entomopathogenic nematodes by grooming and/or avoidance of a contaminated substrate; that they respond differentially to different nematode species, and that they are more likely to respond to a more virulent than a less virulent species (Steinernema carpocapsae and Heterorhabditis downesi, respectively; Girling et al 2010). To understand what might cause differences in weevil response to different nematode species we also examined the behaviour of both nematode species on the body of the insect host.…”

The impact of entomopathogenic nematodes on a non-target, service-providing longhorn beetle is limited by targeted application when controlling forestry pest Hylobius abietis

“…Larvae were the least susceptible, with LC 50 (24-42 IJs) and LT 50 (4-7 days) similar to what has been reported for larvae of other xylophagous beetles, including the target pest H. abietis Burman, 1977, 1978;Lindegren et al, 1981;Solter et al, 2001;Fallon et al, 2004). R. bifasciatum adults were especially vulnerable to EPN infection (LC 50 < 10 IJs, LT 50 6 3 days at 1000 IJs), considerably more so than adults of H. abietis, which had an LT 50 of over 4 days at concentrations of up to 4000 IJs for both S. carpocapsae and H. downesi and showed only 57% mortality after 10 days exposure to 4000 H. downesi IJs (Girling et al, 2010). In our laboratory and field experiments, larvae inside deadwood logs were less frequently infected with H. downesi and S. carpocapsae than pupae and (for H. downesi) also adults, reflecting a trend similar to that observed in the Petri dish assay.…”

Behaviour and Population Dynamics of Entomopathogenic Nematodes Following Application

“…Adult pine weevil H. abietis, which are capable of flight, transported EPN on their elytra (Kruitbos, Heritage, & Wilson, 2009). Since they are also susceptible to EPN infection and survive for several days post-infection (Girling, Ennis, Dillon, & Griffin, 2010), internal transport in these weevils is also possible. Following application of S. scapterisci to control mole crickets, infected insects were collected as far as 23 km from the nearest site of application, and this method of dissemination was important in establishing the species as part of a strategy of mole cricket suppression in Florida (Parkman & Smart, 1996).…”