Association of the native parasitic nematode Deladenus proximus with individuals and populations of the native woodwasp Sirex nigricornis

Abstract

Parasitic stages of the North American native dimorphic nematode Deladenus proximus develop within larvae of the native North American siricid Sirex nigricornis and sterilize many of the eggs in adult females. Sirex nigricornis were reared from felled trees or trapped in Arkansas and Louisiana in 2017–2019 and parasitism by D. proximus was evaluated. Deladenus proximus parasitized 0.16–0.43 of S. nigricornis individuals, among sites and years. Parasitized males were smaller than healthy males, and more males were parasitized than females emerging from the same trees. Rate of parasitism increased with increasing S. nigricornis density per tree, and, as per tree density increased, so did the proportion that was male. Egg sterilization (woodwasp eggs killed and inhabited by nematodes) was partial, with an average proportion of 0.66–0.88 woodwasp eggs sterilized by D. proximus per female S. nigricornis, differing significantly by site. As the numbers of eggs in a female increased, the proportion of eggs sterilized declined. Comparing data from this study with others, a trend is reported for variable levels of egg sterilization in Sirex females, with 0 or 100% sterilization often occurring in novel host/parasite associations and partial (averages between 50% and 100%) sterilization occurring when host/parasite associations have co‐evolved.

Population level interactions between an invasive woodwasp, an invasive nematode and a community of native parasitoids

“…Species of nematodes were not evaluated using molecular methods for this study but, based on past results (Kroll et al 2013;Williams and Hajek 2017), we presume that the vast majority . However, D. proximus are usually found in the host eggs (Hartshorn 2021;van Nouhuys et al 2022), and in our study nematodes were only seen within eggs from 4 S. noctilio from 2 sites in 2011. This was the first year of this study and there S. nigricornis were more abundant (Suppl.…”

“…To evaluate nematode parasitism, subsamples of S. noctilio from each site and year were dissected as described in van Nouhuys et al (2022) and presence of parasitic nematodes within bodies was recorded. A total of 3154 randomly chosen S. noctilio were dissected to detect whether they were parasitized by nematodes.…”

“…The impact of nematodes on host woodwasps is determined by both nematode and woodwasp species and genotypes (Bedding 1972). Different species and strains of Deladenus parasitizing differing species and strains of Sirex are known to either totally sterilize (i.e., kill all host eggs in adult females), partially sterilize, or not sterilize parasitized hosts (van Nouhuys et al 2022). The INA (= Introduced to North America; formerly referred to as 'non-sterilizing') strain of D. siricidicola that was putatively accidentally introduced with S. noctilio to North America does not sterilize eggs of S. noctilio but causes a decrease in adult size and fecundity (Kroll et al 2013;Hajek and Morris 2021).…”