Interactions Among Latitude, Nematode Parasitization, and FemaleSirex nigricornis(Hymenoptera: Siricidae) Fitness

Abstract

Sirex nigricornis F. (Hymenoptera: Siricidae) is an innocuous pine-inhabiting woodwasp native to eastern North America, utilizing dead or dying pine trees as hosts. Although S. nigricornis F. does not cause economic damage, a closely related species, Sirex noctilio, was discovered in New York in 2004 and has continually spread throughout the northeastern United States and southern Canada, threatening the multi-billion-dollar pine timber industry of the southeastern United States and raising interest about potential interactions with native woodwasps and associated mortality agents. A non-sterilizing strain of the biological control agent, Deladenus siricidicola Bedding (Tylenchida: Neotylenchidae), was introduced along with S. noctilio but is not inhibiting the spread or establishment of S. noctilio A North American congener, Deladenus proximus Bedding, has been recently isolated from S. noctilio and shows promise as a biological control agent. To better understand the potential of D. proximus as a control agent for S. noctilio, we measured and dissected nearly 1,200 S. nigricornis females from Arkansas and Mississippi and evaluated differences among collection location with regard to nematode virulence, woodwasp body size, and egg load. Body size and egg load were related to collection location, and nematode infestation resulted in significantly smaller females who produced significantly fewer eggs. Female woodwasps, especially those collected in Arkansas, were often fully sterilized by nematodes, and a higher percent sterilization was inversely related to body size and fewer eggs. We propose field studies to test the nematode's ability to sterilize S. noctilio in the northeastern United States.

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

“…We found that parasitized S. nigricornis females had on average 21% fewer eggs than unparasitized females, a general trend also reported by Hartshorn, Chase, et al (2016). Among the parasitized females, an average of 79% of their eggs contained nematodes.…”

“…This rate of sterilization may assist in retention of the nematodes in a host population; none of the locations we studied averaged 0 or 100% egg sterilization, which has been found in other associations (Table 1). Prevalence of nematode parasitism in S. nigricornis populations differed by site, agreeing with Hartshorn, Chase, et al (2016) that Arkansas had high prevalence; however, this level was nowhere near the high prevalences reported for S. noctilio in Australia (Bedding & Akhurst, 1974). Deladenus proximus had a positively density dependent relation to the host within trees and parasitized more males than females.…”

“…In this study, approximately a third of naturally occurring S. nigricornis were parasitized by D. proximus , ranging from 0.16 to 0.43 of individuals between sites and years (Figure 1). Previous studies conducted in five southeastern and midwestern states reported parasitism of S. nigricornis by D. proximus ranging from 0% to 44% (Fisher et al, 2017; Hartshorn, Chase, et al, 2016; Zieman et al, 2015). Thus, it appears that less than 50% of S. nigricornis are parasitized by D. proximus under natural conditions.…”

“…The average sterilization of S. nigricornis eggs by D. proximus has been over 50% in all studies to date (Table 1). When 20–30 eggs were examined per female, some parasitized females had 70–100% sterilized eggs (Fisher et al, 2017; Hartshorn, Chase, et al, 2016). In our study, where 100 eggs were examined per female (or all of the eggs, if there were fewer than 100), two females had 10% sterilization, while all others had ≥37%.…”

“…Both of these fungi are vectored by S. nigricornis (Hajek et al, 2013). Studies of D. proximus parasitizing S. nigricornis have documented between 0% and 44% of the individuals in the population parasitized in five southeastern and midwestern states, with highly variable egg sterilization (Fisher et al, 2017; Hartshorn, Chase, et al, 2016; Zieman et al, 2015) (Table 1).…”