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

Abstract

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.