Demography of an invading forest insect reunited with hosts and parasitoids from its native range

Abstract

The Sirex woodwasp Sirex noctilio Fabricius (Hymenoptera: Siricidae), a widespread invasive pest of pines in the Southern Hemisphere, was first detected in North America in 2004. This study assessed the impacts of life history traits, host resistance and species interactions on the demography of S. noctilio in New York, Pennsylvania and Vermont, then compared key metrics to those found in the native range in Galicia, Spain. Many trees naturally attacked by S. noctilio in North America produced no adult woodwasps, with 5 of 38 infested trees (13%) sampled across six sites yielding 64% of emerging insects. Reproductive success was highest in the introduced host scots pine, Pinus sylvestris, but native red pine, Pinus resinosa, produced larger insects. Sirex noctilio required one or sometimes two years to develop and sex ratios were male biased, 1:2.98 ♀:♂. Body size and fecundity were highly variable, but generally lower than observed in non-native populations in the Southern Hemisphere. Hymenopteran parasitoids killed approximately 20% of S. noctilio larvae and 63% of emerging adults were colonized by the parasitic nematode Deladenus siricidicola, although no nematodes entered eggs. Demographic models suggested that S. noctilio in the northeastern USA have a higher potential for population growth than populations in the native range: estimated finite factor of increase, λ, was 4.17–4.52 (depending on tree species colonized), compared to λ = 1.57 in Spain.

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

“…On average, 37% of the Sirex from a tree was parasitized, ranging from 0 to 100%. This overall parasitism is similar to what was found by Krivak-Tetley et al (2022) in a smaller study in this region, and by Haavik et al (2015) using experimentally-exposed logs. We found the frequency of parasitism by I. l. ensiger increased with host density, and the frequency of parasitism by rhyssines decreased with increasing host density, leading to no association of overall parasitism with host density (Table 3).…”

“…See text for explanation of the statistical models. The nematode D. siricidicola was present in 11% of the S. noctilio which is less than reports from many smaller studies in North America (see Krivak-Tetley et al 2022), though nematode parasitism levels were variable and were much higher in some sites (Fig. 4).…”

“…Indeed, I. l. ensiger has been shown to be attracted to the fungus associated with S. noctilio (Martinez et al 2006;Faal et al 2021) and to increase residence time on trees at increased S. noctilio density (Corley et al 2010). However, in this complicated system, Foelker et al (2016a) found no association of rate of parasitism by I. l. ensiger with host density while Krivak-Tetley et al (2022) found a host density association for I. l. ensiger in Pinus resinosa, the pine predominantly sampled in this study, but not in Pinus sylvestris. Such differences between studies are not surprising where there is density dependence, especially when studies are conducted at different host densities.…”