Re-assessing the infection strategies of the entomopathogenic nematode Steinernema feltiae (Rhabditidae; Steinernematidae)

Abstract

Previous studies have indicated that between 60 and 80 % of a population of entomopathogenic nematodes do not infect their insect hosts at any one period in time. Two hypotheses explain this behaviour : the first that there is a subpopulation of non-infectious nematodes and the second that the non-infectious group is created by inhibitory cues derived from infected insects. Through an experimental approach with the Galleria mellonella-Steinernema feltiae system we show that both mechanisms operate together. When conditions for infection were optimized, the sum of individual infection behaviours was similar to the number infecting as a population, implying observed infection rates are driven by intrinsic mechanisms. In addition, there was evidence that an infected host released a chemical cue into the environment which inhibited subsequent levels of infection. This degree of inhibition was independent of the number of infecting nematodes. Both these mechanisms are dynamic, so the observed proportion of infectious nematodes depended heavily on the time of exposure. The implications of these findings for both the design of laboratory trials and the use of entomopathogenic nematodes in biological control are discussed.Key words : entomopathogenic nematodes, Steinernema feltiae, subpopulation, inhibitory cue, biological control. Entomopathogenic nematodes are used as biological control agents of insect soil pests such as sciarid mushroom flies (Lycoriella spp.) and the black vine weevil (Otiorhynchus sulcatus). Nematodes are typically applied in high numbers, for example 3 million infective juveniles per square metre are recommended to control the mushroom sciarid fly (MicroBio, UK). Although such a large number of nematodes are used to protect crops, laboratory studies have shown that the majority of the freeliving larvae are not infective (Bednarek & Nowicki, 1986 ; Fan & Hominick, 1991 a ; Bohan & Hominick, 1995 a, b, 1996, 1997 Glazer, 1997). The aim of this study is to understand the infection process that causes this phenomenon.Following the work of Bednarek & Nowicki (1986) two hypotheses have arisen in the literature to describe the invasion strategies of these nematodes. The first states that infective juveniles can be divided into two subpopulations where one is infective and the other is not. To demonstrate this Bohan & Hominick (1995 a, b, 1996 showed that the overall proportion of the nematode Steinernema feltiae (Filipjev) infecting in a given time-period did not change and never exceeded 40 %, even when the

Existence of infective juveniles in the offspring of first- and second-generation adults of Steinernema rarum (OLI strain): evaluation of their virulence

“…Factors related to the generation of origin of the IJs may cause some of these different behaviors. It has also been reported that the population of IJs able to infect a host is unlikely to be a homogenous group, exhibiting variations in aspects such as infection process, sensitivity to various cues, host-searching strategies, and differences among individuals in their condition of primary or secondary invaders (Campbell et al, 1999;Fairbairn et al, 1999;Hay and Fenlon, 1995). These variations found within a single group of IJs that emerged from a host could be explained, in part, by factors related to the generation of IJs.…”

“…Knowledge of the biological characteristics of various steinernematid species and strains, such as virulence, is useful in order to select the most appropriate biocontrol agent for a specific pest (El-Sadawy and Saleh, 1999;Fairbairn et al, 1999). Biological characteristics vary among nematode species (Campbell and Kaya, 2002), and even among nematode strains (Hazir et al, 2001;Simões et al, 2000).…”

Soil mediates the interaction of coexisting entomopathogenic nematodes with an insect host

“…4A) and a highly signiWcant negative impact of S. feltiae on H. marelatus infectivity overall. Although heterospeciWc responses are not always negative (Lewis et al, 2006), these results suggest the hypothesis that H. marelatus IJs is repulsed by the cues of ongoing S. feltiae infection in tubes with hosts (Grewal et al, 1997;Fairbairn et al, 2000). Because we did not eliminate all organic material from Weld-collected soils (except by coarse sifting), we cannot determine whether nematodes recruited to carbon dioxide generated by increased microbial activity in wetted soils.…”

Laboratory susceptibility tests of Aedes aegypti and Culex quinquefasciatus larvae to the entomopathogenic nematode Heterorhabditis bacteriophora

“…Aún hay mucho que explorar en este campo si se tienen en cuenta las diferencias intraespecíficas que pueden afectar la capacidad infecciosa de los nematodos entomopatógenos, y que estarían relacionadas con su comportamiento (25), con el insecto y con factores ambientales (26).…”